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A small planet may hold gas only if it is a very cold planet far away from its sun. But the gas would be a liquid or frozen, too cold to sustain life. But an atmosphere should be gaseous, not liquid or solid. The thermal movement of the gas atoms should be so slow that they could not escape the planet's gravity.


In addition to the answers above, one should add that assuming you can get to Mars very, very fast, you will therefore arrive there with a very high velocity relative to Mars. This makes the problem of landing on Mars much harder. Also, if something goes wrong at Mars insertion, the Martian explorers are hurtling off into deep space with no way back. The ...

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