# On the Need for a Near-Earth Object Characterization Constellation in Low-
Earth Orbit

Nathan Golovich

Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550,
USA<EMAIL_ADDRESS>
###### Abstract

In 2005, the United States Congress passed a bill mandating the detection,
tracking, cataloguing and characterization of 90% of the 140 meter and larger
near-Earth objects (NEOs) by 2020. At the deadline $\sim$35% were detected,
tracked and catalogued, but only a small fraction were characterized. At the
present rate, it will take 40 years to meet the detection mandate, and there
are insufficient global facilities dedicated to NEO characterization to come
close to the characterization threshold. The major surveys focus mainly on
detection and initial orbit determination, which must be refined in order to
fully be tracked and catalogued. Characterization requires observations
spanning multiple wavelengths, cadences, and instruments, so it is challenging
for observers to acquire the requisite data in a timely manner for planetary
defense. Two upcoming surveys will easily meet the 90% threshold for
detection, but each will require separate facilities to tip and queue to
refine orbits and characterize new discoveries, and they will provide too many
discoveries for ground and space-based assets to keep up with. Here, I argue
for a constellation of proliferating small satellites carrying visible and
infrared sensors that would offer the needed coverage and flexibility to
follow up detections from current and upcoming surveys in a timely manner.
Such a constellation would enable NASA to move beyond the detection focused
investments and fully meet the 2005 Congressional mandate.

##  1 Introduction

In the past few years, satellites in low-Earth orbit (LEO)111LEO generally
implies roughly circular orbits ($e<0.25$) with altitudes below 2000 km. have
proliferated. These serve many purposes including scientific, civil, and
commercial. The explosion in numbers has occurred due to the confluence of two
primary factors:

  * •

Small satellite platforms have gotten cheaper due to competition and economies
of scale. Off-the-shelf and space-ready options exist for satellite buses,
communications systems, on-board processors, science instrumentation, cooling
systems, attitude control, and other components.

  * •

The introduction of commercial space-launch companies has opened the door to
ride-sharing, which has vastly decreased the cost to get to space (Jones,
2018).222The cost to LEO was $20\times$ less for a SpaceX Falcon 9 compared
with NASA’s space shuttle in 2011.

Indeed, the number of satellites orbiting in LEO worries ground-based
astronomers. Satellites are an ever-present contaminant in ground-based images
(Figure 1 shows a recent example from Lick Observatory). This concern is due
to not only the assumption of continued proliferation of LEO satellites, but
also the increase in LEO satellite size (and thus brightness). Starlink and
similar constellations are bright enough to saturate typical photometric
images taken by professional telescopes. Numerous studies in the recent
literature have explored the impact to ground based astronomy caused by large
LEO constellations (e.g.; Hainaut & Williams, 2020, Tyson et al., 2020, Lawler
et al., 2022, Mróz et al., 2022). The general consensus is that wide field
surveys from large apertures could have as many as a half of their images
compromised in the first and last hour of observing each night. Such surveys,
unfortunately, are typical of surveys for near-Earth objects (NEOs333NEOs can
be either asteroids or comets, but are predominantly asteroids. I will use NEO
and asteroid interchangeably.), which invariably have large etendue (product
of aperture and field of view).

Figure 1: Recent Nickel telescope image showing satellite streaking through
the top-right quadrant of the camera during the exposure time. Meanwhile, a
NEO streaks through the center of the camera with a much smaller angular
speed. Note that the vertical lines are dead pixel columns, not streaking
objects.

Nevertheless, large ground-based observatories dedicated to detecting NEOs
discover the vast majority of NEOs today. This will be true through the 2020s.
In the 2030s, major ground and space based surveys will discover more faint
NEOs than ever before. The two major next generation surveys will stress our
ability to follow up NEOs for vital tracking and characterization studies. In
this paper, I will assess the need for and benefits of a network of
optical444I will use optical and visible interchangeably to describe light in
the $\sim 400-800$ nm range. and near-infrared (NIR) sensors operating in
space for planetary defense. The impending deluge of new NEO discoveries by
next-generation survey instruments will overwhelm current assets, and such a
constellation in LEO would be able to tip and queue the satellites to cover a
larger range of orbital phase space, which would deliver the oft-overlooked
characterization mission for NEOs. Tracking and characterizing NEOs quickly
after discovery offers timely and vital information for planetary defense
mitigation strategies, especially as more NEOs are discovered in the future
that could have a higher probability of impacting Earth on a shorter timeline
than has been typical.

Such an occurrence happened following the 2004 discovery of 99942 Apophis,
which was assessed to have a few percent chance of impacting Earth in 2029 or
2036 (Giorgini et al., 2005, Chesley, 2006). Ultimately, later observations
refined the orbit and retired the risk (see e.g., Giorgini et al., 2008,
Chesley et al., 2009). However, the experience led numerous astronomers to
study the asteroid carefully for features that could inform planetary defense
mitigation strategies (Benner, 2006, Cellino et al., 2007, Delbò et al., 2007,
Rubincam, 2007, Binzel et al., 2009, Žižka & Vokrouhlický, 2011, Pravec et
al., 2014, Müller et al., 2014, Breiter & Murawiecka, 2015, Brozović et al.,
2018, Reddy et al., 2018). Simultaneously, numerous studies developed
mitigation strategies rapidly after discovery (Maccone, 2006, Kahle et al.,
2006, Izzo et al., 2006, Bailey et al., 2007, Englander, 2008, Fargion, 2008,
Mengali & Quarta, 2009, Quarta & Mengali, 2010, Margulieux, 2010, Hyland et
al., 2010, Wagner & Wie, 2013, Foster et al., 2013, Zeng et al., 2014, Kim et
al., 2021). This experience is possible to occur again because new surveys
will be discovering NEOs more rapidly than ever before. This will increase the
probability of another scenario that requires quick orbital track refinement
and characterization. Ultimately, the 99942 Apophis scenario was used for a
planetary defense exercise conducted during the 2020–2021 close approach to
Earth (Reddy et al., 2022a).

Led by fact that upcoming surveys will increase the likelihood of another
scenario like 99942 Apophis while ground-based tracking and characterization
assets are insufficient to serve this task, I decided to discuss the challenge
at hand in this paper. In §2, I will discuss the language of the NASA
Authorization Act of 2005, which mandated NASA to detect and characterize
large NEOs in the first place. I will map the text of the bill onto a set of
what I call NEO “missions” for planetary defense that must be satisfied in
order to fully meet the mandate. In §3, I will highlight the major NEO
detection surveys of the present and future. I will also discuss how these
meet the various NEO missions and what gaps remain for follow-up instruments.
In §4, I will discuss the assets available to astronomers for follow up
observations of NEOs to fill the gaps in information that the surveys are ill
equip to fill themselves. In §5, I will discuss how a constellation of small
LEO satellites offer a technically effective and feasible solution to fully
meet the mandate set by Congress. Finally, in §6, I will discuss the key
takeaways of this exercise and offer my conclusions.

##  2 Dissecting the Congressional Mandate

Section 321 of the NASA Authorization Act of 2005 (Public Law No.
109-155555https://www.congress.gov/109/plaws/publ155/PLAW-109publ155.pdf)
contains the text originally written as the George E. Brown Near Earth Object
Survey Act666https://www.congress.gov/bill/109th-congress/house-bill/1022
(hereafter NEO Survey Act), which mandates NASA to “detect, track, catalogue,
and characterize the physical characteristics of near-Earth objects equal to
or greater than 140 meters in diameter in order to assess the threat of such
near-Earth objects to the Earth.”

The NEO Survey Act is often discussed as a mandate related to detecting NEOs.
However, this overlooks the additional aspects of the NEO Survey Act’s text,
which explicitly lay out a series of actions (namely the verbs detect, track,
catalogue, and characterize). Taken collectively, these actions enable the
full assessment of the threat to Earth posed by NEOs larger than 140 meters. I
will refer to these verbs in terms of a corresponding “mission” with each
working toward the full mandate laid out in the bill. NASA has funded various
efforts over the past few decades that respond to these missions, but the
clear priority has been in funding the detection mission.

###  2.1 Size Threshold in the NEO Survey Act

The NEO Survey Act includes a size threshold of 140 meters and larger, which
is a relatively arbitrary cut off. It corresponds to the size of a typical NEO
with an absolute magnitude $H=22$ (assuming an albedo of 0.15; see Bowell et
al. p. 524-556 in Binzel et al., 1989, Harris & Harris, 1997). The population
of NEOs with $H<22$ has been estimated to contain around 25,000 objects
(Heinze et al., 2021). There are many asteroids with albedo lower than 0.15,
and since the NEO Survey Act was written in terms of size, it will be
necessary to discover many additional NEOs with $H>22$ to fully meet the
mandate. The absolute magnitude for an asteroid is equivalent to $V$ magnitude
under the assumption that the asteroid is 1 AU from both the Sun and the Earth
with zero phase angle777Note this is technically not physical..

