When the dandelions raise their heads to the sun this spring, NASA will be paying close attention a little comet that could raise a mess at Mars. Comet C/2013 A1 Siding Spring will pass just 86,000 miles (138,000 km) from the planet on October 19 this year, raising the possibility that dust sputtering from its nucleus could pepper the three orbiting spacecraft there and two additional probes expected to arrive about three weeks before the comet’s closest approach. That’s a close shave!
NASA’s taking precautions starting with an intensive observing campaign already underway using the Hubble Space Telescope and the NEOWISE probe. Scientists hope to characterize the comet, carefully monitor the size (and production rate) of dust particles that are released when the comet’s ices vaporize and also refine its orbit. As A1 Siding Spring moves closer to Mars, NASA’s Mars Reconnaissance Orbiter could provide pictures with enough resolution to image the actual nucleus.
No one knows exactly how big the nucleus is yet but already NEOWISE has revealed that A1 Siding Spring is active and dusty even though it’s still three-quarters as far from the sun as Jupiter.
While it’s too early to know whether dust could pose a threat to the orbiters, the two rovers are protected by Mars’ atmosphere, which is thick enough to burn up any dusty meteors released from vaporizing chunks of comet ice. Cameras on the rovers might be used to photograph what could be a spectacular meteor shower, though it appears for the moment that the geometry of the flyby would put the best of the shower in the daylight sky.
“During April and May, the comet will cross the range of distances from the sun at which water ice on a comet’s surface typically becomes active – vaporizing and letting dust particles loose. Dust ejected then could get far enough from the nucleus by October to swarm around Mars,” according to a NASA press release.
Dust shed by the comet is traveling faster than the typical dust that causes meteors – about 35 miles (56 kilometers) per second, relative to the Mars orbiters – because the comet is orbiting in almost the opposite direction as Mars and the other planets. It’s the difference between a car hitting you head-on versus from the side or behind. The two velocities – comet and Mars – add up to a significantly higher speed for dust particle impact.
If NASA decides evasive maneuvers are necessary for the five orbiting probes, plans are being discussed to position them on the backside of the planet away from the brunt of the meteor storm and orient their “delicate” sides away from the comet’s direction to avoid impacts.
Comet 2013 A1 Siding Spring was discovered on Jan. 3, 2013 from Siding Spring Observatory in Australia. Although it will pass 10 times closer than any identified comet has ever flown past Earth, it won’t come anywhere near as close to our planet. That said, it should still be fun to see it involved with Mars in binoculars and telescopes around Oct. 19 when the comet’s expected to glow around magnitude 8.