Earth’s First Trojan Asteroid Discovered


The first known “Trojan” asteroid in Earth’s orbit has been discovered. A Trojan asteroid shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of two Lagrangian points. Trojans sharing an orbit with Earth have been predicted but never found until now. Astronomers analyzing data from the asteroid-hunting WISE telescope – which ceased operations in February 2011 – found the asteroid, named 2010 TK7, and followup observations with the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii confirmed the discovery and the object’s stealthy orbit.

In our solar system, we know of Trojans that share orbits with Neptune, Mars and Jupiter. Two of Saturn’s moons share orbits with Trojans. Astronomers have known that Earth Trojans would be difficult to find because they are relatively small and appear near the sun from Earth’s point of view.

But 2010 TK7 proves that Trojans associated to Earth can be found, and astronomers predict that since one has been found, perhaps they’ll find more, as we’ll learn more about their dynamics and characteristics of their population from this first one.

“These asteroids dwell mostly in the daylight, making them very hard to see,” said Martin Connors of Athabasca University in Canada, lead author of a new paper on the discovery in the July 28 issue of the journal Nature. “But we finally found one, because the object has an unusual orbit that takes it farther away from the sun than what is typical for Trojans. WISE was a game-changer, giving us a point of view difficult to have at Earth’s surface.”

The animation below shows the orbit of 2010 TK7 (green dots).

The asteroid is roughly 1,000 feet (300 meters) in diameter. It has an unusual orbit that traces a complex motion near the L4 point. However, the asteroid also moves above and below the plane. The object is about 50 million miles (80 million kilometers) from Earth. The asteroid’s orbit is well-defined and for at least the next 100 years, it will not come closer to Earth than 15 million miles (24 million kilometers).

“It’s as though Earth is playing follow the leader,” said Amy Mainzer, the principal investigator of WISE’s extended mission called NEOWISE that looked especially for Near Earth Object “Earth always is chasing this asteroid around.”

Asteroid 2010 TK7 is circled in green, in this single frame taken by NASA's Wide-field Infrared Survey Explorer, or WISE. Image credit: NASA/JPL-Caltech/UCLA

A handful of other asteroids also have orbits similar to Earth. Such objects could make excellent candidates for future robotic or human exploration. Asteroid 2010 TK7 is not a good target because it travels too far above and below the plane of Earth’s orbit, which would require large amounts of fuel to reach it.

“This observation illustrates why NASA’s NEO Observation program funded the mission enhancement to process data collected by WISE,” said Lindley Johnson, NEOWISE program executive at NASA Headquarters in Washington. “We believed there was great potential to find objects in near-Earth space that had not been seen before.”

The WISE telescope scanned the entire sky in infrared light from January 2010 to February 2011. The NEOWISE project observed more than 155,000 asteroids in the main belt between Mars and Jupiter, and more than 500 NEOs, discovering 132 that were previously unknown.

Sources: Canada-France-Hawaii Telescope, NASA

47 Replies to “Earth’s First Trojan Asteroid Discovered”

  1. Leaving aside the going so far above and below the orbital plane, I gather that Earth’s Lagrange points should be 1 AU away (equilateral triangles), but the article says it’s now roughly half that distance and will get down to roughly 15% of that distance. Does it count as a Trojan when it wanders so very far from the Lagrange point?

      1. To be precise about meanings, the Lagrange points are nevertheless points (“exactly what it says on the tin”) and there are stable orbits around L4 and L5.

        I see by looking at Wikipedia’s article on “Neptune trojan” that “Jupiter trojans have inclinations up to 40 degrees”, and the image for Jupiter in “Trojan (astronomy)” shows that Jupiter’s Greeks and Trojans can come halfway from the Lagrange point to Jupiter. But I don’t see one that’s 15% of the way from Jupiter to the Lagrange point.

        So Is 2010 TK7 unusual or not? (Aside from being unusual for being the first known Earth trojan, of course.)

