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).
Another comet coming
After ISON, scientists will look forward to Comet Siding Spring, Meyer said. That comet will make a very close approach to Mars in October 2014, skirting just 76,428 miles (123,000 km) from the planet, according to the current best estimates.
"That promises to be pretty exciting," Meyer said. "Right now, in all honesty, what we know about it and what sort of calculations can be done … the error bars are extremely large."
The comet poses risks to orbiters circling Mars, Meyer said, a prospect that may lead to re-orienting and maneuvering of the craft to protect them from comet particle strikes. But whether it's a 10 percent, 1 percent or 0.1 percent risk remains unknown at the moment, he said.
"You can't get too worked up about it until you get some measurements as the comet gets closer," Meyer said. "It promises to be quite a show — if we're able to look at it." [Photos: Amazing Comets of 2013]
In early August, JPL issued a request for proposals to help characterize the cometary environment of Comet Siding Spring, with proposals due on Sept. 11.
"The intent is to provide data products useful for risk assessment and mitigation-strategy development for the Mars orbiter missions, due to possible impacts from dust and ion tail particles as this comet encounters Mars," the JPL request stated.
Model simulations are needed to characterize the evolving dust and ion particle distributions around Comet Siding Spring, as well as their motions with respect to Mars, as this comet approaches the Red Planet.
Because Comet Siding Spring will come so close to Mars, it's likely that the planet, along with its associated spacecraft, will pass through the coma of the comet, Zurek said. But NASA's rovers will probably be relatively well protected, he said.
"As thin as the Mars atmosphere is, it should still shield the rovers from infalling particles," Zurek said, "so the risks to be assessed are to the orbiters."
Zurek told SPACE.com that scientists won't have an idea of how big a risk the comet environment will pose until they make more observations of the comet's variability as it nears the sun.
"We have put out a call for modeling of the cometary environment," Zurek said. Such modeling, he said, is dependent on the developing comet activity.
Passage through the comet's coma could result in a wide range of effects: anything from a modest enhancement to the background meteoritic flux experienced by the spacecraft, which is deemed most likely, to something more substantial, Zurek said.
"So stay tuned!" he said.
Siding Spring's Mars visitation will also overlap with that of another Mars newcomer, NASA's Maven spacecraft. To be launched this November, it will arrive at the Red Planet in late September 2014, with Comet Siding Spring set to make its closest approach to Mars on Oct. 19.
However, that will likely be too soon for the Maven orbiter to analyze the comet.
"Maven will still be in the middle of its commissioning phase at that time and will not be ready to take regular measurements," said Bruce Jakosky, principal investigator for Maven at LASP.
"Although we'd like to be able to observe the comet as it passes by and how it affects the upper atmosphere, our first priority will be spacecraft and instrument health and safety," Jakosky told SPACE.com.
The Maven team is working with the Mars Program Office (MPO) to predict the likely dust environment as the comet passes by, and how impacts from the dust might affect the spacecraft, Jakosky said. The MPO is coordinating the activities, he said, as that group is concerned about Mars Odyssey and MRO, in addition to Maven.
Minimizing the risk
"After Maven's launch, we'll be looking in detail at what mitigations we can take to minimize any risk," Jakosky said. "We'll look at things such as turning the least-vulnerable face into the flow of the dust, putting the solar panels edge-on to the flow and so on."
Jakosky said that, at this point, the best analysis indicates a minimal risk to Maven.
The number of dust impacts expected, and the effect they'll have on the spacecraft should be within the range of what Maven researchers anticipated for a normal mission run, Jakosky said. That is, the dust impacts should not exceed what researchers had already planned to absorb just from interplanetary dust over the lifetime of the mission, he said.
"I do expect that telescopic observations of the comet in the spring and more-detailed modeling of the dust environment in the coma and tail will help us to refine our analysis," Jakosky said.
Leonard David has been reporting on the space industry for more than five decades. He is former director of research for the National Commission on Space and is co-author of Buzz Aldrin's new book "Mission to Mars – My Vision for Space Exploration," published by National Geographic. Follow us @Spacedotcom, Facebook or Google+. Originally published on SPACE.com.