Every comet we’ve ever known has originated within our solar system either in the distant realm beyond Neptune called the Kuiper Belt or even further, in the great comet repository of the Oort Cloud. Astronomers have never seen an interstellar comet or asteroid, one arriving here after being kicked out by another star.
That may be about to change. An object originally called C/2017 U1, picked up last week by the PanSTARRS 1 survey telescope from atop Mt. Haleakala on Maui, appears for the moment like an escapee from another star system. It entered the solar system at a zippy 16 miles a second (26 km/sec), passed 23.2 million miles from the sun on Sept. 9th and then raced just 18.3 million miles from Earth on Oct. 18. Originally thought to be a comet, it’s now more than likely an asteroid, the reason its name was recently changed to A/2017 U1.
What makes the comet/asteroid unique is its orbit. Most comets circle the sun on elliptical orbits — “squashed circles” that resemble fat cigars. Some elliptical orbits are so stretched out they’re nearly open-ended or parabolic. But this comet is neither. Instead, A/2017 U1 is moving so fast that it will escape the gravitational pull of the sun altogether. Even now, it’s ripping along on a hyperbolic orbit straight out of the solar system never to return.
What’s even more intriguing is that the interstellar object came our way from the direction of the bright star Vega that’s part of the Summer Triangle and just 25 light years from Earth. Just 6° southwest of Vega in the constellation Hercules lies the solar apex, the direction in which the sun, with solar system in tow, is traveling at a speed of 12 miles per second (20 km/sec). This heading would be the most logical from which to expect an alien asteroid or comet to approach us since we’re already barreling that way.
Astronomers around the world are busy making observations to refine the object’s orbit to confirm it’s truly interstellar. They’re also dissecting its light with spectrographs to determine its composition. Neither will be easy as the object is small, faint to begin with and rapidly fading. When discovered, it was only magnitude 20 and will dim to 24 by late November.
But what a rare opportunity to study an alien asteroid or whatever it is up close. Even an sun-bound astronomer gets a bone once in a while!