It was first seen Oct. 19, when a postdoctoral researcher at the University of Hawaii Institute for Astronomy, Rob Weryk, witnessed a small bright object streaking across the sky.
Weryk looked through the archives for the Pan-STARRS telescope, which conducts nightly sky surveys in search of celestial objects moving through the space near Earth, and found the mysterious body in images as far back as early September. The space rock followed a path like nothing he'd ever seen. Instead of circling around the “ecliptic” — the plane on which planets, asteroids, comets and other solar system objects orbit the sun — this new thing approached from above. It seemed to be coming from the direction of the constellation Lyra and had been cruising through the chilly void of interstellar space at nearly 16 miles a second.
On Sept. 2, it crossed the ecliptic plane inside Mercury's orbit. A week later, it made its closest approach to the sun. Tugged by the sun's gravity, it reversed course and hurtled back above the ecliptic at an angle, passing about 15 million miles from Earth on Oct. 14. It is now headed for the constellation Pegasus.
“This is the most extreme orbit I have ever seen,” Davide Farnocchia, a scientist at NASA's Center for Near-Earth Object Studies at the Jet Propulsion Laboratory, said in a statement. “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.”
Because no interstellar asteroid has ever been seen before, the International Astronomical Union has no rules for naming the new object. For now, it's been provisionally dubbed “A/2017 U1,” though we're partial to this moniker:
Scientists have suspected for a while that a visit from “exoroids” could be possible. So much material is flung about during the chaotic process of forming planets that it's likely some bits and pieces might escape and make their way to other solar systems.
What's most surprising is that we've never seen any, said Karen Meech, an astronomer at Hawaii's IfA who specializes in small bodies. At least, not one of any significant size; NASA's Stardust spacecraft has collected some dust particles with suspected interstellar origins.
Astronomers aren't entirely sure what this object is. They initially called it a comet, but after failing to spot its coma — the cloud of gas and dust that surrounds a comet's core — they revised their designation. It's an asteroid, they declared Thursday.
But they're fairly certain that the rock doesn't come from our solar system. Their biggest clue is its hyperbolic orbit: rather than endlessly circling the sun in an ellipse, the object's path extends into the unknown far beyond our solar system. Astronomers have seen objects with open-ended orbits like this before, but they were always nudged onto a hyperbolic path by outgassing (the warming and release of gasses as an object flies close to the sun, like air slowly slipping from a balloon) or gravitational interactions with planets. Neither seems to be the case for A/2017 U1.
Now, astronomers around the world are rushing to get a good look at the asteroid before it vanishes into the black. They will try to determine its exact size, shape and spin rate, as well as analyze the colors of light emitted and absorbed by the object to determine its composition.
“We have been waiting for this day for decades,” said Paul Chodas, manager of NASA's Center for Near Earth Object Studies. “It's long been theorized that such objects exist . . . 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.”