For the first time ever an object that seems to have originated from outside the Solar System has been detected passing though our neighbourhood. Temporarily designated as A/2017 U1, the extrasolar asteroid (or possibly comet) was recorded after approaching from interstellar space and heading out again on a parabolic trajectory.
For all its size, the Solar System is a very busy place, where planets, moons, asteroids, comets, and assorted dust and debris whirl about in their orbits. Astronomers have been watching this cosmic ballet for millennia, but so far, all the objects seen have been children of the Sun – until now.
On October 19, University of Hawaii's Pan-STARRS 1 telescope on Haleakala, Hawaii first sighted A/2017 U1 as part of a NASA survey to identify and plot near-Earth objects. This in itself wasn't anything unusual, but when postdoctoral researcher Rob Weryk plotted the object's trajectory and confirmed it with the European Space Agency's telescope on Tenerife in the Canary Islands, something very odd was apparent.
The vast majority of objects that orbit the Sun do so in a narrow band called the ecliptic, which is defined as the plane of the Earth's orbit. Planets and asteroids may orbit at an angle to this plane, but it's usually to within a few degrees. This conformity is so strict that even the dust in the Solar System remains in this plane. In addition, all the objects in the system have elliptical orbits. That is, they cover oval paths that swing out from the Sun and then loop back again in a repeating fashion.
But A/207 U1 was different. The 400 m (1,300 ft) wide object is moving very fast at 27 miles per second (44 km/s) in a very extreme orbit, tracing in from the constellation of Lyra at an angle far out of the norm. Instead of being in line with the ecliptic, it came in like a marble dropped on a plate. In addition, instead of an ellipse, the trajectory was parabolic. That is, it was open ended, meaning that the object was making a one time visit to the Solar System
"This is the most extreme orbit I have ever seen," says Davide Farnocchia, a scientist at NASA's Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory in Pasadena, California. "It is going extremely fast and on such a trajectory that we can say with confidence that this object is on its way out of the Solar System and not coming back."
By the time A/2017 U1 was seen, it had already made its closest approach to the Sun, coming within the orbit of the planet Mercury before arcing around the Sun and heading out of the solar system toward the constellation Pegasus.
What's in a name?
One problem posed by this first time event is that astronomers have no way to officially designate the object. A/2017 U1 is the international designator based on the system for recording asteroids, but no one has agreed on how to number interstellar objects, so the moniker in the Minor Planet Center (MPC) catalog is only temporary until the International Astronomical Union can make an official decision.
"We have been waiting for this day for decades," says CNEOS Manager Paul Chodas. "It's long been theorized that such objects exist – asteroids or comets moving around between the stars and occasionally passing through our solar system – but this is the first such detection. So far, everything indicates this is likely an interstellar object, but more data would help to confirm it."
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