Then, they will duck and hide.
Comet Siding Spring will pass within 132,000 km of Mars on Sunday, Oct. 19. That distance is closer than our moon is to Earth, which by cosmic standards is as good as a hit. Fortunately, we have a fleet of mechanical eyes on Mars that will have a front-row seat to the spectacle as the comet passes.
Five spacecraft in orbit around Mars — the European Mars Express, India’s Mangalyaan and Nasa’s Mars Reconnaissance Orbiter, Mars Odyssey and the recently arrived MAVEN — will turn their cameras outwards to snap images of the celestial visitor, while two rovers on the surface, Curiosity and Opportunity, will get the perspective from the ground.
Their close vantage point is a dangerous place to be as the comet makes its closest approach. As comets near the sun, ices buried within them vapourize, producing ice geysers that blow off the surface and form a long, beautiful tail stretching millions of kilometres into space, giving comets their distinctive angel-hair form. But that tail also includes dust and dirt particles that become cosmic bullets when they hit something like an orbiting spacecraft.
Traveling 56 km/sec
The tail of Comet Siding Spring will sweep right over the entire planet of Mars, traveling at 56 km/sec. To put that in perspective, a rifle bullet travels a little more than 1 km/sec. So even though the dust particles are small, they can do considerable damage at those speeds.
To protect their expensive spacecraft, flight controllers have adjusted the orbits such that when the comet tail passes by, all vehicles will be huddled on the opposite side of the planet, using the body of Mars as a shield. The two rovers on the ground will not be blasted, because they are protected by the Martian atmosphere. However, there is a chance they might see some spectacular shooting stars streaking across the sky.
This is a wonderful example of nature providing a free mission to a comet. All we need to do is point our robotic cameras in the right direction and we get a close-up view. Later in November, another European spacecraft called Rosetta, currently in orbit around a different comet, will release a small probe that will land on the comet’s surface to watch the process of gas release as the comet swings around the sun.
Comets are intriguing objects to study because they are very old, dating back to the original giant cloud of gas and dust that gave birth to the sun and all the planets. Think of them as leftover crumbs from baking a cake. Most comets reside in a gigantic cloud well out beyond Pluto, and every now and then one of them gets nudged by the gravity of a passing star and falls in towards our part of the solar system.
Comets may bring life, or death
Siding Spring is a first-time visitor to the inner part of the solar system, so the water and organic chemicals it carries are relics of a time before Earth existed. Some scientists believe comets may have brought the water that fills our oceans and even seeds of life to Earth when it was young.
While comets may have brought life, they have also brought death. This close encounter with Mars is a reminder that planets and large objects from space do occasionally meet, sometimes head-on, with calamitous results. That’s what gave the dinosaurs a bad day 66 million years ago.
After passing Mars, this comet will swing around the sun on Oct. 25, then head back to the remote outer solar system. It will not pass close to the Earth. But there are many more undiscovered comets and asteroids out there that could head our way, so it’s important that we keep a look out.