Comet ISON is sweeping past Mars more closely than it will ever get to Earth on Tuesday, providing an observing opportunity that could show whether the comet will be visible in our night sky later this year.
The comet is passing within 0.07 astronomical units of the Red Planet, six times closer than it will come to Earth. (An astronomical unit, or AU, is the average distance between the Earth and sun, about 93 million miles.)
NASA will view the comet from its Mars Reconnaissance Orbiter to see how much gas it is releasing, something that could help indicate how large its nucleus is. If the nucleus, a hunk of rock, dust and ice, is larger than half a kilometer wide, it is thought to be able to survive a pass through the sun's corona in November, meaning it could shine brightly enough to be seen with the naked eye from Earth in December.
More than a dozen other spacecraft and space telescopes are trained on the comet as it zooms from outside the solar system toward its brush with the sun.