An impact by a 140 meter asteroid would certainly be problematic. These are
typically referred to as “city killers,” but objects as small as 20 meters can
cause substantial damage on the ground. This was made apparent in 2013 when a
$\sim$20 meter asteroid exploded in the atmosphere above Chelyabinsk, Russia.
The 500 kiloton air-burst generated a shock wave that injured over 1000 people
and damaged thousands of buildings (Brown et al., 2013a). This event led to
revised estimates that shifted the residual risk substantially toward NEOs of
10–50 meters in diameter (Popova et al., 2013). The NEO size distribution
generally follows a broken power-law form (see e.g., Heinze et al., 2021).
While there are $\sim$30,000 NEOs 140 meters or larger, there at least a few
million NEOs as large as the Chelyabinsk meteor or larger (Granvik et al.,
2018, Heinze et al., 2021). For the majority of this paper, I will focus on
the NEOs mandated by Congress; however, where appropriate, I will discuss the
implications of the large number of NEOs that are dangerous by not covered by
the mandate.

###  2.2 NEO Detection Mission

Here I will briefly describe the NEO detection mission as it relates to the
NEO Survey Act. Later in §3, I will discuss in more detail the major surveys
that carry out this mission. Planetary defense starts with the detection (or
discovery) mission. For a detailed review of the NEO detection mission, I
refer readers to National Academies of Sciences & Medicine (2019). One key
finding of this report suggests that a combination of optical and IR systems
should continue to be funded since neither modality offers full information
necessary for planetary defense. Objects are generally detected by major
surveys, which have cadences designed to recover the same object a few times
in a short span to issue a discovery and initial orbit determination (IOD). In
this respect, prioritizing funding for surveys makes sense. The detection list
can be triaged based on the IOD; however, detections are typically made while
NEOs are close and only recoverable for a short duration, so missing the
opportunity to track and characterize any newly discovered NEO could mean
years of waiting for another opportunity.

###  2.3 NEO Tracking and Cataloguing Missions

NEO discoveries are generally posted to the Minor Planet Center
(MPC)888https://minorplanetcenter.net/, which keeps a list in need of new
discoveries in need of refinement for astronomers to observe until the orbit
is sufficiently well-tracked. Meanwhile, the MPC and other cataloging services
such as NASA’s Center for Near Earth Object Studies
(CNEOS)999https://cneos.jpl.nasa.gov and the European Space Agency’s Near
Earth Objects Coordination Centre101010https://neo.ssa.esa.int maintain
catalogs and offer ephemeris services and other observing tools that enable
astronomers to plan follow-up observations that help satisfy both the tracking
and characterization missions. Many amateur astronomers contribute
meaningfully to the tracking mission by observing new discoveries after they
are posted to the MPC.

###  2.4 NEO Characterization Mission

Of the roughly 10,000 NEOs 140 meters or larger discovered today, only a small
fraction are fully characterized. This is because constraining the physical
characteristics of NEOs is an expensive task. Parameters of interest include
the size, shape, rotational, and material properties. In this subsection, I
will primarily discuss the methods astronomers use to constrain these
features, which are important for planetary defense mitigation strategies and
impact threat assessments (e.g.; Asphaug et al., 1998, Bruck Syal et al.,
2016, Feldhacker et al., 2017, Daou & Johnson, 2019, Dearborn et al., 2020,
Burkey et al., 2020, Graninger et al., 2021, King et al., 2021). Readers are
also encouraged to review the United State’s NEO Preparedness and Action
Plan111111 https://www.nasa.gov/sites/default/files/atoms/files/ostp-neo-
strategy-action-plan-jun18.pdf.

In 2022, NASA carried out the first planetary defense experiment to deflect an
asteroid’s trajectory with a collision with the Double Asteroid Redirection
Test (DART; Rivkin et al., 2021). The success or failure of such a mission in
a real-world deflection attempt could very well require precise of knowledge
of these characteristics listed above so that the maximal impulse may be
imparted upon impact (Asphaug et al., 1998, Bruck Syal et al., 2016). Other
possible mitigation strategies exist ranging from painting asteroids white
(Katz, 2022) to successive impulsive impacts or even deflecting or destroying
a NEO with a nuclear explosion (Dearborn et al., 2020). The methods to
optimally defend against a likely impact depend strongly on the lead time as
well as knowledge of the asteroids physical properties. Thus, it is important
to consider the gaps in our present investments in meeting the mandate’s full
list of requirements. For the remainder of this subsection, I will discuss
methods to constrain various NEO features following discovery. I will point
out when one method is more constraining than another and also when one method
is more expensive (usually in terms of telescope time but also in terms of
instrument complexity or difficulty of obtaining the measurement).

####  2.4.1 Size

The size typically refers to the diameter, but it can also refer to the mass,
volume, or cross-sectional area. Spherical geometry is often assumed, but the
vast majority of minor planets have far too little mass to self-gravitate into
a sphere. Rocky objects must be $\sim$500 km in diameter to be spherical. The
second largest asteroid in the Solar System (2 Pallas) has a mean diameter of
$510$ km. It is slightly tri-axial with $c/a\approx 0.79$ and $b/a\approx
0.95$. Larger objects in the Solar System are all more spherical.

Asteroid size is important for several aspects of planetary defense. First,
the entire cross-sectional area of an asteroid is able to absorb radiative
impulses (Dearborn et al., 2020). Second, the drag of an asteroid entering the
atmosphere depends on this area. Similarly, the non-Keplarian components of
asteroid motion through the inner Solar System are key contributors to both
the sourcing and the long-term instability of NEOs. The YORP (e.g., Rubincam,
2000, Taylor et al., 2007, Lowry et al., 2007) and Yarkovsky (e.g., Chesley et
al., 2003) effects cause asteroids to rotate and increase their semi-major
axis due to effects with Solar radiation. Each of these is dependent on the
size of the asteroid.

Ideally, astronomers estimate size with resolved imagery. This is typically
done with radar for asteroids; however, adaptive optics has enabled resolved
imagery of NEOs with large aperture telescopes (Reddy et al., 2022b).
Alternatively asteroids can be resolved with interferometry observations (see
e.g., Li et al., 2011). For each of these methods, the number of asteroids
available are few, and each method is expensive to carry out. Most
practically, diameter can be constrained with thermal emission in the mid-IR
(MIR) wavelengths (4–25 $\mu$m) to $\sim 10\%$ accuracy (Mainzer et al.,
2011a); however, sensors that cover this wavelength range are expensive HgCdTe
technology, so these cameras have not been proliferated for rapid follow up
purposes. They have been used for surveys of NEOs from space, which I will
discuss in detail in §3.

####  2.4.2 Mass

Mass is among the most important measure of an asteroid’s size for planetary
defense since most mitigation strategies involve deflecting the asteroid’s
trajectory with an impulse, and the ability to deflect the asteroid directly
depends on the mass; however, it is challenging to estimate the mass based on
size estimates since the density of asteroids span a wide range. It is trivial
to estimate the mass of a parent asteroid in a binary with a large mass ratio,
but these are uncommon for smaller NEOs, and the secondary for a smaller NEO
becomes more difficult to observe in such a scenario. Mass can be estimated
for a large asteroid by the impact or close encounter of a smaller asteroid
(Siltala & Granvik, 2022) or a man-made satellite (Kumamoto et al., 2022).
Simulation work suggests that the mass can be estimated for loosely bound
“rubble-pile” NEOs that fly by Earth (Richardson et al., 1998). However,
efforts to measure the mass of asteroids are not common since the mass is less
interesting scientifically than the density, which better probes material
properties and constrains important questions of the formation of the Solar
System.

####  2.4.3 Material Properties

The material properties of an asteroid are the most difficult to constrain
with remote sensing. Material properties generally refer to chemical
composition, but also composite nature, density, bulk strength, and albedo
among other features. The most direct method to constrain the chemical
composition with remote sensing is spectroscopy, which reveals absorption
lines of chemicals on the asteroid; however, these are only indicative of the
surface layer, which is often covered in a layer of fine regolith and mixed
sizes from dust to boulders (see Figure 2).

