Imagine watching a comet that flits past Earth well inside the orbit of the moon.
That's a view as many as five orbiters circling Mars are going to get in October, courtesy of comet C/2013-A1 Siding Spring. The comet, on a once-in-a-million-year swing around the sun, currently is slated to pass within 82,460 miles of the red planet – roughly a third of the distance between the Earth and moon.
The cometary flyby comes with equal doses of excitement and trepidation. It represents a rare opportunity to observe a comet at close quarters. But the dust the comet spews could represent a potentially serious risk to the small flotilla of craft orbiting Mars on Oct. 14, the day of closest encounter.
"This is a first-time comet; we haven't seen it before," says Richard Zurek, chief scientist for the Mars Program Office at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "This is a very exciting opportunity. But it is one of those things where you go: Oh, this is great. And then you go: Wait a minute; do I need to worry about this comet?"
Scientists and engineers are now in the process of trying to figure out how much trepidation should accompany the excitement and how to minimize risks to the orbiters from dust the comet will shed.
The team is especially interested in dust particles larger than about 100 microns – the average diameter of a human hair. The comet will be approaching nearly head-on as seen from Mars, so the velocity of the particles reaching the planet will be much higher than they would be if the two objects were moving in the same general direction.
Even hair-thin dust particles can put the hurt on delicate instruments when the particles and spacecraft meet at 125,000 miles an hour.
Help in assessing that risk is coming from the Hubble Space Telescope.
On Thursday, astronomers using the orbiting observatory released images showing two jets of dust and gas erupting from the nucleus. These features allow astronomers to figure out where the nucleus's poles are and the direction of the nucleus's axis as it rotates. Combined with observations from late January, these latest images also allow researchers to estimate the how fast dust particles are traveling as they leave the nucleus.
Three independent modeling teams are using the information to determine where those particles are likely to be during closest approach, Dr. Zurek says.
Although the teams are converging on an answer, "they aren't quite there yet," he says. The teams are expected to have their best estimates available by mid-April.
As for NASA's Mars rovers, Opportunity and Curiosity, the atmosphere of Mars is thick enough to incinerate the dust before it reaches the surface.
In the meantime, researchers are laying the groundwork for their comet-flyby observing campaign.
Although spacecraft from Europe, the US, and India are expected to be orbiting Mars at the time, some of the most exciting science may come from NASA's Mars Reconnaissance Orbiter. The craft carries a high-resolution camera that will allow it to identify features as small as 500 feet across on a comet nucleus that could be more than half a mile across. It should be able to identify active areas on the nucleus's surface and, through images taken before and after flyby, give researchers a better estimate of the nucleus's rotation.
In addition, visible and infrared observations of the dusty halo around the nucleus, as well as of the comet's tail, is expected to yield information on the size of the dust particles.
The team also will be studying the effect the arriving particles could have on the depth of the atmosphere as the upper atmosphere absorbs the heat from the passage of dust as it burns up.
Preliminary estimates suggest that the dust will take about 90 minutes to arrive at the planet after the comet's closest approach – the amount of time the orbiters will have to reach the sheltered skies behind the planet, if the estimates of dust emissions point to an unacceptable risk of damage.
Planners are looking at ways to slowly change spacecraft orbits so that they can make the observations scientists would like to make and still allow the craft to remain on the dust-free side of the planet during the height of the meteor shower, explains Soren Madsen, the Mars Exploration Program's chief engineer.
"We're basically hiding the orbiters behind Mars," he says. "Since all of the dust is coming from one direction, there will be a dust shadow."
In addition, planners are exploring ways to reorient the craft to shield sensitive instruments or solar panels from excessive pummeling from the dust.
Since NASA's Deep Space Network is the lynchpin for communications with the orbiters, planners also are trying to figure out how to manage communications with up to five orbiters, all of which will be seeking safe haven behind the red planet.
Controllers could wait until September to begin altering orbits to ensure the craft are shielded from the worst of the dust from comet C/2013 A1. But the maneuvers would be fuel intensive at that late date.
"The earlier we initiate the maneuvers, the cheaper they are in terms of fuel," Mr. Madsen says. The team aims to have a risk reduction plan in place by late June so that controllers can begin tweaking orbits in late July.