In ten days, a comet that has never before ventured into the inner Solar System will come barreling in, on a path that will bring it dramatically close to Mars. At a distance of approximately 87,000 miles from the red planet, it will be less than half the distance between the Earth and the Moon. Comet Siding Spring is visiting from the Oort Cloud, thought to contain thousands of icy bodies, in a cluster perhaps 50,000-100,000 Astronomical Units from Earth. One AU is the distance from the Sun to the Earth, or 93 million miles. Because this comet has never before come near, and been disturbed by, the Sun, scientists expect that its composition will reflect that of the earliest bodies in the Solar System, billions of years ago.
Siding Spring was most likely "kicked out" of the Oort Cloud by the pull of a passing star, and has traveled more than one million years to get to Mars, NASA scientists report. At a briefing at NASA headquarters this afternoon, Jim Green, head of NASA's planetary science programs, described this as an event that happens "once every million years." Fortuitously, for this rare event, a fleet of orbiters and rovers at Mars will have a front-row seat.
At today's briefing, scientists discussed the images of the comet that have already been taken by the Hubble Space Telescope, ground-based telescopes, and other instruments. With the array of especially space-borne instruments available, the comet is being examined in various wavelengths. But in the path it is following, the best view and measurements that will be taken will be from Mars. Because the encounter is so close, precautions are being taken to protect the orbiters from debris. NASA has already maneuvered Mars Odyssey, Mars Reconnaisance Orbiter, and MAVEN, in order to reduce the risk of the orbiters being damaged. Of most concern is the 90 minutes after the closest approach to the planet, when Mars will come closest to the center of the comet's widening tail of dust, which will be flying off the comet's nucleus.
The entire planetary community is mobilized to take advantage of the cometary fly-by. In answer to a question from EIR, the scientists reported that workshops have been held with international participation to maximize the scientific return. They have pledged that images and updates will be quickly posted online before and after the encounter.
In addition to what we learn about the comet and history of the early Solar System, the impact of this event on the atmosphere on Mars is of great interest. MAVEN, for example, has as its mission a study of the upper atmsophere of Mars, to try to help reconstruct the process that led to the loss of the planet's atmosphere. How the atmosphere interacts with cometary material will be another window into that process.