Figure 2: Final image taken by the DART mission before impact showing the
material properties of the surface of Dimorphos from 7 km. The image shows a
patch of the surface approximately 100 meters on a side. The largest boulders
are $\sim$15 meters across, but there is a mix of sizes and regolith filling
the gaps between large boulders. Image credit: NASA/Johns Hopkins APL. This
and more images are available at https://www.nasa.gov/feature/dart-s-final-
images-prior-to-impact.

The interior of asteroids are much more difficult to constrain. There have
been a three sample return missions of NEOs, but each only extracted material
from the surface layer. The Japanese Aerospace Exploration Agency rendezvoused
with 25143 Itokawa and 162173 Ryugu with their Hayabusa and Hayabusa2
missions, respectively. Most recently, NASA’s OSIRIS-REx mission explored
101955 Bennu and extracted material, which will be returned later in 2023.
Hayabusa’s target 25143 Itokawa was shown to be an S-type asteroid similar to
ordinary chrondite meteorite samples. It is a contact binary with two lobes
with different densities, and it was shown to have water present (Abe et al.,
2006, Fujiwara et al., 2006, Durech et al., 2008, Nakamura et al., 2011, Jin
et al., 2018). Its surface seemed to be exposed to space weathering for eight
million years suggesting it was originally part of a larger asteroid and had
broken off and recombined (Abe et al., 2006). Hayabusa2’s target, 162173
Ryugu, was shown to have a similar surface age as 25143 Itokawa; however, it
is more similar to C1 chrondite meteorites and is a Cb-type asteroid. The
surface was shown to contain water, and the asteroid has a low density and
uniform spectral features covering the surface. Its low density and porous
surface boulders suggest it was formed from the recombination of loose
material (Sugita et al., 2019, Grott et al., 2019, Jaumann et al., 2019,
Michikami et al., 2019).

The young age of the surface layers suggests that some NEOs formed recently
compared with the age of the Solar System with the common picture being that
they are formed from various escape routes from the asteroid belt (Wetherill,
1979, Wisdom, 1983, Bottke et al., 2000, Morbidelli et al., 2002, Vernazza et
al., 2008, Carry et al., 2016). NEOs are stable in the inner Solar System for
a few million years (Bottke et al., 2002, Morbidelli et al., 2002, Granvik et
al., 2016). However, they also weather on timescales $<1$ Myr (Vernazza et
al., 2009). Space weathering refers to an array of processes that alter the
surfaces including high energy particles from the Sun and cosmic rays as well
as impacts from micrometeoroids. Some NEOs also appear to have unweathered
surfaces, which has been explained by a number of processes including
planetary encounters, YORP spin-up, small impacts, and thermal degradation.
DeMeo et al. (2023) showed that none of these processes can explain all trends
observed in Q-type asteroids in the inner Solar System. A few broad trends
were observed including fewer fresh surfaces on smaller asteroids, which could
be explained by smaller asteroids being more likely to have higher bulk
strength.

However, since rendezvous and sample return missions are not economically
feasible for more than a handful of NEOs, spectral studies of asteroids often
reveal the most information we can obtain for the chemical structure of NEOs.
DeMeo et al. (2009) provides the canonical asteroid spectral classification
system based upon visible to NIR spectroscopy of a few hundred asteroids. The
24 spectral classes are shown in the top panel of Figure 3 with the relative
flux normalized at 550 nm. Many of these spectral classifications have been
linked to various meteorite samples (DeMeo et al., 2022). Furthermore, many of
these spectral classes have also been linked to escape mechanisms from the
asteroid belt (Marsset et al., 2022). These studies constitute an important
line of work that will continue to increase the usefulness of spectral
classification of NEOs for planetary defense purposes.

However, spectroscopy across these wavelengths are difficult to measure for
the majority of NEOs, especially rapidly after detection by large aperture
survey instruments, which are likely to be detected further away. Since
spectroscopy requires binning photons by wavelength, longer exposure times are
needed to obtain sufficiently high signal-to-noise ratio (SNR) measurements to
identify spectral features and classify asteroids. An alternative is multi-
band photometry covering the visible to NIR wavelengths to coarsely resolve
the spectral energy distribution (SED). A number of broad-band filters are in
common use that span these wavelengths. Such a measurement is analogous to
using broad-band filters to estimate the redshift of galaxies used for extra-
galactic astrophysics and cosmology as opposed to the more expensive
spectroscopic redshift measurements. In fact, most modern photometric filter
sets are designed with this science in mind. A useful endeavor for planetary
defense and Solar System science would be to design a filter set that
maximizes the ability to approximate spectral classification with broad-band
filters.

In the bottom panel of Figure 3, I show the variability in visible and NIR
colors across the 24 spectral classifications. These were determined using the
filters that will be used for the Vera C. Rubin Observatory Legacy Survey of
Space and Time (hereafter referenced together as Rubin/LSST or seperately
referring to the telescope or the survey, respectively; Ivezić et al., 2019)
plus an unfiltered InGaAS NIR camera. Colors involving the NIR filter are most
constraining. This method has been used in the literature (de León et al.,
2013) to identify a NEO as an L-type asteroid during a close approach, which
is a good approximation of rapid characterization after discovery.

Figure 3: _Top:_ SEDs for all 24 asteroid types from DeMeo et al. (2009). The
flux in each curve is normalized to the flux at 550 nm. There is a much more
prominent spread in the NIR than visible wavelengths. The NIR colors are
estimated assuming the quantum-efficiency curve for a InGaAs sensor
unfiltered. _Bottom:_ The standard deviation of color measurements made with
Rubin/LSST filters (omitting the u-band filter, which is bluer than the SEDs
above) and an unfiltered NIR quantum efficiency curve. For each color index,
the two relevant broad-band filters were convolved with all 24 spectral energy
distributions and apparent magnitudes were estimated assuming a nominal flux
and the standard deviation is reported.

####  2.4.4 Rotational State

Similar to asteroid size measurement, asteroid rotation is most easily
constrained with a series of resolved images; however, similarly, this is
expensive compared with a more practical and proliferating approach made
available by optical photometry generated light curves. The period of rotation
is observable in a light curve as the period of a repeating pattern. Asteroids
have a wide range of rotational frequencies that form a peculiar pattern when
looked at as a population as a function of diameter (see Figure 4, prepared by
the Asteroid Lightcurve Database Warner et al., 2009). This is referred to as
a spin barrier, which is interpreted as the maximum frequency an asteroid can
have before its gravitational strength is overcome by centrifugal forces
caused by rotation. This suggests that solid asteroids larger than a few
hundred meters in diameter are extremely rare. The common interpretation is
that large asteroids are “rubble piles” loosely bound by gravity.

Figure 4: Rotation frequency versus diameter for known asteroids. The plot is
made from data from the Asteroid Lightcurve Database available at
https://minplanobs.org/mpinfo/php/lcdb.php. The black points are all asteroid
in the database. Yellow points are NEOs with red and green points denoting
binary NEOs and tumbling (NPAR refers to non-principle axis rotation),
respectively. Smaller asteroids are more likely to be NEOs given their
proximity.

####  2.4.5 Shape

Similarly to rotation and size, the shape of an asteroid can be resolved, but
only for large and nearby objects. Similar to the rotational state, the shape
can also be estimated with time-domain analysis of a light curve, though with
added complexity. Light curves are simplest to measure with optical photometry
given their ability to gather enough SNR in short exposures compared with
other wavelengths. Since optical photometry measures the brightness of
reflected sunlight, the observed flux is proportional to area of the
reflective surface apparent to the observer during the exposure time121212This
also changes with the Sun–asteroid–Earth angle, but much more slowly. The
relative flux observed over a period of rotation allows the shape to be
constrained via the amplitude of the variations. Since asteroids often have
complex shapes, tri-axial convex spheroid reflection models are typically
assumed. A method to constrain the shape and rotational properties of an
asteroid by inverting the light curve was developed by Kaasalainen et al.
(1992a, b) and Lamberg (1993). Optimization techniques advanced the method
(Kaasalainen & Torppa, 2001, Kaasalainen et al., 2001), and the model was
incorporated into a Bayesian statistical framework more recently with ground
and space-based data of varying sparsity (Muinonen et al., 2020).

##  3 NEO Detection Surveys

In the previous section, I discussed the various NEO missions that flow from
the text of the NEO Survey Act. In this section, I home in on the detection
mission and discuss the major surveys of the present and future. I will assess
their abilities to address the characterization and tracking missions as well.

