Comet ISON was introduced to the public earlier this year with much fanfare about a potential “comet of the century." As observations have continued, and especially as recent measures show it to be fainter than predicted, there has been much public speculation as to whether it will flourish or fizzle. While no one can definitively answer that question, it is pretty safe to say that Comet ISON will at least be the "comet of the year."
On Nov 28th, 2013, Comet ISON will reach its closest point to the Sun, called perihelion. It will swing about 680,000 miles above the surface of the Sun, which is extremely close when you consider that the Sun’s diameter is only a bit larger at about 860,000 miles. Scientifically interesting events will certainly unfold before, during, and/or after that perihelion passage. Whether or not the passage is visually interesting is the big unknown, and one that is hampered by the fact that Comet ISON’s closest approach to Earth is about 40 million miles on Dec 26, 2013.
However, an encounter with Mars occurs well before either of those milestones. On Oct 1, 2013, Comet ISON will fly by Mars at a distance of about 6.7 million miles. Though it is about 10 times farther than the closest approach to the Sun, it’s also about one-sixth the closest approach to Earth. This Martian encounter brings some intriguing opportunities.
There are five active Mars missions: the rovers Opportunity and Curiosity as well as the orbiters Odyssey, Express, and MRO (Mars Reconnaissance Orbiter). In a recent blog post, I noted how Curiosity had observed a solar eclipse, but its instruments are not as well suited for observing a much fainter comet. If it does succeed, that would mark the first comet observation from the surface of another planet.
The Mars mission best equipped to see Comet ISON is MRO. Its instruments include a 20-inch telescope, the largest ever carried into deep space. MRO should easily detect the comet, although there are some complications -- it was designed for rapid observations of the surface, not long exposures of the sky. To create an analogy with Earth satellites, MRO is intended to be used like Landsat, but is being re-purposed to be used like Hubble. Comet ISON encounter observations are slated to begin this weekend.
These views from Mars will be the closest snapshots yet, and should provide clues to the size of the comet. A larger comet is more likely to survive its close passage by the Sun, while a smaller one may break into pieces. In addition, as Comet ISON passes inside the orbit of Mars and continues to warm, the chance of outbursts from vaporizing ices has grown considerably. Surprises would not be unexpected.
The Martian encounter data will join the accelerating campaign of Comet ISON observations. Also scheduled for October are more Hubble observations. In total, 16 NASA spacecraft will study the icy visitor as it sweeps through the inner solar system. Add to that thousands of ground-based telescopes and this will be one heavily scrutinized comet. What would be nice is if billions of human eyes could join the party in a couple months. Stay tuned.