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SYFY WIRE Bad Astronomy

An asteroid is about to embark on a very long voyage to interstellar space

By Phil Plait
Artwork of a distant asteroid on its way in toward the inner solar system. Credit: NASA/JPL-Caltech

A tweet by my colleague Ron Baalke alerted me to some interesting news. Astronomers have just announced the discovery of an asteroid that's highly unusual: It's on a hyperbolic orbit.

That’s a technical term for the shape of the orbit, which I’ll explain in a sec, but in more general terms it means the asteroid is not gravitationally bound to the Sun. Once it swings around and heads back out, it ain’t coming back.

It'll be an interstellar asteroid.

If that sounds familiar, it may be because of the recent news about 'Oumuamua, a rock that we know came from deep interstellar space, literally from another star. However, this newly discovered rock — called A/2017 U7 — is almost certainly an original member of our own solar system. How can that be?

Position of the asteroid A/2017 U7 in March 2018. Credit: NASA/JPL-Caltech

It likely started out life as a member of the Oort Cloud, a roughly spherical volume of space around the Sun that starts around 400 billion kilometers out (100 times farther out than Neptune!) to a staggering distance of a light-year or so: 10 trillion kilometers! There may be trillions of objects out there, some of them hundreds of kilometers or more across, composed of rock and ice, left over from the formation of the solar system.

Sometimes, something disturbs one of these objects (perhaps a star passes us nearby, or it suffers a collision with another Oort Cloud object), and it drops down into the inner solar system. Sometimes when it does we see a spectacular comet, the ice on the surface turning into a gas as the Sun warms it and blowing off to form the tail. That usually happens when the object gets around the distance of Jupiter or so, though under some circumstances it can happen farther out or closer in (depending mostly on the surface composition of the object being able to protect it from sunlight).

As of right now, U7 (for short) isn't showing any activity like that, so it's classified as an asteroid (hence the A in its name). If it does start to display gaseous emission, it'll be reclassified and the A will become a C. At the moment, U7 is 1.2 billion km from Earth and 1.1 billion from the Sun. That puts it outside the orbit of Jupiter, and it won't get any closer to the Sun than Jupiter is, either.

The size of U7 is hard to know precisely, but by its brightness and colors it looks like it's pretty big, at 25–50 km wide. Happily, it doesn't get anywhere near Earth; it stays well outside Earth’s orbit during this apparition, so there’s no danger of a collision.

Nor will there ever be. Like I said, this is it. It'll swing around the Sun in September 2019 and head back out into deep space.


So what’s the deal? How can it start off in the solar system and then leave us forever?

The short answer is Jupiter. This object is falling from a long way off, and given no outside influence other than the Sun, it’d probably head back out to some maximum distance of a trillion kilometers or so, then the cycle would start again. Instead, though, as it gets nearer the Sun there are other influences. Jupiter is the biggest, literally: It has enough gravity that it can give U7 a little bit more speed, a little bit more orbital energy. It doesn’t take much to give it the kick it needs to leave the solar system forever.

And it looks like Jupiter will be doing exactly this; looking at the orbital numbers, U7 was bound to the Sun before it dropped down, but on its way out the orbit changes, and it becomes unbound, free to roam the galaxy.

In slightly more technical terms, orbits can be closed (bound to the Sun, so they orbit around it forever) or open (unbound, so they head out into interstellar space). Closed orbits can be circular or elliptical. All planets have elliptical orbits, most of which are nearly circular. Asteroids in the main belt do too. We say these orbits have "negative total energy," meaning they don’t have enough energy to escape the Sun.

Open orbits are hyperbolic, literally shaped like a mathematical hyperbola. These have positive total energy, meaning they have more than enough to escape the Sun’s grasp.

There’s another kind, too, that balances on the cusp of open and closed. These are parabolic orbits, and they have zero net energy. If you have an object infinitely far away from the Sun, and let it drop in, it will have a parabolic orbit. It’s more of an idealization than anything else.

