"Green" comet makes closest approach to Earth

Originating from the Oort Cloud, comet is on its first trip to the inner solar system.
By Laurel Kornfeld | Dec 19, 2018
A green-colored comet is making its closest approach to Earth on Tuesday, August 7, as it travels on its first ever journey through the inner solar system.

Discovered with the PanSTARRS telescopes in Haleakala, Hawaii, on September 23, 2017, and designated C/2017 S3, the comet has undergone two recent outbursts, one on June 30, and the other two weeks later.

Many comets experience outbursts in which they brighten. For this unusually large comet, which came from the Oort Cloud, a reservoir of comets in the outer solar system beyond the Kuiper Belt, these outbursts give it a greenish tinge. Its second eruption produced a large gas cloud surrounding the comet that swelled to 161,000 miles (260,000 km) in diameter.

C2017 S3's green color is the result of the ionization of its carbon and cyanide molecules due to warming from the Sun. Ionization involves the separation of electrons and protons, which produces the trademark green glow, as explained by Brian Koberlein of the Rochester Institute of Technology (RIT) in New York.

At closest approach, the comet will come within 70 million miles (112 million km) of the Earth, then head toward the Sun. It will swing around the Sun on August 16 and then head back to the distant Oort Cloud.

While C2017/S3 is currently too close to the Sun to be visible, observers will get the chance to see it after it begins heading back toward the outer solar system later this month, noted Paul Chodas, manager of the Jet Propulsion Laboratory's (JPL) Center for Near-Earth Object Studies (CNEOS).

Regular outbursts by comets were long thought to be caused by surface heating and pressure buildup that produce explosions on their surfaces as they head toward the Sun. However, this theory was called into question by the Rosetta spacecraft's close observations of Comet 67P/Churyumov-Gerasimenko, whose data pointed to dust kicked up and out into space by landslides on the comet's surface as the outbursts' cause.

The comet's close approach involves no threat to Earth.

A 3D interactive visualization of the comet produced by JPL is available for public viewing.

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