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NOV. 2014

An Icy Traveler Sweeps Past Mars

Comet Siding Spring passing close to the planet Mars

Caution: Comet crossing!
Hubble captures comet as it whizzes past Mars

About a million years ago, an icy comet began its journey from its home in our distant solar system, hundreds of times farther out than any planet. Now, the comet has finally reached our inner solar system neighborhood as it follows its long path around the Sun. Like the planets in our solar system, the icy comet, nicknamed Siding Spring, is bound by the Sun's gravity.

Comet Siding Spring

Comet Siding Spring
March 11, 2014

Comet Siding Spring was discovered in 2013 by astronomers at the Siding Spring Observatory in Australia. Since Comet Siding Spring's discovery, astronomers have observed it with many telescopes, including the Hubble Space Telescope. Hubble images taken in October 2013 and March 2014 show two jets of dust coming from the comet's core, or nucleus. The jets arise as the comet nears the Sun. The Sun's heat warms the comet's ice, turning it into gas that is blown off as jets.

Partial diagram of comet anatomy

Comet anatomy
Nucleus, coma, and tail explained

The comet has created plenty of excitement among astronomers because it passed very close to Mars. In fact, the comet's record-breaking flyby on Oct. 19 was only one-third of the distance between Earth and the Moon.

Collisions with solar system planets

Jupiter showing signs of a comet impact

Jupiter, 1994
Smudges show a comet's impact sites

Astronomers, however, have actually witnessed a comet striking a solar system planet. In 1994, two dozen pieces of Comet Shoemaker-Levy 9 smashed into Jupiter. The collision didn't damage the planet, but it left behind black scars in Jupiter's clouds. The Hubble telescope took images of the mushroom-shaped fireballs of hot gas sent into the planet's upper atmosphere by huge explosions as the pieces disintegrated.

Tunguska River valley impact site with flattened trees

Russia's Tunguska River valley site
Shown 20 years after cataclysmic 1908 explosion

Astronomers also have evidence of solar system objects striking Earth. More than a century ago, a comet or a meteor may have exploded in the sky above the Tunguska River valley in Russia. The explosion knocked down millions of trees over hundreds of miles. More than 65 million years ago, a larger comet may have hit Earth, leading to the demise of the dinosaurs and most of the planet's other species.

Visitors from the distant solar system

Besides Comet Siding Spring's close brush with Mars, astronomers are interested in the icy visitor because of its birthplace. Many of the famous comets are short-period comets, which orbit the Sun in less than 200 years. These comets originate in the Kuiper Belt, a broad region beyond the orbit of Neptune containing perhaps 100 million small objects made of ice and rock.

Partial diagram of Oort Cloud

The Oort Cloud
Source of long-period comets

Comet Siding Spring is one of the long-period comets, those that require more than 200 years to orbit the Sun. These comets come from the Oort Cloud, a sphere of about a trillion ancient, icy objects located hundreds to thousands of times farther than the planets' orbits. Although astronomers have never seen objects at the great distance of the Oort Cloud, they have plenty of evidence of its existence. Every year astronomers observe comets that come from the distant solar system and travel back out again.

Astronomers suggest that material in the Oort Cloud remains relatively unchanged since the birth of our solar system 4.5 billion years ago. The Oort Cloud is so far away from the Sun that its icy objects are not warmed much by the Sun's heat. The objects are frozen fossils from the early solar system.

Comet Siding Spring, therefore, may be a time capsule that gives astronomers a better understanding of our solar system's beginnings, including the birth of Earth and the other planets. Once Comet Siding Spring leaves the inner solar system, it will not come back for a couple million years.


The Star Witness

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