      2. The other body is, indeed, the sun. Also the moon (aka Luna) interferes with the straight forward dynamics.

    1. Trojans populate a large volume, wandering around the L4 and L5 points on the libration orbits over about 150 years. For Jupiter trojans, the widest libration orbits end in the L3 point (180 deg) and come as close as 24 deg to Jupiter, if my memory serves right. That’s still farther away than the 15% figure, but I guess it depends on the Sun-planet mass ratio.

    1. If your weekly/monthly housemaid missed one toenail clipping in your bedroom, does that mean you don’t have to pay her?

      1. No,,,, But if the darn thing sticks into the bottom of my foot I’ll sue her or him, for the paycheck. So how do y’all like me now!!!!!!

      2. Apples to oranges, I was just quoting what a planet is now defined as, and kidding, but for the maid question, maybe, I am answering as me, or Joan Crawford?

  2. This I feel maybe a left over piece from the formation of Terra? Maybe worth that asteroid visit. Yo Elon we need a reasonably priced launch for a probe. Maybe a University to build a Micro Probe, Hey Lehigh University need a Challenge. Show us you are the elite engineering school.

  3. Objects at Lagrange points execute lyssajous orbits around the libration point due to the gravitational perturbation of other bodies. Libration points are not stable, and as attractor points they are saddle point pseudopotentials. It is a pseudopotential because this potential is not defined on a proper inertial frame. In the case of a star and a planet, two body problem, a satellite or test mass may sit exactly at the libration point. If it is perturbed along one direction (angular) it will execute an oscillatory motion as it move along the “upwards” part of the saddle. If it is perturbed in the other direction it falls off the saddle potential and runs away. The presence of a third planetary body however means that the saddle is deformed some so that stable orbits around the unperturbed Lagrange point can exist.


    1. “In the case of a star and a planet, two body problem, a satellite or test mass may sit exactly at the libration point.”
      In other words, the odds of a natural body settling exactly at the L4 or L5 point are essentially infinity to one unless someone deliberately puts a satellite there. Any other objects that are gravitationally captured will get caught in a gravity tug of war between the star, planet, and other orbiting bodies and either get flung off elsewhere or settle into a somewhat stable oscillation around the libration point. I think.

      Also, I can’t help reading “libration point” as “libation point.” The same basic physics seems to apply, however – you often find people circling around bars in stable, predictable, and highly eccentric orbits. And they, too, have LOTS of pseudopotential.

      1. If you have a potential function that has a maximum at some point, say given a radial distance r from the center the potential is V(r) = -kr^2 is such a potential, the force is F = -?V(r)/?r = kr, so the force pushes away as r increases. Right at r = 0 the force is zero. So if you put a marble exactly on the top of a hill it will remain there. This is a fixed point, but it is not stable. If you change the sign of the potential it is a “bowl” and the bottom is stable. On the accelerated frame of the geo-coordinate system the Lagrange point L_1 for instance is a saddle. Along the radial direction it is locally V(r) = -kr^2 and along the radial direction tangent to the orbit it is V(r) = k’x^2. So if it is pushed along the radial direction exactly it will oscillate around the Lagrange point. If you push it along that radial direction it falls away.

        In the solar system there is the moon and other planets. This perturbs the potential in the accelerated frame. There are then semi-stable orbits that are lissijous orbits. This gets into a complicated matter of perturbation theory, which is curiously more difficult in classical mechanics than it is in quantum mechanics. So at this point I have to leave this at a rather vague descriptive level.


  4. Pity these Lagrange orbits are too far away for current technology. I google that of 2010 we knew of 2 asteroids that were possible targets for a CEV mission (low delta-v; < 180 days travel time; "larger than the craft") before 2025; and a few more after that period.

    I also assume that a Hohmann transfer orbit to Mars and visiting Deimos and Phobos is easier (and more fruitful) than visiting that Trojan. But could it be up for a visit after that?

