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On a comet’s first visit to the inner solar system, astronomers’ first task is to determine its orbit to see where it’s going, and where it’s come from.

Our solar system’s comet reservoir, the Oort Cloud, is a vast, spherical volume of space with a hollow middle in which the sun and planets reside, stretching almost halfway to the nearest star. At such distances, an orbital snail’s pace is sufficient to balance the Sun’s feeble gravity.

Oort Cloud comets reach the inner solar system after free-falling for millennia. Unless they pass a planet (or impact the sun), their outbound loss of speed mirrors their inbound gain, until they arrive at their apex at the same slow pace they began.

At any distance from the sun, there is an escape velocity below which bodies remain captive. Until recently, no solar system object had ever been known to possess a significant excess of solar escape velocity.

Asteroid ’Oumuamua, discovered in 2017 only when outbound, possessed speed in excess of escape velocity, which it must have had inbound — the first interstellar object. The second, Comet Borisov, appeared five weeks ago.

At its closest approach to the sun in early December, Borisov will peak at over 25 miles per second. At twice the earth’s distance, no body orbiting the sun has come anywhere close to having this much speed — 40% more than escape velocity.

These peripatetic projectiles mesh perfectly with the solar system’s origin story: comets formed among the giant planets, then crashed into the sun or planets or got flung to the Oort Cloud — or beyond. So, somewhere in our galaxy, aliens may currently be studying one of our escapee comets as it whizzes past their world for the first and last time.

Next column: Mercury in solar silhouette.

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Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or canderson@csi.edu.

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