On Sunday, October 19 around 2:30 P.M. Eastern time about a dozen of those interplanetary robots and hundreds of Earthbound scientists will be watching as that kilometer-wide hunk of dusty ice approaches the end of its sunward plunge. Nicknamed “Siding Spring” after the Australian observatory where the astronomer Robert McNaught first sighted it in January 2013, the comet will pass Mars at a distance of 138,000 kilometers—only about a third of that between Earth and the moon—at 56 kilometers per second. About a week after Siding Spring’s close encounter with the Red Planet, the comet will be outward bound, scheduled to return again only after at least another million years.
“Normally we go to the comets to observe them but this time the comet is coming to us,” says Kelly Fast, a program scientist in NASA’s Planetary Science Division. Siding Spring’s high velocity and high-inclination orbit, far out of the ecliptic plane containing the planets, places it well out of reach of even our most powerful rockets. Our ability to see it up close is a matter of sheer luck, and scientists have taken full advantage of the opportunity, spending more than a year exquisitely choreographing their observations.
Even using a relatively large consumer telescope, the view from Earth will be lackluster. But in outer space the Hubble, Spitzer, Chandra, Kepler and Swift telescopes will all have excellent views. Each will turn its gaze toward Mars to remotely monitor Siding Spring’s encounter, investigating the comet’s composition and activity as well as its interactions with Mars and the solar wind. With luck, Hubble might see Martian aurorae as ionized particles perturb the planet’s weak magnetic field.
Around Mars three NASA orbiters as well as ones from Europe and India, will gain much closer views, capturing gorgeous wide-angle mosaics of the comet and much more. NASA’s MAVEN orbiter, freshly arrived only weeks ago, will watch as gas venting off the comet slams into Martian airspace, potentially heating the planet’s rarefied exosphere and ionosphere by 30 degrees Celsius. Watching how the Martian atmosphere responds to the shock will help MAVEN researchers calibrate their future observations, which seek to reveal how and why Mars lost most of its atmosphere and surface water eons ago.
From the Martian surface, the Curiosity and Opportunity rovers will attempt to observe the comet stretching wide across the heavens, although its closest approach will occur during daylight hours for both rovers. They might see a sudden burst of meteors sparkling in the sky—dust from Siding Spring’s tail burning in the Martian atmosphere—and snap the first images of a comet from the surface of another world.
Its chance encounter with the Red Planet isn’t the only reason Siding Spring is special. Humans have seen and virtually visited several comets before. In November the European Space Agency’s Philae probe will even land on one. But all the comets we’ve closely studied circle the sun in orbits of only tens or hundreds of years, slowly boiling away beneath intense sunlight. Siding Spring is our first close-up glimpse of something different. It comes to us directly from the Oort Cloud, a giant, spherical swarm of comets assembled early in our solar system’s life when Jupiter and the other giant planets flung leftovers of planet formation into the outer dark. Like all the comets of the Oort Cloud, Siding Spring is a pristine relic from that bygone time, carrying otherwise-inaccessible secrets from the infancy of our solar system. “This will be our first chance to image and closely study the nucleus of an Oort Cloud comet,” says Richard Zurek, chief scientist for NASA’s Mars Exploration Program at the Jet Propulsion Laboratory (JPL). “We want to know what the core is made of, what it looks like, and how it’s behaving.”
Although it will only appear as seven or eight pixels in each image, Zurek and his colleagues hope to snap several pictures of Siding Spring’s nucleus using HiRISE—a high-resolution camera onboard the Mars Reconnaissance Orbiter—and assemble them later to make a short, crowd-pleasing movie.
Most short-period comets have surfaces black as coal, perhaps due to baking near the sun. But if Siding Spring has a dark surface, too, it would suggest the phenomenon is due to something besides solar cooking. Discerning the composition of its nucleus and its gassy shroud, known as the coma, could also help pin down where exactly the comet first formed and whether its icy siblings helped deliver water to Earth and the other rocky inner planets long ago. Other insights could emerge from close examination of Siding Spring’s rotation, surface activity and interaction with Mars’s upper atmosphere.
The comet’s gassy coma now spans more than 19,000 kilometers—big enough to envelop the Red Planet for several hours during its passage—but poses no significant risks to the spacecraft there. The comet’s dusty tail is another story. After careful studies from three separate groups selected by NASA, the consensus is that the very edge of Siding Spring’s tail will sweep through the orbiters’ paths, a glancing blow that will pepper the surrounding space with sparse showers of millimeter-size dust grains.
“At 56 kilometers per second even a small dust particle can do serious damage to a spacecraft,” says Chad Edwards, chief telecommunications engineer for NASA’s Mars Exploration Program. The rovers will be safe on the ground beneath the atmosphere but every Martian orbiter will be exposed. To minimize the risk, NASA and the European and Indian space agencies have phased the orbits of each spacecraft so that they will hide on the opposite side of the planet when the dust shower is at its peak. About an hour and a half after the comet’s closest approach the orbiters will fly through what remains of the dust trail.
Back on Earth at JPL, Edwards and his colleagues will be sitting uneasily in front of their computer screens monitoring each orbiter through NASA’s worldwide Deep Space Network of radio telescopes, trying not to hold their breath. Even one orbiter’s malfunction could pose a dire threat to the entire Mars Exploration Program. “Most of these orbiters do communications as well as science,” Edwards says. “Essentially every bit of data from our two rovers comes through the orbiters’ relay radios. No one wants to lose that capability…. Once we come out from around the planet and complete another orbit, people will be breathing easier.”
The risk, however, should be worth the reward of witnessing this once-in-many-lifetimes cosmic coincidence as it unfolds. “If this comet was coming to the Earth, everyone would drop everything, and we’d all be observing it night and day,” says Carey Lisse, a senior research scientist at Johns Hopkins University and chairman of the worldwide campaign to monitor Siding Spring. “It’s amazing to think that this thing with a multimillion-year orbit got kicked down here at the dawn of humanity and we’re only able to investigate it right now, thanks to our recently launched space telescopes and our Martian orbiters and rovers. You don’t usually realize when you’re in the midst of history being made, but here it’s unmistakable.”