On October 26th, 2017 we learned about

For the first time, astronomers spot an interstellar asteroid flying through our solar system

Our solar system had a guest this week. A small object, most likely an asteroid, was detected zipping past the Sun, dipping below the orbital plane of our local planets and asteroids, then being rocketed up past the Earth at over 97,000 miles-per-hour. It’s exact path is still being pinned down, but at this point it looks like A/2017 U1 is the first object to be observed passing by our Sun while originating somewhere outside our solar system.

Sharp turn just past the Sun

A/2017 U1 was first spotted by the University of Hawaii’s Pan-STARRS 1 telescope on October 19th. It was actually photographed the night before as well, but it wasn’t recognized as something special until the following day when it’s unusual trajectory was detected and analyzed. Rather than following the orbital plane that most of the objects in our solar system move along, this new object was seen diving almost directly at the Sun, and it was doing so very quickly, at around 56,000 miles-per-hour. This high speed is probably what allowed it to slip past the Sun at inside Mercury’s orbit without getting pulled in the Sun, or being vaporized by its heat.

The Sun’s gravity did get a hold of the small visitor, which is estimated to only be around 525 feet in diameter. Even with all the momentum it had to get past the Sun, that small mass was whipped around in a wide, hyperbolic orbit. This means that rather than continue straight through our solar system, the asteroid went “beneath” the planets’ orbital plane then curved around to fly back “up” between Earth and Mars. The hyperbolic shape of this path is open enough that A/2017 U1 isn’t expected to ever return, basically being accelerated in a gravity assist from the Sun without getting caught in a permanent orbit.

First observed object without an obvious orbit

Obviously, we’re not going to get a ton of data about this small rock that, as of this writing, is already over 31 million miles away from Earth. Still, even during its quick visit, we were able to observe it enough to determine that A/2017 U1 is most likely an asteroid, and not a comet as originally assumed. Beyond its physical characteristics, simply spotting this object is a big step for astronomers. With the mind-boggling number of objects in the universe, it’s long been assumed that there must still be some objects that weren’t caught in orbit around larger planets or stars, traveling between solar systems with no real point of origin to speak of. Scientists have observed at least one comet that was in the process of exiting our solar system thanks to a push from Jupiter’s gravity, but that comet may have been home grown. A/2017 U1 finally provides prove that this untethered objects are out there.

Source: Astronomers Spot First-Known Interstellar “Comet” by Kelly Beatty, Sky & Telescope

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