A2017 U1 Path
A/2017 U1 is most likely of interstellar origin. Approaching from above, it was closest to the sun Sept. 9. Traveling at 27 miles per second (44 kilometers per second), the comet is headed away from the Earth and sun on its way out of the solar system. NASA/JPL-Caltech

It is smaller than a quarter of a mile (400 meters) in diameter, but A/2017 U1 is perhaps the first known object from outside the solar system that has made its way inside. Some more data-crunching and analysis is still needed to confirm the interstellar nature of the object, which could be a comet or an asteroid.

A/2017 U1 made its closest approach to the sun Sept. 9 but was discovered Oct. 19 by Rob Weryk, a postdoctoral researcher at the University of Hawaii Institute for Astronomy (IfA), using the Pan-STARRS 1 telescope on Haleakala, Hawaii. It had appeared in the images taken the previous night as well, but had not been identified as a near-Earth object by the telescope’s moving object processing software.

The strange orbit of the object made Weryk realize it was very unusual, and after combining data from observations taken at the European Space Agency’s telescope on Tenerife in the Canary Islands, he understood A/2017 U1 for what it was.

"Its motion could not be explained using either a normal solar system asteroid or comet orbit. This object came from outside our solar system," Weryk said in a statement Thursday.

NASA’s Center for Near-Earth Object Studies (CNEOS) calculated the object’s current orbit and also projected which way it was headed. From the direction of the constellation Lyra, it approached almost perpendicular to the ecliptic — an approximate plane of the solar system in which the planets and most asteroids orbit the sun — thereby avoiding any encounters with the eight planets during its inward journey.

The object crossed the ecliptic between the sun and Mercury on Sept. 2, and under the influence of gravity during its close approach to the sun, changed its direction drastically. Making a hairpin turn under the ecliptic, it passed below Earth at a distance of about 14 million miles (24 million kilometers) Oct. 14 and has now moved back over the ecliptic. It is headed in the direction of the constellation Pegasus.

NASA also designed an animation to show the path of A/2017 U1.


When A/2017 U1 approached the solar system, it was moving at the speed of 15.8 miles a second, and as if that wasn’t fast enough already, it is now traveling at 27 miles per second, likely a result of the slingshot effect of the sun’s gravity.

"This is the most extreme orbit I have ever seen. 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," Davide Farnocchia, a scientist at CNEOS, said in the statement.

But the object cannot be branded interstellar until further observations confirm it to be such.

A notice by the Minor Planet Center on Wednesday which provided astrometry data from the observations, said: "Further observations of this object are very much desired. Unless there are serious problems with much of the astrometry listed below, strongly hyperbolic orbits are the only viable solutions. … If further observations confirm the unusual nature of this orbit, this object may be the first clear case of an interstellar comet."

And while we still don’t know if A/2017 U1 is an asteroid or a comet, it is not entirely surprising that an object from beyond the solar system should have entered it.

"We have long suspected that these objects should exist, because during the process of planet formation a lot of material should be ejected from planetary systems. What’s most surprising is that we’ve never seen interstellar objects pass through before," Karen Meech, an astronomer at the IfA specializing in small bodies and their connection to solar system formation, said in the statement.