NASA just prevented a collision in Mars's orbit. Earth's could prove more challenging.

March 3, 2017 --On March 6, NASA's MAVEN Mars orbiter is expected to cross paths with the Red Planet's moon Phobos. At first, computer models showed the two satellites missing each other by just seven seconds. Mission controllers decided that was too great a risk for the $671 million spacecraft, whose name stands for Mars Atmosphere and Volatile EvolutioN. So, on Tuesday, they fired its rocket engine enough to increase its velocity by 0.4 meters per second. Controllers say that small correction will yield a safe 2.5 minutes between MAVEN and Phobos on the 6th.