Mission planners have devised an unusual strategy for protecting orbiting spacecraft when Comet Siding Spring passes very near the Red Planet in October.
As Comet Siding Spring (C/2013 A1) hurtles toward Mars, NASA is taking steps to protect its Martian orbiters. The plan? Use the planet itself as a shield between the spacecraft and the comet’s potentially dangerous debris.
As part of its long-term Mars Exploration Program, NASA currently has two spacecraft orbiting the Red Planet, Mars Reconnaissance Orbiter (MRO) and Mars Odyssey, with Mars Atmosphere and Volatile Evolution (MAVEN) set to arrive in late September. Teams of scientists at the University of Maryland, the Planetary Science Institute, and NASA’s Jet Propulsion Lab (JPL) have used data from both Earth-based and space telescopes to model Siding Spring’s journey through the inner solar system, and determined that there is no risk of the comet colliding with Mars.
However, at its closest approach to Mars on October 19, 2014, Comet Siding Spring will come within 82,000 miles of the Red Planet, which is about a third of the Earth-Moon distance. The closest comets ever to whiz by Earth have been at least ten times more distant.
Such a close encounter means the dust tail left in C/2013 A1’s wake might graze Mars’s upper atmosphere. The smallest particles are only about a half millimeter across, but even these could severely damage a spacecraft when striking at 35 miles per second. Scientists predict that the time of greatest danger for the orbiters will occur about 90 minutes after Comet Siding Spring’s closest approach and last about 20 minutes.
To avoid the threat of Siding Spring’s debris, NASA engineers will manipulate the orbiters’ trajectories so that all three will end up on the opposite side of the planet during the flyby. The MRO team executed one maneuver at the beginning of July, with another planned for the end of August. The Mars Odyssey team took similar steps on August 5th, and the MAVEN team will perform a precautionary maneuver shortly after the spacecraft enters orbit around Mars.
An Exercise in Caution
Scientists point out that it’s also possible that Siding Spring’s dust tail will present no threat at all. In June, the University of Maryland issued a press release stating that Siding Spring poses little danger to the orbiters. In the NASA press release that announced plans to move the orbiters, Rich Zurek, chief scientist for the Mars Exploration Program at JPL, affirmed, “Mars will be right at the edge of the debris cloud, so it might encounter some of the particles — or it might not.”
“Although our best prediction is that the most dangerous time is actually not dangerous at all, putting all the spacecraft in this shielded position at that moment is relatively easy, so it makes sense to play it safe,” says Michael Kelley (University of Maryland).
Indeed, the fuel expended to adjust the orbiters’ positions for the flyby is fairly insignificant. Mars Odyssey’s maneuver used about 0.1 kg of fuel, leaving the spacecraft with around 16.5 kilograms, which can support another decade of operation. MRO’s two maneuvers will require a total of 0.15 kg of fuel, which is similar to the amount used for monthly maintenance maneuvers. MRO carries 254 kg of fuel, enough to run for another 25 years.
Seizing a Rare Opportunity
Threat of destructive dust particles aside, Siding Spring’s close flyby is a unique opportunity for close-up study of a dynamically new comet— one that will zoom past the Sun for the first time. Since the planets formed, Siding Spring has been chilling in the Oort Cloud, the spherical collection of comets on the outermost fringes of the solar system. As Siding Spring closes in on Mars, it’s bringing with it some of the most ancient material around the Sun. NASA plans to take full advantage of this event by using its Mars orbiters and rovers to study Siding Spring in the days before and after its flyby.
MRO, whose usual mission is studying the history of water on Mars, will observe the gases in Siding Spring’s coma. The MRO team also hopes to get a detailed-enough view of the comet’s nucleus to determine its rotation rate and distinguish some of its surface features. Mars Odyssey will study Siding Spring’s coma and tail and examine their effects on the atmosphere. MAVEN will study the gases streaming off Siding Spring’s nucleus as the Sun warms the dirty iceball, before beginning its investigation of the Martian climate and atmosphere in early November.
The European Space Agency is taking similar precautions to protect its Mars Express (MEX) orbiter. MEX has a highly elliptical orbit that would leave it exposed to Siding Spring's debris longer than MRO or Odyssey. On June 22nd the MEX team altered the orbiter's track around the planet so that it will be hidden behind Mars for 27 minutes during the comet's closest approach.
According to MEX Spacecraft Operations Engineer Andy Johnstone, if observations closer to the comet's arrival show a high risk of impact, then the MEX orbiter will adopt a special attitude that uses its high-gain antenna as a shield and will shut down as many onboard systems as possible. Otherwise, MEX will study the comet and its interactions with Mars.
The Indian Space Research Organization also will have its Mars Orbiter Mission (MOM) swinging around the Red Planet starting in September, but the agency has not announced any plans yet for special maneuvers to avoid Comet Siding Spring.
Fascinated by the Red Planet? Check out Sky and Telescope's special issue Mars: Mysteries and Marvels of the Red Planet.