Does Earth have a Trojan horde?

[image credit: University of Arizona]

Here’s a fun question: Are there asteroids sharing Earth’s orbit around the Sun?

Cutting to the chase: Yes, there is at least one. But how many are there? There might be a lot, but we don’t know. The cool news is, we may very well know more soon.

In general, it’s actually a bit tough to have a small object like an asteroid share an orbit with a big object like a planet. Even though it orbits at the same distance from the Sun at the same speed, that kind of orbit is unstable. The weak but persistent tug of gravity from other planets would tend to alter the asteroid’s orbit, moving it in closer or pulling it farther out from the Sun. Over time, the asteroid will catch up to Earth (or vice versa), and our planet’s gravity would change the asteroid’s orbit to a much larger degree, flinging it away (or causing it to impact us, which is not the optimal outcome).

But it turns out the story is more complicated and fun than that (which is almost always the case in science). In the 18^th century, two mathematicians, Leonhard Euler and Joseph-Louis Lagrange, discovered that there are five points along a planet’s orbit where gravity and centrifugal force balance, such that an object placed in one of those positions will stay there. We call those the Lagrange points.

The first Lagrange point, called L1, is inside the planet’s orbit toward the Sun. L2 is outside the planet’s orbit, and L3 is on the opposite side of the planet from the Sun. These points are metastable: If you put an object there, it’ll stay there, but if you poke it somehow, it’ll fall away from that point. Think of these regions as being the tops of hills. Put a ball there, and it’ll stay, but a gust of wind will cause the ball to roll downhill*.

L4 and L5 are different. Those act like valleys, gravitational dips. Poke an object that’s in one of those points and it’ll actually fall back into it. Those points are stable. The L4 point is 60° ahead of the planet in its orbit, and the L5 point 60° behind.

Jupiter has a lot of gravity, and it turns out that its L4 and L5 points are very stable. Asteroids that wander in there stay there. In 1906, a 135-km-wide asteroid was discovered in Jupiter’s L4 point. It was named Achilles, and it soon became customary to name all the asteroids in Jupiter’s Lagrange points after figures in the Trojan War (Greek ones at L4 and Trojans at L5), and we generically call them Trojan asteroids. We know of thousands of Jupiter Trojans!

We’ve discovered Trojan asteroids in the orbits of Venus, Mars, Jupiter, Uranus, and Neptune. Mercury is too close to the Sun to find them, and Saturn Trojans may not be stable due to Jupiter’s influence.

And Earth? Well, that’s a funny thing: It’s really hard for us to observe any Earth Trojans from Earth, because they’re 60° away from the Sun in the sky. That means they set not long after sunset, and rise not long before sunrise. That makes them very hard to find, and in fact we know of precisely one, called 2010 TK7. As its name implies, it was discovered in 2010, in observations taken by the Wide-field Infrared Survey Explorer. WISE orbits the Earth, and has a better view from space than we do stuck on the planet.

So, how do we find more? Why, I’m glad you asked.

The spacecraft OSIRIS-REx (potential winner of the “most tortured acronym name”: Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is currently on its way to the near-Earth asteroid called Bennu. It launched on September 8, 2016, and is taking two years to get to the rock. Right now, it’s about 120 million km from Earth, ahead of us and just inside our orbit around the Sun.

Perhaps you see where this is going.

Starting on February 9, and continuing on through February 20, OSIRIS-REx is in the perfect position to look for Earth’s L4 Trojans. It’s passing near that region of space, so any faint rocks will be easier to spot, and the spacecraft’s position means the geometry is good as well —it’ll have the Sun to its back, so any L4 asteroids will be fully lit and as far from the Sun in the sky as they can be. So, for a dozen days, the spacecraft is scanning the sky, looking for any asteroids that are in our L4 point. It will gaze at the same fields over and over again, looking for any objects that move the right way against the background stars to be Earth Trojans.

Bonus: That region of the sky also includes Jupiter and three main-belt asteroids (Pandora, Victoria, and Aglaja), so we get free science. Also, the techniques used to look for the Earth Trojans will give the engineers back here on Earth practice and knowledge that will be valuable once OSIRIS-Rex reaches Bennu.

This is pretty exciting. There are two ways for an asteroid to be in the Earth’s L4 point: It can wander in from another orbit and settle into the gravitational divot — which is interesting enough — or it could have been there since the formation of the solar system. That’s potentially very cool, indeed. A primordial asteroid, relatively untouched since the birth of the Sun and planets 4.6 billion years ago, would be an astronomical time capsule, allowing us to see what conditions were like back then. Trojan asteroids aren’t terribly hard to send spacecraft to, either, so if we do find any substantial rocks there, I would dearly love to see a mission planned to explore them (a mission called Lucy has already been announced by NASA to look at some of Jupiter’s Trojans up close).

I love stuff like this, using spacecraft to do incidental -- but important -- science. And, like so many such endeavors, it’s just so dang cool. Whether it finds Earth Trojans or not, some important science will be learned, and that’s why we do these missions in the first place. If we knew what was out there, we wouldn’t call it exploring.

* It turns out that there’s yet another weird trick to the L1 and L2 points: Due to the complex nature of gravity and centrifugal force, you can actually put an object into orbit around one of those two points, and it’s kinda sorta stable. You need to tend to it, using a thruster to keep it there, but it’s a lot easier than being at the L1 or L2 themselves. We put lots of satellites there; the James Webb Space Telescope will be in such a halo orbit around the Earth’s L2 point after it’s launched in 2018.