###  3.1 Present Surveys and Their Limitations

Today, two ground-based surveys discover over 75% of all the NEOs. These are
the Catalina Sky Survey (hereafter CSS; Larson et al., 1998) and the Panoramic
Survey Telescope and Rapid Response System (hereafter Pan-STARRS; Kaiser et
al., 2002). CSS has three telescopes, two with wide field of view cameras
optimized for survey-based discovery and one that targets NEOs for IOD
refinement. CSS has detected around 30% of the large NEOs over the past few
years. Pan-STARRS has two telescopes with wide field of view cameras optimized
for survey-based discovery. Pan-STARRS has detected around 55% of the large
NEOs over the past few years. Both CSS and Pan-STARRS operate from the
Northern Hemisphere, which limits the overall NEO detection rate since the
ecliptic plane extends into the southern sky. The remainder of 140 meter and
larger NEOs are discovered by a handful of asteroid and synoptic surveys. The
largest contributors are the Asteroid Terrestrial-impact Last Alert System (or
ATLAS; Tonry et al., 2018), the Zwicky Transient Facility (ZTF; Bellm et al.,
2019), and NEOWISE (Mainzer et al., 2011b), which each detect a few percent of
the large NEOs. ATLAS has a smaller aperture and is better at nearby NEOs
given its rapid cadence. ZTF is not a dedicated NEO survey, but given its 47
square degree field of view and high cadence, it frequently detects them.
NEOWISE, is an extended NEO-focused mission using the Wide-field Infrared
Survey Explorer (WISE; Wright et al., 2010), which was an all-sky survey in
four MIR channels before its mission ended. The telescope was revived for
NEOWISE with just the two shorter IR channels since the two longer wavelength
channels ran out of refrigeration.

At the present rate, it would take 40 years to meet the 90% mandate without
accounting for the slowing detection rate as the pool of undetected asteroids
becomes saturated with those that are undetectable with the current surveys.
This is evident by comparing the increasing rate of discovery in the bottom
panel of Figure 5 versus the top panel. There are two primary drivers making
the remaining population more difficult to detect over time. First is the
albedo, which is the fraction of incident light reflected by the surface of
the asteroid. The left panel of Figure 6 shows the albedo distribution of NEOs
derived with WISE data (Wright et al., 2016). There are numerous with albedo
of only a few percent. Since most NEOs have been detected with reflected
light, undetected NEOs are likely over-represented by those with low albedo.
This over-representation will grow with time. The second difficulty is that
many NEO orbits are inherently challenging to observe, especially with ground-
based surveys, which contribute the vast majority of discoveries. The orbit of
a given NEO will dictate the distances and reflection angle, which, along with
the albedo, govern the apparent brightness of the asteroid for the observer.

Figure 5: Detection statistics of 140 meter and larger NEOs (top panel) and
NEOs of all sizes (bottom panel) by survey and year through 2022-Dec-28. The
continued increase in detections of NEOs of all sizes suggests increased
survey capabilities; however, the slower increase in larger NEO discoveries
indicates that the pool of remaining large NEOs is becoming harder to detect.
Images courtesy of the Center for Near-Earth Object Studies NASA/JPL-Caltech
available at https://cneos.jpl.nasa.gov/stats/.

NEOs can be split into four groups depending on just two of their orbital
parameters. The groups are named after the original archetypes (1221 Amor,
1862 Apollo, 2062 Aten, and 163693 Atira). According to CNEOS, as of the end
of 2022, there are 11,110 known Amor asteroids, 17,451 known Apollo asteroids,
2427 know Aten asteroids, and 28 known Atira asteroids. Apollos and Amors have
semi-major axes larger than Earth. Apollos have large enough eccentricities
such that they are Earth crossing, or put another way, their perihelia are
smaller than Earth’s aphelion ($q<1.017$ AU). Meanwhile, since Amors perihelia
are larger than Earth’s aphelion ($q>1.017$ AU), they are not Earth-crossing.
Atens and Atiras are analogous with semi-major axes smaller than Earth, and
Atens are the Earth crossing sub-population. See the right panel of Figure 6
for a schematic of each of these types of NEOs. Potentially hazardous
asteroids (PHAs) are Apollos and Atens larger than 140 meters that cross
within 0.05 AU of Earth’s orbit. Most NEOs originate from ejected MBAs, so it
is not surprising Amors and Apollos are the most populous types given their
proximity to the MBA source. They are also the easiest to detect with ground-
based surveys since they spend most (or all for Amors) of their time outside
of Earth’s orbit. Atens and Atiras, on the other hand, are under-represented
in the detected population because they are more difficult to observe from the
ground. Simulations also suggest Atens and Atiras are intrinsically rarer
types of NEOs (Granvik et al., 2018).

Figure 6: _Left:_ Albedo distribution for NEOs derived from WISE (Wright et
al., 2016). _Right:_ Example orbits for the four NEO types in relation to
Earth and Mars. Apollos and Atens (solid ellipses) are Earth-crossing, whereas
Amors and Atiras (dashed ellipses) are not. PHAs are those that cross within
0.05 AU and are larger than 140 meters.

Beyond these orbital groupings, the other orbital parameters can make some
asteroids evade detection for long periods of time. For example, a very Earth-
like orbit exactly out of phase from Earth would be positioned behind the Sun
for many years. On the timescale of the NEO surveys to date, such an asteroid
could be undetectable with night-time surveying from the ground. As another
example, an asteroid with a large inclination could evade detection from many
asteroid surveys since they preferentially observe the ecliptic plane where
asteroids are most numerous. Furthermore, for all except NEOWISE, the vast
majority of NEOs are discovered via ground-based surveys, and most of these
are carried out in the northern hemisphere. Ground-based optical surveys are
unable to observe on the Sun side of the ecliptic plane (with the exception of
twilight observing or high airmass observing just after evening twilight and
before morning twilight; however, these observing modes are sub-optimal given
the variable bright sky background and higher atmospheric extinction. Indeed,
the majority of NEOs discovered today are close to Solar opposition.

###  3.2 Planned NEO Surveys

While the planned NEO surveys discussed in this section are primarily for
detection, I will also discuss their contributions to the detection, tracking,
and characterization. I will continue to return to this breakdown throughout
this paper to highlight capabilities and gaps of the various surveys and
follow-up instruments over the next two sections.

####  3.2.1 NEO Detection Mission

The observational challenges that the current NEO surveys are plagued by helps
explain the designs of the next generation of asteroid detection missions.
There are two primary surveys that will satisfy the NEO detection mission. The
first is Rubin/LSST, which will have a 3.2 gigapixel camera that will cover
the entire available night sky every few nights for a decade to extremely
faint detection limits. Median single-epoch $5\Sigma$ detection limits are
$u=23.9$, $g=25.0$, $r=24.7$, $i=24.0$, $z=23.3$, and $y=22.1$ in a 30 second
exposure. First light is expected in 2024, and although the survey is not
optimized for asteroid detection, the leap forward in sensitivity compared
with ZTF (which carries out a similar survey) means a large fraction of the
remaining NEOs will be discovered with Rubin/LSST alone.

Several studies estimated that if the other surveys continue running at the
current capabilities, $\sim$75% of the 140m and larger NEOs will be discovered
after a decade of Rubin/LSST (Ivezić et al., 2007, Grav et al., 2016, Chesley
& Veres, 2017, Jones et al., 2018). Rubin/LSST will solve a few of the
detection challenges described above. First, its increased sensitivity and
wide area coverage will enable fainter asteroid detections over a broader
portion of orbital space, which will probe lower albedo NEOs as well as NEOs
with sub-optimal viewing geometry for ground-based observing. This will
especially be true for the proposed twilight survey (Seaman et al., 2018),
which will observe lower Solar elongations than CSS and Pan-STARRS do with
much higher sensitivity than other instruments that carry out such surveys
such as ZTF (Ye et al., 2020). Second, additional survey time with Rubin/LSST
has been proposed to finish the 90% detection threshold; Jones et al. (2018)
estimated that two additional years would finish the task, which is explained
by the point raised above where out-of-phase asteroids will have additional
time to get into phase with the ground-based observing scene. However, this
proposal is an expensive one. The estimated yearly operating budget for LSST
operations is nearly $100M.

