Two comets will buzz Mars over the course of the next year, prompting excitement as well as some concern that cometary particles could hit the spacecraft orbiting the Red Planet and exploring its surface.
Three operational spacecraft currently circle Mars: NASA's Odyssey and Mars Reconnaissance Orbiter (MRO), as well as Europe's Mars Express. NASA also has two functioning rovers, Curiosity and Opportunity, on the ground on Mars.
All of these spacecraft will have ringside seats as Comet ISON cruises by Mars this year, followed by Comet 2013 A1 (Siding Spring) in 2014. (Photos of ISON, a Potentially Great Comet)
Crossing the sublime line
The MRO spacecraft has been on the lookout for Comet ISON, said Richard Zurek, MRO project scientist and chief scientist in the Mars Program Office at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.
On Aug. 20, MRO looked for Comet ISON, which experts say could put on a dazzling sky show here on Earth shortly after the icy wanderer zips a scant 724,000 miles (1.16 million kilometers) above the surface of the sun on Nov. 28.
During last month's observation by MRO, ISON was 1 astronomical unit (AU) from Mars and 2.5 AU from the sun. (One AU is the distance from Earth to the sun — about 93 million miles, or 1.5 million km.)
Given ISON's distance from the sun, the comet should have crossed the solar system's "snow line" by that time, Zurek told SPACE.com. At the snow line, many comets brighten as ice more rapidly sublimes into gas due to increasing solar radiation.
"The MRO instruments did not see anything," Zurek said, and evidence suggests the instruments "were pointed accurately. Thus, the current conclusion is that the comet had not brightened quite enough to be seen at that range with the MRO instruments."
Comet ISON's current luminosity is a topic of much discussion among astronomers and skywatchers alike. The icy wanderer was branded a "comet of the century" candidate almost immediately after its discovery in September 2012, but recent observations suggest that it's not brightening as much as expected or hoped on its trek toward the sun.
More observations ahead
MRO will look at ISON again, Zurek said, with observations scheduled for Sept. 29, Oct. 1 and Oct. 2 (when the comet will be closest to Mars). At those times, ISON will be roughly 14 times closer and will likely be relatively easy to detect. (Comet of the Century? Sun-Grazing Comet ISON Explained (Infographic))
"At the closest passage distance, there is no concern that cometary particles from ISON will affect the orbiters or Mars," he said.
NASA's 1-ton Curiosity rover and its smaller, older cousin, Opportunity, will also image ISON from the Martian surface later this month, Zurek said. However, those plans are still being formulated.
The spacecraft in orbit around Mars and on the planet will give scientists a better chance of investigating Comet ISON, though that is not their primary function, said Michael Meyer, lead scientist for NASA's Mars Exploration Program at the agency's headquarters in Washington, D.C.
"Mars has a better view than Earth does right now," Meyer said. However, it is "challenging for orbital and landed assets as they are not really designed to do this sort of thing. They are supposed to be looking at Mars."
Meyer spoke via Skype Aug. 25 during a New Media Practitioners Professional Development Workshop on the upcoming launch of NASA's Mars Atmosphere and Volatile Evolution orbiter (or Maven for short). The workshop took place at the University of Colorado Boulder's Laboratory for Atmospheric and Space Physics (LASP).