Earth Has A Companion Asteroid With a Weird Orbit


There are plenty of near-Earth asteroids out there, but this latest one studied by two researchers at Armagh Observatory in Northern Ireland is extremely rare in that it has a weird, horseshoe-shaped orbit. Not that Asteroid 2010 SO16 does an about-face and turns around in mid-orbit — no, the asteroid always orbits the Sun in the same direction. But because of its unique orbital path and the gravitational effects from both the Earth and the Sun, it goes through a cycle of catching up with the Earth and falling behind, so that from our perspective here on Earth, its movement relative to both the Sun and the Earth traces a shape like the outline of a horseshoe: it appears to approach, then shift orbit, and go farther away without ever passing Earth.

This asteroid was discovered on September 17, 2010 by the WISE Earth-orbiting observatory.

There are only a handful of other asteroids known to have a horseshoe orbit. But astronomers Apostolos Christou and David Asher say 2010 SO16’s absolute magnitude (H=20.7) makes this the largest object of its type known to-date. It is just a few hundred meters across, so the other asteroids are extremely small, and none of the other horseshoe asteroids have orbits that are likely to survive for more than a few thousand years. But the researchers did computer simulations of SO16’s orbit, which showed it could stay in its orbit for at least 120,000 years, maybe more.

For an asteroid to have such an orbit means it is in almost the same solar orbit as Earth, and both take approximately one year to orbit the Sun.

The Technology Review Blog explained it this way:

“Two points are worth bearing in mind. First, objects further from the Sun than Earth, orbit more slowly. Second, objects that are closer to the Sun orbit more quickly than Earth.

So imagine an asteroid with an orbit around the Sun that is just a little bit smaller than Earth’s. Because it is orbiting more quickly, this asteroid will gradually catch up with Earth.

When it approaches Earth, the larger planet’s gravity will tend to pull the asteroid towards it and away from the Sun. This makes the asteroid orbit more slowly and if the asteroid ends up in a orbit that is slightly bigger than Earth’s, it will orbit the Sun more slowly than Earth and fall behind.

After that, the Earth will catch up with the slower asteroid in the bigger orbit, pulling it back into the small faster orbit and process begins again.

So from the point of view of the Earth, the asteroid has a horseshoe-shaped orbit, constantly moving towards and away from the Earth without ever passing it. (However, from the asteroid’s point of view, it orbits the Sun continuously in the same direction, sometimes more quickly in smaller orbits and sometimes more slowly in bigger orbits.)”

Right now, SO16 is near one of its closest points of approach, chasing the Earth on its inside orbit. It will be tagging along near Earth for the next few decades until it is pulled all the way over into the outside orbit and it slowly recedes from view.

The researchers say the existence of this long-lived horseshoe raises the twin questions of its origin and whether objects in similar orbits are yet to be found. Additionally, they suggest that SO16 may be a suitable test target for the direct detection of the Yarkovsky acceleration as it makes frequent close encounters with the Earth during the next decade.

Paper: “A long-lived horseshoe companion to the Earth”

Sources: Technology Review Blog, Wikipedia

12 Replies to “Earth Has A Companion Asteroid With a Weird Orbit”

  1. That’s pretty nifty. Allow me to be the first person to make the leap from cool little asteroid with an interesting orbit straight to…

    It’s a remnant of Theia! IT’S THEIA! DOOOOOOOOOOOOOOM!

    I reserve the right to change my tone from sarcastic to petrified if someone finds an ancient Mayan temple on 2010 SO16. In the interim, I’ll stick with that’s pretty nifty.

    1. Or maybe it’s an impact escape mass that barely made it and then got caught by L5 (as per below).

    2. Though then its more likely a Moon escaper, come to think about it,

  2. This is great to know, of course, but it’s not too different from 1685 Toro, which is in an 8:5 resonance with Earth AND a 13:5 resonance with Venus. (The Earth is in its own 8:5 resonance with Venus.)

    There are SO many objects in the Solar System and it’s SO old, I’m beginning to think that if we look closely enough, we’ll find objects in practically any gravitationally sheltered niche, just like we find life in every possible niche on Earth.

  3. The graphic is quite misleading – it is the orbit as seen from Earth but in this case you have to draw the Sun orbiting the earth as well. What actually happens is far better shown by the animation on the NASA Orbit Diagrams page at which shows very well how it catches up with Earth and then is deflected into an outer and initially slower orbit which then crossess Earth’s orboit to catch up again.

    1. @ John – I agree with you. If we try to drow it’s orbit ‘as seen from Earth’ as per JPL’s diagram, the orbit need to be drown around L5 point.

    2. The picture is not misleading! maybe the explanation is. Where it’s said: “its movement relative to both the Sun and the Earth” it was better to say: “its movement relative to a surface rotating with the earth around the Sun”. 🙂
      actually the Sun’s movement is just rotating around it’s own center(not around the Earth)

  4. If I read things correctly, the orbit is not the light blue horseshoe shape. This represents the limiting closed orbit. In fact, you are much more likely to have one of the bean-shaped orbits around L4 or L5, which to me looks closer to what I see on the NASA Orbit Diagrams page, rather one that loops past L3 and back again.

    Have a look at Same diagram.

  5. The orbit shape does occur only in the reference frame of the Earth’s orbit. It is a manifestation of a third body problem, and the orbit is in an accelerated reference frame. The loop, which is this distended horseshoe shape, has no central gravitational source inside the loop. As a result the orbit is a “pseudo-orbit.”

    From the perspective of an inertial frame in heliocentric coordinates this asteroid is in a circular orbit (topologically a circle) around the sun. When the orbit is closer to the sun than the Earth’s orbit the asteroid has a smaller orbital period, or equivalently it has a larger velocity. It will eventually catch up with the Earth, but it is not necessarily gravitationally drawn into the Earth. It interacts with the Earth’s gravity field in its frame with an effective and repulsive potential L^2/2mr, L = angular momentum r = distance from Earth, which pushes the asteroid into a higher orbital radius. Its orbital velocity is now smaller and recedes away from the Earth. Eventually the Earth approaches the asteroid and the process is repeated.

    This is a “hunter-chaser” type of orbit. The thinner the horseshoe is the small there angular momentum L is with respect to the Earth at close approach. This means the gravitational attraction can become larger. There is then a prospect for impact. These orbits are not stable over an indefinite time period, as is the case with general three body problems. As a result there is a probability this asteroid will at some time impact the Earth.


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