The second major survey NASA’s proposed NEO Surveyor (Mainzer et al., 2022),
which is a space-based MIR system, which will operate at Earth-Sun L1 in a
halo orbit (see Farquhar, 1966, for the original concept of a halo orbit
around Lagrange points). This position inside of Earth’s orbit will allow for
optimal viewing of the regions that are inherently challenging to observe from
Earth (see left panel of Figure 7). Furthermore, the MIR sensor will detect
thermal emission of NEOs rather than reflected light, so it is less
susceptible sensitivity losses from the low angle of reflection and low
albedos. NEO Surveyor will launch no earlier than 2028, but together with the
ground-based surveys it will enable the completion of the NEO detection
mission by the early 2030s. Acting by itself, NEO Surveyor will detect about
two thirds of the target NEOs in five years or 90% in twelve years (Mainzer et
al., 2022). The survey has limited lifetime given its halo orbit, which
requires constant station keeping. In the right panel of Figure 7, an example
thermal and reflected emission spectrum is shown under the near-Earth asteroid
thermal model (NEATM; Harris, 1998).

Figure 7: _Left:_ Schematic showing the NEO Surveyor field of regard, which is
complimentary to ground-based surveys, which prioritize zenith observations
during the night and thus most easily observe the white sector to the bottom
of the figure. Rubin/LSST has a twilight survey planned, which will cover much
of NEO Surveyor’s field of regard, but with diminished capabilities due to the
bright and variable twilight sky background. _Right:_ Reflected (dashed) and
thermal (solid) contributions to the IR emission of NEOs assuming a large NEO
with typical albedo and emissivity under the NEATM (Harris, 1998).

####  3.2.2 NEO Tracking Mission

The observing cadence for Rubin/LSST is designed such that NEOs will be self-
recovered a few times per lunation such that a discovery and IOD may be
reported to the MPC (Jones et al., 2018, Ivezić et al., 2019). Rubin/LSST will
implement the Moving Object Processing System (MOPS) originally developed for
Pan-STARRS for track linking (Denneau et al., 2013). Typical NEOs will be
rediscovered a couple hundred times over the 10-year survey such that orbits
may be refined and updated with Rubin/LSST alone; however, the gap between the
discovery exposures and new recovery exposures will often be large. Thus,
additional follow-up instruments will be important for refining the
trajectory. However, Rubin/LSST will require very sensitive follow up
observations given its faint detection limits. This is especially the case for
nearby NEOs that are intrinsically small and quickly move away from Earth to
lower Solar elongation.

NEO Surveyor is still in the planning and mission design phase of its life
cycle, but it has a similar operational plan as Rubin/LSST in that it is
prioritizing survey-based discovery, and IODs will be issued based upon self-
recovery over a short period of time. However, there is a need for higher-
fidelity IODs given that follow-up instruments on the ground will not be able
to observe newly discovered NEOs until they move to larger Solar elongations
(see left panel of Figure 7).

####  3.2.3 NEO Characterization Mission

Here I will discuss the ability of Rubin/LSST and NEO Surveyor to satisfy the
NEO characterization mission. Both surveys will have new capabilities beyond
that of the current state-of-the-art surveys; however, there are some
capability gaps, which I will discuss.

Rubin/LSST is an optical and NIR photometry instrument with six broad-band
filters covering the visible to NIR wavelengths. Its unique capability will be
color determinations across between 350 nm and 1050 nm. LSST’s cadence design
cycles through its six filters and also returns to recover NEO discoveries a
few times per lunation, so it is likely that a discovery and IOD will be made
with multiple filters. As I discussed in §2.4.3, these color-indices are
useful for constraining the type of asteroid. In Figure 3, I showed the large
spread in color indices and broad-band magnitudes for typical asteroids from
all 24 asteroid types derived via visible to NIR spectroscopy. Given the
photometric precision of Rubin/LSST, the colors measured with just the few
observations needed to discover and establish an IOD will be sufficient for
rough asteroid characterization. Colors that span larger wavelength baselines
such as $g-y$ and $r-y$ are the most constraining. However, many NEOs rotate
many times per day, and the colors will be confounded with potential
periodicity in the light curve, which will not be measured enough to correct,
so color indices could be off by as much as the peak-to-trough difference in
magnitude. Thus, the Rubin/LSST derived colors estimated during detection are
at best only useful for roughly constraining material properties of a newly
discovered NEO. They will become much more accurate after the rotational state
is determined and the discovery measurements can be phase-wrapped. On longer
time scales, Rubin/LSST will detect the same NEO tens to hundreds of times.
This could potentially allow for time-series analysis of the light curve;
however, this will require extremely accurate phase wrapping, which is likely
sub-optimal compared to more timely follow up from dedicated instruments. An
exception to this is possible for NEOs that track through a deep-drilling
field, which may have the requisite cadence to enable light curve analysis.

NEO Surveyor is a MIR imager with two pass-bands. Beyond about 4.5 $\mu$m, the
thermal emission dominates the reflected sunlight (see right panel of Figure
7). NEO Surveyor’s instrument will effectively measure the heat signature of a
NEO, which means the flux will be proportional to the size of the asteroid.
This is unique to MIR sensors, and is a valuable piece of information for
planetary defense. An important follow-up for any NEO Surveyor discovery will
be an optical flux estimate. When combined with the thermal IR measurement
that gives the size of the asteroid, the albedo can be deduced (see Figure 8).
For NEO Surveyor acting alone, this will be useful for when known objects
(predominantly discovered via optical photometry) are recovered. However,
given the orbital regime that NEO Surveyor will observe, new discoveries will
need timely follow up with optical photometry to quickly determine these
additional properties. As discussed above, this is challenging for ground-
based follow up instruments. NEO Surveyor will not be as useful for shape or
rotational property estimation since the MIR flux is not as strongly dependent
on phase as optical photometry. NEO Surveyor’s two MIR pass bands will give
some indication of potential emissivity variation of asteroids as they rotate,
which could be used to constrain the material properties, but it will not be
as strong as multiple broad-band visible to NIR photometry (DeMeo et al.,
2009).

Figure 8: Schematic showing the value of combining visible and MIR (thermal)
observations of NEOs. With visible light alone, there is a degeneracy between
albedo and size. With MIR alone, only the mass is determined but not the
albedo. By combining the two, the mass is determined with the MIR and the
albedo is determined with the combination. If there is also an estimate of the
material properties than the size and thus density may also be determined.
Image credit: NASA/JPL-Caltech available at
https://www.nasa.gov/mission_pages/WISE/multimedia/gallery/neowise/pia14733.html.

###  3.3 Gaps in the NEO Mission Space

Previously in this section, I discussed the major NEO surveys of the present
and future, and I broke down their contributions to the detection, tracking,
and characterization missions of the NEO Survey Act. Here I summarize the
discussion to highlight the components of the various missions that remain for
follow up facilities to tackle. This information is summarized in Figure 9.

Figure 9: Breakdown of the two major present and two major future NEO surveys.
For the characterization mission check marks are placed in cells where the
survey plausibly contributes to the specified feature within normal survey
operations in a timely manner. The level of shading indicates the relative
performance of the survey compared with a dedicated follow up instrument. CSS
and Pan-STARRS are largely similar except for the third CSS telescope, which
offers some time-domain characterization in addition to its purpose of
refining NEO tracks. Rubin/LSST uniquely offers multi-band photometry enabling
material constraints based on color information. NEO Surveyor independently
constrains the size of an asteroid..

No single survey is capable of detecting all NEOs. Ground based surveys are
limited by their aperture size; though, Rubin/LSST will be sensitive enough to
detect far more NEOs that enter its field of view that CSS or Pan-STARRS. They
are also limited by their inability to observe during the daytime, and so low
Solar elongations are unobserved. Conversely, NEO Surveyor can observe lower
Solar elongations, but it cannot observe at Solar opposition due to the Earth.

Only CSS is capable of refining the track of NEOs; though, its capability is
niche because it has a third telescope dedicated to this task. However, the
throughput of a single ground-based sensor is limited to fewer than 100 NEOs
per night. While this likely satisfies refining the orbits of discoveries
today, future surveys will detect far more NEOs that are too numerous to
follow up with a single telescope or are too faint to follow up from 1 meter
class optical telescopes.

Once both Rubin/LSST and NEO Surveyor are operational, asteroids observed by
both surveys will have a good track, size, and rough material property
constraints. However, this characterization will not be timely since NEO
Surveyor’s and Rubin/LSST’s detections will be temporally separated due to
their different survey regimes. Furthermore, much of the characterization from
these two surveys will be serendipitous in nature; that is, the surveys are
designed for rapid recovery of a few exposures in order to issue a discovery
and IOD, but detections that come afterward are serendipitous and may come
months or years later. This is most especially true for the time-domain
analysis required to estimate shape and rotational properties. Neither
Rubin/LSST nor NEO Surveyor will provide high fidelity estimates of the shape
or rotational properties.

