On Oct. 19, a visitor from outer space was discovered by the PAN-STARRS telescope in Hawaii. No, it wasn’t a spaceship full of alien creatures. The visitor was a space rock from outside our solar system.
Many comets and asteroids that we see come from the same materials that formed the sun and the planets. We know this because some fall to Earth as meteorites and can be taken to the lab, where the chemical composition is determined.
Almost all of the comets move toward the sun in the same plane as the planets orbit. If the comets came from outside the solar system, you might expect that they would come in from all angles, some from above the planetary plane and some from below.
Because many comets move nearly parallel with the planetary plane, we assume that these rocks are part of the stuff that made our solar system.
The space rock just discovered, first named A/2017 U1 (appropriate for an asteroid) and then renamed I1 (for interstellar object No. 1) came in from above the planetary plane, dipped below as it orbited the sun, flew past Earth and is exiting above the plane. This trajectory is almost perpendicular to the planetary plane.
Having a weird trajectory is not enough to prove it’s from outer space. The real zinger is that it’s traveling at high speed — so fast that its orbit takes it straight out of the solar system. At its highest speed, it was going nearly 200,000 mph.
In contrast, all comets and asteroids have elliptical orbits, which take them repeatedly around the sun. Some comets, like the famed Halley’s comet, return at regular intervals.
It takes 75 years for Halley’s comet to complete each orbit, but the space rock I1 will never come back. Like a rocket with enough velocity to escape Earth’s pull, I1 has enough velocity to escape the sun’s pull.
So where did this space rock come from? Presumably, it was ejected from another solar system eons ago and traveled across interstellar space until it was pulled in by the sun.
This is what makes it so interesting. Is the chemical composition of other planetary systems similar to that of our solar system? If we could zoom out to collect a sample of this rock, we could see whether it’s like our asteroids or something completely different.
Another way of finding out about its composition is to look at the light scattering off it. Unfortunately, it is now far enough away to be too dim. All that we can tell is that it has a reddish hue.
Fortunately or not, the rate of extra-solar rocks visiting our solar system is rare. In fact, this is the first one ever discovered, hence its designation as I1.
PAN-STARRS (the Panoramic Survey Telescope and Rapid Response System) was built by a multinational consortium with the major goal of discovering and characterizing objects approaching Earth. If a massive asteroid were headed toward Earth, this telescope should give an early warning.
One of the reasons that PAN-STARRS didn’t see I1 earlier is that it came in from the other side of the sun at such a high speed. Luckily, I1 wasn’t going to hit Earth.
The chances that a random space rock of sufficient size would come from outer space and hit Earth are astronomically small. But with PAN-STARRS and other telescopes looking out for us, we’ll know if an asteroid is on a collision course with Earth.
And just maybe we’ll get another opportunity to grab a chunk of rock from the next visitor from beyond our solar system.