Astronomers will have a virtual front-row seat to study a pristine comet in October, when it squeaks past Mars and a flotilla of spacecraft orbiting the red planet. NASA scientists are finalizing their plans to observe the rare event.

NASA/Swift/D. Bodewits (UMD), DSS

On 19 October, Comet Siding Spring will swoop just 135,000 kilometres above the Martian surface. That’s less than half the distance between Earth and the Moon. And because the comet is on its first trip to the inner Solar System, the gas and dust that have been frozen to its surface for billions of years are finally warming up and spraying off.

“This is our first chance to see the nucleus of a long-period comet up close,” says Richard Zurek, chief scientist for the Mars programme office at the Jet Propulsion Laboratory in Pasadena, California. “Mars will be right at the edge of the debris cloud.”

Zurek leads a team that has been analysing whether dust particles flying off the comet could damage spacecraft around Mars. Three probes currently orbit the red planet (NASA’s Mars Odyssey and Mars Reconnaissance Orbiter, and the European Space Agency’s Mars Express), and two more are slated to arrive in September (NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission and India’s Mars Orbiter Mission). Cometary dust is zipping towards Mars at a relative speed of 56 kilometres per second — fast enough to ding protective shielding.

But observations this spring from the Hubble Space Telescope, the asteroid-hunting Near-Earth Object Wide-field Infrared Survey Explorer spacecraft, and other telescopes show that the comet is not spewing out quite as much debris as astronomers had feared, says Zurek. New modelling studies suggest that the most dangerous period will come about 1.5 hours after the comet’s closest approach to Mars, when the planet whizzes within just 27,600 kilometres of the comet’s path. During the most crucial half-hour when the comet dust comes fast and furious, all the orbiters will hunker down on the other side of Mars.

MAVEN, which was built to study the Martian atmosphere, is planning to take science data two days before and two days after the comet’s closest approach. Its ultraviolet spectrometer will take images and spectra of the comet, and other instruments will monitor any changes in the upper atmosphere before and after the comet hits. It’s possible that the comet may dump enough hydrogen into the atmosphere to be seen, says mission leader Bruce Jakosky, a planetary scientist at the University of Colorado in Boulder.

“It’s going to be a spectacular data set,” he says. Jakosky spoke to Nature during a Mars conference this week in Pasadena, California.

As seen from the Martian surface, the comet’s dust cloud will cover a huge amount of sky. The Opportunity and Curiosity rovers will attempt to take pictures of it, but it will be daytime from the rover perspective. They may have a shot at seeing meteors the night before or after the comet passes, Zurek says.

Discovered in January 2013 by astronomers in Australia, Comet Siding Spring is on its first trip to the inner Solar System. More than a million years ago, gravitational interactions probably kicked it out of the frigid cometary reservoir at the edge of the Solar System known as the Oort cloud. It has been travelling towards the Sun since; after this pass it won’t return for about 1 million years.