##  4 NEO Follow Up

Today, after discovery and IODs are reported by the discovery survey
(typically CSS or Pan-STARRS), they are posted to the MPC, which maintains a
list of NEOs in need of further
observation131313https://minorplanetcenter.net/iau/NEO/toconfirm_tabular.html.
The tracking mission is often carried out by amateur astronomers around the
world, who observe and report astrometry and photometry back to the MPC, which
recomputes orbital states frequently and moves confirmed NEOs off this list.
In this process, most large NEOs are observed rapidly for a few days after
discovery. In principle, this could enable time-domain analyses for shape and
rotational properties; however, the MPC does not have strict requirements on
data quality. MPC photometry precision is often unreported and assumed
accurate only to 0.1 magnitudes. Many amateur astronomers do not calibrate
their observations to the level that is done at professional facilities.
Furthermore, their observations typically come from sites with poor
atmospheric conditions with poor seeing, high levels of aerosols from air
pollution, and bright sky backgrounds from scattered terrestrial light; thus,
it is challenging to estimate uncertainties properly for the relevant time-
domain analysis and color index estimate.

For color index estimation, two magnitude estimates with an uncertainty of 0.1
mag from the MPC database leads to an uncertainty in the estimated color index
by of $\sqrt{2\times 0.1^{2}}\approx.14$ magnitudes. Most amateur astronomers
observe with visible sensors and filters, so such an uncertainty is comparable
to the spread of colors for asteroid spectral classes presented in the bottom
panel of Figure 3; thus, typical MPC photometry is insufficient for color
index estimation to the level of precision needed for constraining the
material properties of NEOs. Nevertheless, the light curve and color
information posted to the MPC has been collected by the Asteroid Lightcurve
Database (Warner et al., 2009).

In the following subsections, I will review the major facilities that
astronomers use for the NEO characterization mission. This will not be a
comprehensive list. I will prioritize systems that are most useful for rapid
characterization, and I will organize the facilities by their wavelength and
observing modality and mention the various asteroid features that are probed
by each facility.

###  4.1 Optical NEO Follow Up Facilities

Optical photometry is the cheapest type of observation to acquire for the NEO
characterization mission. The relatively low-cost for ground-based optical
telescope time makes them particularly useful for building light curves for
shape and rotational analysis. A system particularly well-suited for this
purpose is Las Cumbres Observatory Global Telescope Network (LCOGT; Brown et
al., 2013b), which is composed of twenty seven telescopes (ten 0.4 meter
telescopes, fifteen 1 meter telescopes, and two 2 meter telescopes) at seven
sites around the world. The telescopes have a variety of instruments including
low and high-resolution optical spectrographs as well as photometric cameras
with moderate field of view ($\sim\,$0.2 sq-deg) and full broadband filter
sets. Each telescope is fully robotic, operates from optical to NIR
wavelengths, and observations are scheduled in an optimized fashion across all
observers to minimize slew time while taking into consideration the timeliness
needed to achieve the observer’s science goals. LCOGT serves planetary science
well and has even dedicated key
projects141414https://lco.global/science/keyprojects/ to asteroid science.
This key project has in part developed
NEOExchange151515https://lco.global/neoexchange, which helps collect and
easily queue observations with LCOGT (Lister et al., 2021). LCOGT also
participated in the global observations of the DART target Dimorphos (Lister
et al., 2022).

LCOGT has recently partnered with the National Science Foundation to include
NOIRLab’s Community Science and Data Center, 4.1 meter SOAR telescope, and 8
meter Gemini-South telescope into a combined follow up network for high-
profile transients detected with Rubin/LSST. This partnership is named the
Astrophysical Events Observatories Network (AEON Street et al., 2020).
However, it remains to be seen how the NOIRLab facilities, which operate in a
traditional model with blocks of consecutive nights that requiring the
investigators to operate the instrument. Some telescopes have begun offering
target of opportunity (ToO) time, where proposers have the ability to borrow
time from traditional proposals if a transient phenomenon occurs. This
methodology became popular after the detection of gravitational waves and
full-spectrum electromagnetic counterparts (Abbott et al., 2017). While AEON
will certainly be good for transient astronomy, NEOs make up only a portion of
alerts delivered by LSST, which will also detect supernovae, all types of
variable stars, minor planets throughout the Solar System, among others
totaling $\sim\,$ 10 million alerts per night (Ivezić et al., 2019). A
drawback for LCOGT and AEON is that they are entirely ground-based, so it will
be unable to follow up most NEO Surveyor discoveries.

Other groups have organized around NEO characterization. One notable group is
the Mission Accessible Near-Earth Objects Survey (Moskovitz et al., 2015).
MANOS has prioritized smaller NEOs and targeted them with 4–8 meter class
telescopes both spectroscopically and photometrically (Thirouin et al., 2016,
Devogèle et al., 2019). To date they have reported 500 visible spectra and an
optical lightcurve for over 700
NEOs161616https://manos.lowell.edu/observations/summary.

Optical cameras generally are very common on telescopes of all apertures
including some of the largest telescopes worldwide. Modern sensors have also
skewed toward wide field of view such as the Dark Energy Camera (Flaugher et
al., 2015) and Hyper Suprime Cam (Miyazaki et al., 2018), both of which were
designed to carry out wide-field surveys for cosmology but have since
transitioned to general observing through their respective telescope proposal
processes. Large aperture, wide field of view systems are particularly useful
for recovering faint asteroids with uncertain orbital states such as faint
NEOs detected nearby Earth which become too faint to recover with the NEO
surveys. Such instruments are typically over-subscribed via their calls for
observing proposals, and the typical minimum unit of time is a half or whole
night; thus, singular observations to recover an asteroid are a challenging
modality.

Asteroid observations have been carried out on numerous smaller aperture
optical telescopes with good success; for example, the 0.9 meter WIYN
telescope was used to precisely detect and estimate orbital parameters of
asteroids to faint magnitudes using a stacking technique known as “shift and
stack” or “digital tracking” (Heinze et al., 2015). Recently, I used the 1
meter Nickel telescope to obtain multi-band photometry covering the visible
wavelengths to estimate asteroid classifications of NEO that came within a few
lunar distances in late 2022 (Golovich et al. in prep). One of the targeted
asteroids exhibited fast rotation apparent in a single five minute exposure
(see Figure 10).

Figure 10: Observation of rotation within a single 5-minute exposure of 2010
XC15 with the Nickel 1 meter telescope at Lick Observatory. Observations were
taken by Connor Dickinson and Srujan Dandu of UC Santa Cruz under Nickel
proposal 2022B-N006 (PI: Kirsty Taggart). _Left:_ 300-s B-band photometric
image in reverse color. Flux variations along the streak are apparent. North
is up and East is left. The proper motion was to the Northeast. The camera was
readout with 2$\times$2 binning such that the pixels are 0.37′′. The image was
convolved with a symmetric two dimensional Gaussian kernel of width equal to
the atmospheric seeing at the time of the exposure
(FWHM$=1.29^{\prime\prime}$). _Right:_ Boxcar projection along the length of
the streak. The width of the boxcar is $2\times$FWHM. The error bars represent
the standard deviation in the pixel ADU values in the vicinity of the asteroid
indicating that the flux variations are statistically significant.

Optical spectroscopy is also available at many observatories. Spectroscopy is
inherently more constraining since a spectrograph gives a data-point for model
comparison per spectral resolution unit. Even a low-resolution spectrum such
as those offered by LCOGT’s offers far more parameters for model comparison;
however, the trade is for exposure time. Using the LCOGT’s exposure planning
tools, 21 minutes (including overhead) are needed to observe the spectrum of a
NEO at g=16 at a spectral resolution of R$=500$ to a SNR of 10 per resolving
unit; whereas, the same asteroid could be observed eight times in six filters
spanning the same wavelength range, which would offer a coarse replication of
the SED as well as dozens of observations spanning 20 minutes for time-series
analysis for shape and rotation.

###  4.2 Infrared NEO Follow Up Facilities

IR wavelengths range from $\sim$ 0.75 to 1000 $\mu$m, but can be further split
into NIR (0.75-3 $\mu$m), MIR (3-25 $\mu$m), and far-IR (25-1000 $\mu$m).
Asteroids are typically studied with NIR and MIR observations with NIR
dominated by reflected light and MIR dominated by thermal emission above 3–4
$\mu$m (see right panel of Figure 7).