  5. I just don’t understand how a body can have that kind of orbit.

    Something tells me if I could read Icrowell’s explanation in Layman’s terms I’d get it. Sorry, but I’m one of those regular people out there thirsty to a high degree for knowledge and updates on findings in the universe, but no formal education on terminology.

    In fact, given such an odd orbit, it surprises me the article doesn’t cover a bit on that.

      1. I was gonna post those links, but you beat me to it! Great minds think alike…

      2. One can only hope as much. Outside, I say, take your thinking outside right now, and stay out of those boxes on the lawn.


      3. … but don’t beat around the bushes.

        That strikes me as quite the opposite of risk taking.

        Of course both approaches have their value. Carroll of Cosmic Variance says, IIRC, that he does one speculative paper for every 5 papers.

  6. This is just the same as Trojans of Jupiter. Probably worth a space mission for observing the Earth from afar, especially with a big telescope for accurate appraisal with Earth colliding minor bodies. I.e. By parallax shifts from two locations for accurate triangulated positions.

  7. It should be named “Tellus”, the Roman goddess of Mother Earth, daylight, marriage, and to pray against any earthquakes.

  8. It doesn’t make sense why they’re coming out with this now? — don’t tell me they’ve just discovered since it’s not too far from earth.

    “This asteroid is among the s“This asteroid is among the safest,” said Wiegert. “It never gets particularly close to the Earth, but it doesn’t wander too far away. There is a distinct gravitational link”afest,” said Wiegert. “It never gets particularly close to the? Earth, but it doesn’t wander too far away. There is a distinct gravitational link”

    —– it’s probably been put there by design by some ET.

    This may sound crazy but if you’re any alien person like me you might think not. If you follow? alex collier, he said planet earth is passing a dense part of the galactic plane and also mention something about extraterrestrial assisting us by putting an object in-front of earth as it passes the plane to reduce the stress/effect it would otherwise have on earth. This could be the reason why Nasa and others are starting to mention about this asteroid.

    1. Extraordinary claims require extraordinary evidence.

      Is it possible that this little rock was put in this highly eccentric orbit by some alien intelligence? Sure. Anything’s possible.

      Is it likely? No. It’s much more likely that it’s just a little space rock left over from the formation of our solar system caught up in a gravity saddle, like the little space rocks caught up in similar gravity saddles in the orbits of other planets in our solar system. It’s also possible that the reason it hasn’t been found until now is that it’s the size of the Empire State Building and 50 million miles away.

      As for this “dense part of the galactic plane,” I’m not following. The solar system is currently passing through the Local Bubble, a very thin gaseous region (.05 atoms per cm^3) in the Local Interstellar Cloud, which itself is a fluffier ( .1 atoms per cm^3) portion of the Interstellar Medium (.5 atoms per cm^3). All of that has no impact on the Earth, as we are well within the far denser heliosphere, which has a density of about 5 atoms per cm^3.

      So all evidence is that we live in a thick solar region of space which is passing through one of the least dense portions of the galaxy. All of which renders the idea that this tiny space rock is somehow protecting us from the interstellar medium rather silly.

  9. They said it wouldn’t be a good target due to how far above and below the orbital plane it moves requiring too much fuel. Is that because they would intend to send an orbiter? I would think this would be a prime target for a robotic lander.

    Seems to me the easiest way to catch this little guy would be to launch the probe so that it will fall behind Earth and then meet up with the trojan as it comes back around. If the orbit is as well defined as they indicate, then meeting it when it is crossing the orbital plane shouldn’t be too hard to calculate. So instead of burning a ton of fuel to try and catch up, it would just use it to keep from getting passed by and to try to land.

    I would think such an orbit would provide invaluable views of the solar system from different angles above and below the ecliptic that the planets are on, as well as possible triangulation with observations made from Earth.

    Not to mention it would make finding other trojans in L4 and possibly other lagrange points that would be hard to view from Earth a bit easier.

