Does Earth have a Trojan horde? – Blastr | Futurist Transhuman News Blog

[image credit: University of Arizona]

Heres a fun question: Are there asteroids sharing Earths 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 dont know. The cool news is, we may very well know more soon.

In general, its 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 asteroids 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 planets gravity would change the asteroids 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 18th century, two mathematicians, Leonhard Euler and Joseph-Louis Lagrange, discovered that there are five points along a planets 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 planets orbit toward the Sun. L2 is outside the planets orbit, and L3 is on the opposite side of the planet from the Sun. These points are metastable: If you put an object there,itll stay there, but if you poke it somehow, itll fall away from that point. Think of these regions as being the tops of hills. Put a ball there, and itll 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 thats in one of those points and itll 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 Jupiters L4 point. It was named Achilles, and it soon became customary to name all the asteroids in Jupiters 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!

Weve 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 Jupiters influence.

And Earth? Well, thats a funny thing: Its really hard for us to observe any Earth Trojans from Earth, because theyre 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, Im 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,its 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 Earths L4 Trojans. Its passing near that region of space, so any faint rocks will be easier to spot, and the spacecrafts position means the geometry is good as well itll 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 Earths 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. Thats 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 arent 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 Jupiters Trojans up close).

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

* It turns out that theres 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 its kinda sorta stable. You need to tend to it, using a thruster to keep it there, but its 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 Earths L2 point after its launched in 2018.