IR astronomy has a long history due to the gaps in the atmospheric absorption
spectrum. The gaps are given names, which follow through to the names of the
photometric filters. The most common NIR bands are I ($\sim 0.8\,\mu$m), Z
($\sim 0.9\,\mu$m), Y ($\sim 1.0\,\mu$m), J ($\sim 1.2\,\mu$m), H
($\sim\,1.6\mu$m), and K ($\sim\,2.2\mu$m). I, Z, and Y-bands are commonly
coupled with optical astronomy facilities. J, H, and K-bands were the filters
used for the Two-Micron All-Sky Survey (2MASS; Skrutskie et al., 2006), which
provide the canonical calibration data set for nearly all NIR astronomical
observations.

There are several major ground-based IR facilities; however, few are open to
NEO observations through general calls for proposals. The most NEO-focused
instrument is NASA’s Infrared Telescope Facility (IRTF), which has a
guaranteed minimum of 50% time toward planetary science. IRTF has both imagers
and spectrographs covering NIR and MIR. As discussed above, NIR is
predominantly reflected light, but it is highly constraining for material
classification, especially when paired with optical photometry. This is
evident from the slope of the SEDs in the top panel of Figure 3 and the large
spread in colors between the various asteroid types, especially the g$-$NIR
color index in the bottom panel of Figure 3. IRTF has been used for much of
the research discussed in this paper regarding remote-sensing enabled asteroid
material studies and classifications. The MIT-Hawaii Near-Earth Object
Spectroscopic Survey (MITHNEOS)171717http://smass.mit.edu/minus.html has had a
long-running campaign of obtaining visible to NIR spectroscopy for large and
nearby NEOs. Much of their data has been made public (DeMeo et al., 2009,
Polishook et al., 2014, DeMeo et al., 2019, Binzel et al., 2019). Over twenty
years, MITHNEOS has measured $\sim$1600 NEO spectra spanning visible to NIR
wavelengths.

Many large-aperture telescopes around the world have IR sensors coupled with
adaptive optics systems. While these enable resolved imagery of large and
nearby NEOs, which give direct measurements of size, shape, and rotational
properties, these exposures are expensive to carry out and time on 8$-$10
meter telescopes such as Subaru, Keck, and VLT are highly competitive.

The remaining notable IR instruments for NEO characterization are in space.
The first is NEOWISE, which is a survey instrument and a precursor to NEO
Surveyor. The second is James Webb Space Telescope (JWST; Gardner et al.,
2006), which needs no introduction. JWST is certainly very powerful for NEO
characterization. Its NIR and MIR spectrographs and imagers would enable
extremely precise spectroscopy; however, it is perhaps overkill for most large
NEOs. When JWST was being planned, white papers discussed NEO observations in
terms of imaging NEOs as small as a meter (e.g., Thomas et al., 2016). While
there is value in studying small objects since most large asteroids are
composite structures, it is clear that JWST is not going to meaningfully
contribute to the NEO discovery, tracking, or characterization missions.

###  4.3 Radar NEO Follow Up Facilities

Radar systems use a radio wave source and detect the reflected signal. Radar
observations of asteroids are among the most useful for the NEO
characterization mission, as I discussed above. Because they are often mounted
on large radio dishes, they offer resolved imagery and can measure size,
shape, and rotational properties. However, radar sensitivity falls as
$r^{-4}$, so it is only viable for very nearby or very large NEOs.
Nevertheless, when NEOs fly near Earth, radar observations should be
prioritized. A recent NEO flyby of 2005 LW3 resulted in an unexpected
discovery. The asteroid’s albedo was unknown prior to the close approach, so a
nominal value of 0.15 was assumed, which suggested an object a $\sim 100-200$
meters in diameter. However, observations with NASA’s Goldstone Solar System
Radar resolved a binary asteroid system181818The discovery was reported in
CBAT Electronic Telegram No. 5198 by S. P. Naidu, L. A. M. Benner, M.
Brozovic, and J. D. Giorgini, of Jet Propulsion Laboratory. with a primary
diameter of 400 meters and a secondary diameter of 100 meters orbiting $\sim$4
km from the primary (see Figure 11). The primary’s albedo was estimated to be
0.02.

Additional radar facilities could be opened in the future. The Green Bank
Observatory has been used as a single dish receiver using the Goldstone Solar
System Radar; however, there are plans to develop radar capabilities for it
and the Very Large Baseline Array (Paganelli et al., 2022). These advancements
are needed to replace the lost capabilities since Arecibo’s demise (Virkki et
al., 2022).

Figure 11: Goldstone Solar System Radar image of 2005 LW3 showing a binary
asteroid pair. The primary was measured at Image available at
https://echo.jpl.nasa.gov/asteroids/2005LW3/2005LW3.2022.goldstone.planning.html.
Reproduced here with permission, credit: NASA/JPL-Caltech.

##  5 Unique Capabilities of a NEO Characterization Constellation

In §2, I discussed the NEO Survey Act and various missions that follow from
the language of the bill. In §3, I detailed the major detection surveys
operating today and the two major surveys that will operate over the 2020s and
2030s. By the end of that time frame, it is very likely that 90% of the 140
meter and larger NEOs will have been discovered, and (especially with
Rubin/LSST) many more NEOs as small as a few meters will be discovered flying
close to Earth. NEO Surveyor, on the other hand, will detect NEOs in portions
of the Solar System that are difficult to observe from the ground. In Figure
9, I detailed the gaps in capability of the four major surveys for the
detection, tracking, and characterization missions. While Rubin/LSST and NEO
Surveyor will offer more characterization than previous surveys, they will not
solely (or together) meet the NEO Survey Act’s characterization mission. Thus,
in §4, I detailed the methods and instruments astronomers use to follow up
newly discovered NEOs. One key takeaway is that the networks of follow-up
instruments are entirely on the ground and are largely composed of only
optical sensors. Another takeaway is that there are no sensors dedicated to
tipping and queuing newly discovered asteroids, and those that do exist (such
as LCOGT) will be inundated with observing requests from multiple science
cases. Asteroid alerts from Rubin/LSST will be only a portion of the millions
of transient alerts released nightly. Finally, there is an inability writ-
large to follow up discoveries made by the NEO Surveyor with low Solar
elongation. This is particularly problematic since optical photometry follow
up of NEO Surveyor’s discoveries will directly measure the albedo (see Figure
8).

There are an estimated 30,000 NEOs larger than 140 meters (Mainzer et al.,
2012, Harris & D’Abramo, 2015, Granvik et al., 2016, Schunová-Lilly et al.,
2017), today about 35% have been discovered. As mentioned in §2.1, this is a
somewhat arbitrary cutoff, especially given that there is enhanced risk from
smaller (and much more numerous NEOs). Granvik et al. (2018) provides a
simulated NEO catalog for objects as faint as H=25 (or 34 meters assuming 0.15
geometric albedo). To estimate the number within proximity to Earth at any
given time, a Monte Carlo simulation was executed. For each sample, the
802,000 asteroid orbits from Granvik et al. (2018) and Earth were propagated
on their orbits to a randomly selected time within the next 100 years of the
orbital epoch (set to be 1 January 2023). The distance was computed to Earth
and the $H$ magnitude was recorded for any asteroids within 0.05 AU. Orbits
were instantiated along with Earth at random times within the next 100 years
to provide for a good amount of mixing between the NEO orbital periods with
respect to Earth. The left panel of Figure 12 shows the number of NEOs within
0.05 AU ($\sim$19.4 lunar distances) as a function of absolute magnitude and
size (assuming an albedo of 0.15).

The right panel of Figure 12 shows the spatial reachability volumes projected
onto the ecliptic plane for various sized NEOs at $V=20$. The HG asteroid flux
model (Harris, 1989, Binzel et al., 1989) was assumed with G$=0.15$. The black
sector is the Solar exclusion regime for the NEO Surveyor, which is positioned
at Earth–Sun L1. Figure 12 demonstrates that a LEO constellation could
characterize NEOs that are difficult to observe with ground-based assets
(white sectors). The gray sectors show the regions that Rubin/LSST will
observe. The choice of $V=20$ for the reachability volumes was made to
demonstrate what would be feasible for a small optical telescope in space.
Using an exposure time calculator assuming modern CMOS sensor characteristics
(low dark current and read noise), and assuming the background was solely from
the zodiacal light (Diaz, 2015), it was estimated that 20${}^{t}h$ magnitude
was obtainable within a few tens of seconds for optical filters and an
unfiltered NIR camera.

