October 17, 2014

Numerous rovers and orbiting probes will break from their usual observations of Mars on Sunday to watch a comet flyby described by NASA officials as a once-in-a-million-year event. Comet Siding Spring–named for the Australian observatory where it was discovered in 2013–will pass within about 87,000 miles (139,500 kilometers) of the red planet. That’s less than half the distance between Earth and the moon. Siding Spring will zip by Mars at a speed of about 126,000 miles (56 kilometers) per hour at about 2:27 p.m., EDT.

Why are astronomers so excited?  For one, Siding Spring will be the first comet from the far-distant Oort cloud to be studied close up by spacecraft. For another, the comet is making its first-ever voyage through the inner solar system. That means it has never passed by sun and so has never been altered by the sun’s powerful heat and radiation. In other words, the comet looks much the same as it did 4.6 billion years ago, shortly after the formation of the solar system. “This is a cosmic science gift that could potentially keep on giving,” said John Grunsfeld of NASA’s Science Mission Directorate. “This particular comet … will provide a fresh source of clues to our solar system’s earliest days.”

Siding Spring, which has a nucleus (core) from 0.5 to 5 miles (0.8 to 8 kilometers) wide, is a well-traveled comet. Scientists believe it formed somewhere between the orbits of Jupiter and Neptune as the solar system was being born. At that time, many similar objects there were colliding and coming together to form the outer planets (Saturn, Jupiter, Uranus and Neptune). Siding Spring, however, apparently was caught by the gravitation pull of one of these planets and thrown out into the Oort cloud at the outer reaches of the solar system. For billions of years, the comet visited only the outer planets on its trips into the solar system. Then about one million years ago, it was jolted from its orbit–probably by a star passing by the Oort cloud–and started its voyage to the inner solar system.

When imaged by the Hubble Space Telescope on March 11, 2014, Comet Siding Spring was 353,000 miles (568,000 kilometers) from Earth. The coma (dust cloud) surrounding the comet’s nucleus (core) is about 12,000 miles (19,000 kilometers) wide. The comet will take about 1 million years to complete one orbit of the sun. (NASA, ESA, and J.-Y. Li [Planetary Science Institute])

“We can’t get to an Oort Cloud comet with our current rockets. These orbits are very long and extended, at very great velocities,” said Carey Lisse, a senior astrophysicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “So this comet is coming to us. It’s a free flyby, if you will, and that’s a very fantastic event for us to study.”

Watching for the comet–and observing its effects on the Martian atmosphere–will be various Mars orbiters, including the Mars Reconnaissance Orbiter, Mars Odyssey, and Mars Atmosphere and Volatile Evolution craft. Other observers will be the Hubble, Spitzer, Kepler, and Chandra space telescope. Scientists are particularly interested in what the Mars rovers Curiosity and Opportunity make of the comet. If the Martian atmosphere isn’t too dusty, they could provide the first images of a comet from another planet.

Additional World Book articles:

• Exploring the Outer Solar System (a Special Report)
• When Worlds and Comets Collide (a Special Report)