However, something dropping in from the Oort Cloud is so far away its orbit is very close to parabolic. It can be so close that measurements of the orbit make it hard to tell if it’s on a long elliptical orbit or a hyperbolic one. U7 has a just barely hyperbolic orbit, which makes it extremely likely it started off in the Oort Cloud and dropped down from there.

That’s why this is different than 'Oumuamua. That object came in to the solar system so fast it was on a very hyperbolic orbit. It had a lot of excess speed, far more than just something falling in. It clearly got kicked out of some other star system and came from true interstellar space.

Another way to think of this is using eccentricity, a mathematical term that denotes the deviation of an orbit from a circle. A circle has an eccentricity of 0. A moderate ellipse might have an eccentricity of 0.1 or 0.2. The longer the ellipse, the higher the eccentricity, up until it becomes a parabola, which has an eccentricity equal to 1.

If you give the object more energy, more speed, it’ll go hyperbolic. The eccentricity will be greater than 1.0. U7 has an eccentricity of a hair more than 1; hyperbolic, but just barely. 'Oumuamua has an eccentricity of 1.2, much higher. It’s clearly not from around here.

By the way, only three hyperbolic asteroids are known: 'Oumuamua, U7, and another recently discovered called A/2018 C2. That's assuming these are asteroids; just because no activity is seen doesn't mean there isn't any. It could be faint, or it could switch on later. C2 gets closer to the Sun than U7 and may yet become active.

Lots of hyperbolic comets are known, mostly because objects from the Oort Cloud are far more likely to be cometary (with ice) than asteroidal (rock and metal). That's why we're still waiting to see if U7 and C2 show any activity. It's likely they will given all this, but not certain.

Position of the asteroid A/2018 C2 in March 2018. Credit: NASA/JPL-Caltech

You may be wondering why I focused so much on U7 when C2 is also hyperbolic. That's because it depends on what frame of reference you're talking about here. It's like watching a car go by when you're standing on the side of the road versus the same thing when you're headed toward it in another car. In the first scenario the car passes you at, say, 30 kilometers per hour; in the second it's 60.

In this case it depends what we call the center of the solar system. You can say the Sun is, or you can use the solar system's barycenter; its center of mass. That's different than the center of the Sun because Jupiter is so massive it pulls the center of mass away from the Sun's center.

U7 is surely unbound, and is on its way to going bye-bye. C2, though, is only hyperbolic relative to the Sun's center. Relative to the barycenter it's elliptical (though a verrrrryyyyyy long ellipse), and that's the one that counts here. In the end, C2 doesn't have the energy it needs to escape the gravity of the solar system. It'll be back ... but not for a long, long time.

I know, this is complicated! I actually got an interesting lesson about all this chatting with amateur astronomer Daniel Bamberger on Twitter. I never took an actual orbital mechanics class in grad school (it wasn't offered), but now I'm thinking of grabbing a book. It's complicated, sure, but it's also fascinating!

As a final note, it’s natural to wonder how often something like this happens; how often Jupiter bullies an object so much it leaves the solar system. The answer is: probably all the time! U7 is big and a relatively rare object, but it's likely many more smaller objects come in and are tossed out by Jupiter that we don't see. Jupiter is massive, and it’s no surprise it has a huge effect on these inbound objects*.

The more of these we find, the better we’ll understand the dynamics of the outer solar system. Oort Cloud objects are extremely distant, and extremely hard to observe. Having them occasionally drop by like this is actually pretty convenient, allowing us to study a part of the solar system that otherwise we know very little about. And when we search for them we’ll find more true interstellar visitors like 'Oumuamua, and who knows what other weird beasties.

If we knew what was out there, we wouldn’t call it discovery. I hope we discover a lot more.

* Incidentally, Jupiter’s reputation as an interplanetary defense mechanism — saving the inner planets from impacts by tossing out such debris — is not entirely true. A study has shown it tends to throw things toward us, too!