Figure 12: _Left:_ Monte Carlo simulation of the cumulative ($N(H)<H$) number
of NEOs within 0.05 AU as a function of size assuming the simulated NEO
orbital catalog of Granvik et al. (2018). The black histogram and points
indicate the mean of the Monte Carlo samples, and the thick and thin vertical
lines indicate the 1$\sigma$ and 2$\sigma$ confidence intervals, respectively.
The size axis at the top of the figure assumes an albedo of 0.15 and the size-
magnitude relationship of Harris & Harris (1997). _Right:_ Spatial
reachability volumes for asteroids of various size. The black sector shows the
Solar exclusion zone for the NEO Surveyor, which is positioned at Earth–Sun
L1. The Moon’s orbit is indicated for reference. The dark and light gray
sectors show where 68% and 95% of Rubin/LSST pointings will occur,
respectively. The white sectors show the regimes where ground-based
observatories do not observe frequently. The Sun is located at $(x,y)=(0,0)$,
which is relevant for the asteroid apparent magnitude contours.

A constellation of small satellites in LEO equipped with visible and NIR
sensors would be capable of filling the gaps left open by Rubin/LSST, NEO
Surveyor, and ground-based follow up facilities like LCOGT. They could be
tasked directly from the discovery lists or alert streams sent out by the
surveys, and the increasing affordability of launching to LEO would enable the
constellation to grow over time and keep pace with discoveries made by
Rubin/LSST and NEO Surveyor. When there are too few new NEO discoveries to
track at any given time, they could be tasked to further characterize known
NEOs in the MPC.

While the detection alert system for NEO Surveyor is not defined at this point
(beside presumably to send discoveries to the MPC), Rubin/LSST will have
alerts sent out within a minute of acquisition. A number of alert brokers have
been proposed to curate the alert stream for individual science cases. One
such broker, Automatic Learning for the Rapid Classification of Events
(ALeRCE) will include asteroid classification labels, and has been tested on
ZTF (Sánchez-Sáez et al., 2021, Förster et al., 2021, Carrasco-Davis et al.,
2021). These curated alerts could be used to tip and queue the sensors from a
central scheduler similar to the way LCOGT schedules their observations across
many telescopes.

With filters such as those presented in Figure 3, a LEO constellation could
identify asteroid classification with a series of colors and magnitudes
measured to infer the underlying SED. Studies linking SED classifications for
NEOs are linked to meteorite and sample return mission studies indicating
known minerals (DeMeo et al., 2009, Nakamura et al., 2011, Pieters et al.,
2012, DeMeo et al., 2022, Marsset et al., 2022, DeMeo et al., 2023).
Furthermore, numerous telescopes could observe and hand-off targets to the
next sensor as they orbit Earth in such a way that would build high fidelity
and well-sampled light curves without gaps in coverage that are sufficient for
shape and rotation inference, as I discussed in §2.4.5.

If the constellation was distributed along a few orbital inclinations in LEO,
just a handful of sensors could essentially observe anywhere in the sky
including at low Solar elongations. For example, I have recently tasked
GEOStare2191919GEOStare2 is a joint LLNL and Terran Orbital satellite used for
space traffic management that we tasked to observe asteroids flying by Earth
in late 2022. See https://www.llnl.gov/news/llnltyvak-space-telescope-goes-
orbit for a press release and outline of the mission. to observe three NEOs
flying by the Earth within a few lunar distances (Golovich et al. in prep).
GEOStare2 is a pathfinder program run at LLNL for tracking space debris for
space traffic management using a wide field of view 8.5 cm optical telescope.
Coupling the space-based light curves with Lick multi-band optical photometry,
we were able to characterize the asteroids rotational state and material
properties. Figure 13 shows an example image taken by GEOStare2 of the
asteroid 2005 LW3. Though our GEOStare2 observations were not at particularly
low Solar elongation, GEOStare2 is capable of observing within 50∘ of the Sun.
Telescopes designed with simple baffling could observe closer, which would
open the door to following up all NEO Surveyor detections in near real time.
This capability would be unique to space-based follow up and tracking of newly
discovered NEOs, so it is the only way to meet the tracking and
characterization portions of the NEO Survey Act. Finally, the wide field of
view of LSST, ATLAS, CSS, and Pan-STARRS makes it likely that small but nearby
NEOs are discovered. These fly by Earth in as short as a few hours. The
ability to characterize these NEOs would begin to unlock information about the
composition of rubble pile NEOs, but it can only be done with a dynamic hand
off between sensors since the LEO orbital period is short compared to even the
fast flyby time of a NEO within a lunar distance.

Figure 13: GEOStare2 images of 2005 LW3 taken on 23 November 2022. Each image
is a reprojected along the asteroid’s relative motion with respect to the
stars, which were tracked during a series of 1 second exposures. We were able
to estimate a $\sim$200 minute rotational period, which was later confirmed by
the Goldstone Radar images.

Space-based small satellites for planetary defense have been proposed recently
(Wang et al., 2022); though, not for all around characterization of
discoveries made by the major surveys. I began by discussing the increasingly
low cost of entry to LEO, and given the hole in our planetary defense
characterization capabilities of our largest investments, it is clear that
there is a need for a NEO characterization constellation in LEO.

##  6 Conclusions

In this paper, I broke down the NEO Survey Act into a series of “missions”
that I used to assess the performance of the major NEO surveys at present (CSS
and Pan-STARRS) as well as the two coming in the next few years (Rubin/LSST
and NEO Surveyor). In Figure 9, I detailed the gaps in the mission space left
by the surveys, and in §4, I discussed the assets available to astronomers to
help track and characterize NEOs after discovery. Finally in §5, I discussed
how a constellation of LEO small satellites would fill the holes left in
particularly the NEO characterization mission. To conclude the paper, below is
a list of the high level takeaways:

  * •

Despite a mandate to detect, track, catalogue, and characterize, NASA’s
investments have largely focused on NEO discovery and cataloguing. Tracking
has been largely been implemented by astronomers (many who are amateur) that
interface with the MPC.

  * •

In 2004 when 99942 Apophis was discovered and determined to have a high chance
of impact 25 years later, it took far too long to characterize the asteroid.
Such a scenario is more likely when Rubin/LSST and NEO Surveyor are
operational because they will detect far more NEOs than have been detected so
far in a short period of time.

  * •

NEO threat landscape is more heavily weighted with objects not covered by the
NEO Survey Act. There are $\sim$200 times more Chelyabinsk meteor and larger
NEOs than there are 140 meter and larger NEOs.

  * •

NEO characterization is best carried out by a combination of visible to NIR
spectroscopy and resolved imagery with radar observations; however, these are
far too expensive for the number of NEOs that have been discovered and
impossible for the majority of NEOs due to their sub-optimal orbits and
intrinsic faintness. Only about 15% of NEOs have had a spectrum measured, and
fewer than percent have been imaged with radar. This is in part because the
majority of NEOs are too faint to observe with the ideal techniques for
characterization.

  * •

In Figure 3, I showed that the development of accurate spectral
classifications with NEO spectroscopy can be used as a model to determine
asteroid classifications with visible to NIR spectroscopy with obtainable
photometric precision.

  * •

A worthy task for the planetary defense community would be to a set of visible
to NIR filters that maximize the characterization potential of photometric
measurements coving these wavelengths.

  * •

Continued work with asteroid sample return missions and meteorite sample
analysis have drawn credible links between the spectral classifications which
further enables photometric classification to infer material properties.

  * •

Many NEO discoveries are impossible or difficult without decades long surveys
due to the infrequency of their ventures into high Solar elongations. This is
especially true for Atens and Atira asteroid (see Figure 6). NEO Surveyor will
be able to detect these asteroids; however, all current follow up instruments
are ground-based, so NEO Surveyor detections will not be able to be
characterized or tracked fully without a space-based option.

  * •

A space based constellation of small telescopes with visible and NIR sensors
with several broad-band filters would enable the characterization of all
discoveries made by NEO Surveyor and Rubin/LSST in near real time.

## Acknowledgements

Thanks to Travis Yeager, Michael Schneider, Alex Geringer-Sameth, Megan Bruck
Syal, Pete Supsinskis, Wim de Vries, Brian Bauman, and Ben Bahney for useful
discussions leading to this paper. I also thank Kirsty Taggart, Connor
Dickinson, and Srujan Dandu of UC Santa Cruz for generously observing the NEO
that flew by Earth in December 2022 shown in Figures 1 and 10. This work was
performed under the auspices of the U.S. Department of Energy by Lawrence
Livermore National Laboratory under Contract DE-AC52-07NA27344. Funding for
this work was provided by LLNL Laboratory Directed Research and Development
grant 23-ERD-044.

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