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Why NASA Has 13 Spacecraft Watching a Single Comet This Weekend

Comet C/2013 A1, also known as comet Siding Spring, will pass about 87,000 miles from Mars, and a whole fleet of spacecraft and rovers have been lined up to image it.

by Amy Shira Teitel
Oct 15 2014, 3:20pm

Mars orbiters will hide behind Mars to protect themselves from Siding Spring's dust trail. (Not to scale.) Image: NASA/JPL-Caltech

NASA is gearing up for an incredible off-world effort this weekend: Comet C/2013 A1, also known as comet Siding Spring, will pass about 87,000 miles from Mars, and a whole fleet of spacecraft and rovers have been lined up to image it.

The coordinated viewing is sure to produce some pretty incredible images and fascinating data, but pulling off the feat is taking a fair bit of practice to make sure everything goes right on the big day.

"This is a cosmic science gift that could potentially keep on giving," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate in Washington, in a release. "This particular comet has never before entered the inner solar system, so it will provide a fresh source of clues to our solar system's earliest days."

The target comet, Siding Spring, is an Oort cloud object, one of those little bodies thrust into the inner solar system from the distant cloud of icy rocks scientists suspect is leftover material from when the solar system formed.

There are a lot of Oort cloud objects out there, but this is the first time astronomers are getting a chance to study one up close. Measuring what Siding Spring is made of, how much water it holds, and what carbon compounds it holds will shed some light on the state of the very early solars system.

Scientists are also interested to see how the debris from the comet interacts with Mars's atmosphere. They will look for meteor trails in the atmosphere, any changes in particle distribution, and whether the comet has any effects on the planet's air temperature and clouds.

Watching the comet's pass are some pretty powerful eyes on the ground and in the skies. NASA's Hubble Space Telescope will watch Siding Spring pass by Mars, as will the Kepler, Swift, Spitzer, and Chandra space-based observatories. From the ground, the Infrared Telescope Facility on Mauna Kea, Hawaii will also watch the comet's pass.

But the really good views will be from Mars, and this is where things get a little tricky. NASA has a number of rovers and orbiters that will be used to track Siding Spring's flyby, but coordinating them all is a logistical—and physical—nightmare. Timing is key, as is practice and preparation.

Mars will be right at the edge of the debris cloud, so it might encounter some of the particles—or it might not

The comet will make its closest pass by the Red Planet at around 2:27 PM EDT on October 19, speeding along at about 126,000 mph. Add to that the fact that the planet is also rotating on its axis, and the rovers Opportunity and Curiosity will only have a limited time in which they will be able to see the comet.

The Curiosity rover will be looking at the comet using its Mastcam science camera for straight imaging and the ChemCam to gather spectroscopic information on the comet's composition. Luckily, this rover will have the best view.

"The closest approach occurs in the mid afternoon for Curiosity, so our best imaging will be that evening," Ashwin Vasavada, deputy project scientist for MSL, told me in an email. "We'll start as early after sunset as we can, based on the brightness of the sky relative to the comet."

For the MastCam images, the Curiosity team has been preparing for the comet encounter by taking images of stars in the early evening sky to get a sense of exposure times.

Getting ready for the ChemCam images has been a little trickier. This is the instrument that blasts rocks with lasers then studies the bounced back light to unravel that rock's composition, but it's not designed to hone in on and measure the nucleus of a comet.

Image: NASA

The team has been developing a complex workaround process which uses Curiosity's gyros to measure the rover's tilt and track the Sun to figure out the rover's orientation. With these measurements known, the team will point ChemCam in a raster pattern across the sky while simultaneously tracking the comet's trajectory. There's no guarantee they'll hit the comet, but they might!

Meanwhile, the orbiters are, well, orbiting. The Mars Odyssey orbiter, the Mars Reconnaissance Orbiter (MRO), and the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft are all moving around the planet with occasional and limited chances to image the comet.

MRO has actually already imaged the comet, though the big observations have yet to start. This spacecraft's big image campaign will begin about 60 hours before the comet makes its closest pass by Mars and continue until about 14 hours after closest approach.

But preparing for images isn't the only thing NASA is doing to get ready. Project scientists have had to adjust their orbiters' orbits slightly to keep them out of the way of the high-velocity dust particles that will blast off the comet. The spacecraft will be at risk for a 20 minute period beginning soon after the comet's closest approach. At this point, Mars will be as close as it will get to the center of the widening trail of dust coming off the comet's nucleus.

"Mars will be right at the edge of the debris cloud, so it might encounter some of the particles—or it might not," said Rich Zurek, chief scientist for NASA's Mars Exploration Program. The rovers, too, could be hit by dust; the thinner atmosphere won't be able to shield them all that well.

The Mars orbiters will measure the comet throughout its pass on the comet's size, rotation, activity in its nucleus, the composition of its gas coma, and the distribution of particles in its tail. They're also going to turn their instruments to look at the planet itself.

All told, it's going to be a really busy comet pass, especially considering it's not passing our own planet. For more on the weekend's events, check out the above video from a JPL social this week. The comet talk, right from the scientists, starts around the 55 minute mark.