sentence: Some people are interested in sending astronauts to visit Mars.
sentences_before: So far, scientists have not found life on Mars, either living or extinct. Several space probes have gone to Mars to study it. There are pictures of the surface of Mars that were sent back to Earth by the probes.
sentences_after: They could do a better search, but getting astronauts there would be difficult and expensive. The astronauts would be in space for many years, and it could be very dangerous because of radiation from the sun. So far we have only sent unmanned probes.
concept_name: people
headings: Mars Life on Mars? Today