Mars is about to get a visitor that comes around only once in a million years or so.
The arrival of a "mountain-sized" comet, Siding Spring (C/2013 A1), is made all the more extraordinary by the fact that humans — who were busy refining their stone-tool-making skills the last time such an event might have occurred — now have spacecraft from multiple countries at the Red Planet to see it happen.
"Think about a comet that started its travel probably at the dawn of man and it's just coming in close now," Carey Lisse, a senior astrophysicist at the Johns Hopkins University Applied Physics Laboratory, said at a news briefing about Comet Siding Spring last week. "And the reason we can actually observe it is because we have built satellites and rovers. We've now got outposts around Mars."
As the nucleus of the comet passes about 80,000 miles from the Martian surface, orbiters from NASA, Europe and India are all being repurposed to quickly observe the comet flyby and then beat a retreat before the comet's tail swings by.
As The Associated Press writes: "The orbiting craft will observe the incoming iceball, then hide behind Mars for protection from potentially dangerous debris in the comet tail. NASA's Opportunity and Curiosity rovers will be shielded by the Martian atmosphere. They should have the best seats in the house."
Emily Lakdawalla, a senior editor for The Planetary Society, says: "There are tons and tons of scientific observations planned by Mars orbiters, Mars rovers, and Earth-based observatories. In fact, most of the facilities that are planning to observe Siding Spring have already begun their work, and will continue observation for days after the encounter."
Lakdawalla adds: "It's not like a Mars landing; there won't be a single moment when a bunch of serious-looking engineers suddenly erupt into cheers. Instead, there'll be many smaller, non-televised moments as instrument teams receive their data from far-flung spacecraft and telescopes, spread out over the next several days. For the most part, the images of the comet won't be instant classics; many will show only a single pixel, or a faint smudge. Some of the data won't even arrive on Earth until the middle of next week."
As astronomer Phil Plait, who writes the Bad Astronomy blog for Slate, notes:
"The NASA comet page says the coma (the big fuzzy cloud of gas surrounding the solid nucleus of the comet) is about 20,000 km across. At closest approach, that means that if you were standing on Mars, the comet would appear to be over 8° across! That means that if you have a big hand, you could just barely block it with your upraised fist.
"That's astonishing. What a view that would be! And while the astronomer part of my brain is envious and wishes we could see something like that from Earth, the human part of my brain is screaming obscenities at the astronomer part of my brain. In real life, it's probably best comets keep their distance from us."