Mars's tiny moons – one small step for mankind?
点击量： 时间：2017-12-07 07:02:01
By Michael Reilly, Moffett Field (Image: ESA/DLR/FU Berlin/G Neukum) Forget Mars – the Red Planet’s moons Phobos and Deimos could be the next stop in the solar system for humanity, according to planetary scientists. During a conference at the agency’s Ames Research Center in Moffett Field, California, US, on Wednesday, scientists said astronauts could make their first footprints on one of them within 10 years. “They are the most accessible planetary bodies in our solar system,” said Pascal Lee of the Mars Institute, a non-profit research organisation based in Moffett Field, California. “It’s counterintuitive, but they’re even easier to get to than Earth’s Moon, for a robotic mission.” True, the Moon is only a short three-day spaceflight from Earth, whereas Phobos and Deimos are several months away using current propulsion methods. But the Moon’s gravity is one-sixth Earth’s, strong enough that a landing craft has to fire retrorockets to slow its descent to the surface, and again to leave. This wastes tonnes of heavy, expensive fuel and adds millions of dollars to the cost of a mission. Mars’s two potato-shaped worlds, on the other hand, are tiny – Phobos is the size of Manhattan and Deimos is about a third as large, just 6.3 kilometres wide. So their gravitational pull is only one-thousandth that of Earth, making landing on them more like docking with another spaceship. That translates to big savings for a crewed mission to the moons compared with one to the Martian surface. Estimates vary, but Lee says putting a person on Mars could cost $200 billion to $300 billion dollars, including the cost of decades of research. The bill for a run to Deimos could be as low as $30 billion, says S Fred Singer of the Science and Environmental Policy Project. Still, it might seem an awful waste of a trip to go all the way to the Martian system and not land on Mars. But Lee says that the moons’ mysterious geologic pasts make them alluring targets. Scientists have theorised that the moons may be asteroids captured by Mars’s gravity, or that they may be remnants of a single moon that was blasted apart by a massive impact. “We know almost nothing about these moons, other than their density, size and their colour, which is black as asphalt,” says Lee. For now, the only planned missions to the moons are robotic. Russia is gearing up to launch the Phobos-Grunt mission in 2009 that will attempt to land a craft on Phobos, collect the first samples from the moon’s surface and return them to Earth. The project could help determine if there is any hydrogen or water present, which astronauts on a later crewed mission could use. If Phobos-Grunt is successful, Lee says he’d advocate that NASA send a robotic mission to Deimos, about which even less is known than its larger cousin. For a crewed mission, though, he would opt to go to Phobos first, because it’s bigger than Deimos and closer to Mars – just 9700 kilometres away. Because of its proximity, Phobos is more likely to harbour ancient meteorites blasted up from Mars’s surface. The rocks could stretch deep into the planet’s past, and would be better preserved in Phobos’s near-vacuum than the harsh oxidising conditions found on Mars. However, a human journey is not without its perils. For one thing, dust could be a huge problem in Phobos’s weak gravity field. “There could be a layer of dust four or five metres thick that would be easily mobilised,” says Michael West of the Australian National University in Canberra. Apollo 17 astronaut Harrison Schmitt reported a nasty cough associated with Moon dust on crew spacesuits. The so-called ‘lunar lung’ could be much worse on Phobos, where even the slightest disturbance could raise a massive dust cloud. Exposure to radiation would also present a hazard. NASA estimates that a three-year mission would expose astronauts to a lifetime risk of premature death due to cancer of 5%, says Marcelo Vazquez of the National Space Biomedical Research Institute in Houston, Texas, US. That is almost double NASA’s acceptable risk of 3%. By comparison, a six-month stay on the International Space Station poses a risk of 0.35%. But Vazquez says radiation shielding and medical countermeasures may be able to reduce that risk, so that radiation will not be a ‘showstopper’ for such a mission. There would also be perks to the journey. Phobos would be fun to jump around on – the moon’s weak gravity means a person could jump many metres in the air and land without a bruise, West says. But the real knockout would be the view. An astronaut looking up would see Mars’s massive red orb filling the sky above Phobos,