Beyond the Moon: Planets and asteroids
U.S. space policy sets a crewed expedition to Mars as NASA’s primary long-term goal. This requires more advanced technologies than now exist, including the ability to make propellant from Martian resources to refuel ships for the return to Earth.
The propellant might be extracted from the Red Planet’s atmosphere, or from its ice deposits. Robotic missions are required to prospect for the best sources and to demonstrate that propellant production actually works.
For science, Mars sample return is the highest priority, according to the National Research Council’s every-ten-year report on planetary science objectives. The Spring 2011 decadal survey said a three-mission campaign should be NASA’s sole Martian science goal: a robot to gather and package rock samples, a follow-up lander to rocket these samples into Mars orbit, and a third mission to catch the orbiting samples and bring them back to Earth. The report said the first mission to gather samples should take place only if its cost can be limited to $2.5 billion.
The solar system has two additional planets with rocky surfaces to support human and robotic exploration: Venus and Mercury. The surface of Venus is so hot that landers or rovers are expected to fail after only a few hours of exposure. On the other hand, the question of how Venus came to be so hostile to life could have important lessons for keeping the Earth at a steady temperature.
The Firdousi Plain on Mercury
Mercury is even closer to the Sun than Venus, but may be easier to explore because it lacks an atmosphere and rotates very slowly. Robots and later human explorers may be able to keep from frying by moving along the dawn terminator – the line between night and day – which takes 175 days to move completely around the planet. They can warm up by lagging the terminator’s progress (and getting more Sun exposure) or speeding up to move deeper into the cooler night.
Robots eventually will explore asteroids, the rubble left over from the solar system’s formation. Thousands exist in the main belt between Mars and Jupiter, while others are easier to reach because they pass by Earth in their orbits. Asteroids are small, and so their gravity fields are very weak. A spacecraft essentially docks with an asteroid rather than landing on it. The weak gravity means the return trip to Earth requires much less fuel than blasting off from the deep gravity wells of the Moon, Mars or Mercury. Finding an asteroid that comes near Earth, is large enough to be interesting, and is tumbling slowly enough to be approachable is a challenge. Candidates have been identified that pass by Earth in 2025 and 2030.