    1. Catching up isn’t the issue here – you just nudge a spacecraft (orbiting the Sun) inwards or outwards from the orbit of Earth and Kepler’s laws – and some patience – will do the rest. That’s what the STEREO crafts did: one speeded up, the other fall behind on slightly narrower and winder orbits than Earth’s.

      Spacecrafts start with the same amount of speed as the Earth itself has, rockets and thrusters only deliver the difference needed. The real issue is the inclination: you either settle for a flyby when the asteroid crosses the Ecliptic, or carry ridiculous amounts of fuel to build up a lot of speed perpendicular to the Ecliptic, to sync your spacecraft’s orbit to the asteroid’s. Another example: Ulysses (the probe) had to travel all the way to Jupiter and do close flyby to get a boost to have an orbit perpendicular to the Ecliptic:

  10. I have to wonder if the upcoming planetary alighnment of 2012 could cause a gravitational change in the orbit of this asteroid that might make it a hazard to earth.

    1. Only if you are trolling “2012” nonsense and/or not thinking about the physics. Alignments happens all the time, yet they don’t correlate with asteroid impacts.

  11. i’m fascinated by the way the earth/sun/moon pulls compete and yank this thing around. i never really thought of an asteroid behaving that way.

  12. i’m fascinated by the way the earth/sun/moon pulls compete and yank this thing around. i never really thought of an asteroid behaving that way.

  13. @Cameron Coolidge Buswell,

    This object never orbits the Earth, it orbits WITH the Earth. In this case at the L4 point of the Earth Sun System. The posts above and the article itself explain this so I shall not go on and attempt to repeat that information from another point of view.

    Glance at it askew, for a view to renew.


  14. @Cameron Coolidge Buswell,

    This object never orbits the Earth, it orbits WITH the Earth. In this case at the L4 point of the Earth Sun System. The posts above and the article itself explain this so I shall not go on and attempt to repeat that information from another point of view.

    Glance at it askew, for a view to renew.


  15. @Cameron Coolidge Buswell,

    This object never orbits the Earth, it orbits WITH the Earth. In this case at the L4 point of the Earth Sun System. The posts above and the article itself explain this so I shall not go on and attempt to repeat that information from another point of view.

    Glance at it askew, for a view to renew.


  16. How much delta-V perpendicular to the earths orbit is needed for 2010 TK7, and how much could be got from a fly-by one of the moons poles ?

    1. I am speculating here, but it is claimed to be too much for a CEV/robot, which roughly translates to “more than a Delta IV/Atlas V can throw”.

      The largest version of Atlas V can throw ~ 9 Mg, with a Centaur restartable upper stage with ~ 140 kN thrust for ~ 400 s. Let us assume half that can be used for leaving Earth orbit; ~ 3 minutes burn time seems reasonable.

      Hence required delta-v should be higher than ∫ (T/m) dt ~ 200*(140*103/9*103) s*N/kg ~ 0.3*104 kg*m/s2*s/kg ~ 3*104 m/s.

  17. What would be the danger of another body, think Apophis, throwing one of these things out of their obits into an Earth collision?

    1. Looks like the delay time before they scatter out of lagrange points would be on the order of ~ 0.1 My. (IIRC what I read the other day.) Let us assume that is valid here; but see below.

      We don’t seem to have seen any of those scatter towards us, assuming 2010 TK7 is typical. A 300 m impactor would make a sizable impact of ~ 20* 300 = 6 km diameter. I don’t think we have a recent such impact scar (but it can perhaps hide in the sea).

      Now, the delay time was dynamic scatter. That should include disturbances such as near collisions or collisions. But in any case the absence of impact scars makes the estimate fairly safe.

      Hence the upper bound of impact rate can be estimated as < 10-5 y-1.*

      * No asteroids or planets were harmed during the production of this estimate. However, I wouldn’t trust it further than I can throw an asteroid!

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