Searching for the Earth-Trojan Asteroids

The OSIRIS-REx mission will take advantage of our fortuitous trajectory to search for an elusive class of asteroids that may be the remnants of the building blocks of the Earth.


The trajectory of OSIRIS-REx as we pass through the Earth-Sun L4 region.

Since our launch on September 8, 2016, spacecraft operations for OSIRIS-REx have gone well. We have exercised the propulsion system by performing regular momentum dumps using the attitude control thrusters, executing the large deep-space maneuver using the main engines, and firing the trajectory correction maneuver thrusters to perform precision targeting of our upcoming Earth gravity assist in September 2017. All science instruments have been checked out, and we have downlinked all their data. The team is learning how to fly this amazing robotic explorer. As part of this process, we decided to perform a full rehearsal of our observation planning system. I thought it would be a good idea to add some excitement to this activity by making observations that have the potential for a real science discovery. Starting on February 9, 2017, the OSIRIS-REx spacecraft will search an important part of our Solar System for an elusive class of planetary objects – the Earth-Trojan asteroids.


Diagram illustrating the Lagrange points in the Earth-Moon system. Credit:

For more than two centuries, scientists have known that asteroids (or any small object) can share the orbit of a massive planet in a dynamically stable state if they remain near the so-called “Lagrange points.” Lagrange points are positions in space where the combined gravitational pull of two large masses provides the exact centripetal force required to orbit with them. There are five such points, labeled L1 to L5, all in the orbital plane of the two large bodies. In any two-body system (e.g., Earth-Sun or Moon-Earth) the three points called L1, L2, and L3 are on the line connecting the two bodies. The other two points, L4 and L5, each form an equilateral triangle with the two bodies.  As a result, the L4 point leads the planet by 60˚ and the L5 trails by 60˚ degrees.


Asteroid (253) Mathilde as observed by the NEAR-Shoemaker spacecraft during a flyby in 1997.

The custom of calling asteroids trapped in a planet’s L4 or L5  “Trojan asteroids” is credited to Johann Palisa. Palisa was a prolific asteroid researcher, discovering 122 asteroids over the course of his career. His first discovery was the asteroid (136) Austria in 1874. Other notable discoveries include (153) Hilda, (216) Kleopatra, (243) Ida, (253) Mathilde, (324) Bamberga, and the Amor asteroid (719) Albert. Asteroid (253) Mathilde is especially famous because of the Near Earth Asteroid Rendezvous – Shoemaker spacecraft, which visited it during a flyby on its way to encounter near-Earth asteroid (433) Eros. Palisa named the asteroids trapped in Jupiter’s L4 and L5 points after famous heroes of the Trojan War. Jupiter-Trojan asteroids in the L4 group are named after Greek heroes, and those at the L5 point are named after the heroes of Troy.


The complete path of 2010 TK7 over its 195-year cycle. Figure from

Significant populations of such “Trojan asteroids” are known for Jupiter, Mars, and Neptune.  However, only one Earth-Trojan is currently known, 2010 TK7, which was discovered by the WISE mission.  This object, of diameter ~300 m (for an assumed albedo of 0.1), has a large libration amplitude that brings it into proximity to Earth every few hundred years. A libration is a perceived oscillating motion of orbiting bodies relative to each other. Asteroid 2010 TK7 was discovered near its closest approach distance of about 20 million kilometers. It is currently oscillating about the Earth-Sun L4 point. However, it has a large amplitude of oscillation that is unstable. Over a long time, the cumulative gravitational pull of all the planets in the Solar System will make this amplitude grow and eventually it will break out of oscillating around L4 and flip over to oscillating about L5. This phenomenon is similar to a pendulum that is oscillating with a large amplitude; a small extra force could push such a pendulum over the top. The orbital motion of 2010 TK7 is chaotic and unstable on billion-year timescales, and it is unlikely to be a remnant from the formation of the Earth.  The existence and size of a primordial population of Earth-Trojans (genuine remnants of the building blocks of our planet) are not well constrained and represents a significant gap in our inventory of small bodies in near-Earth space.


The area of the sky (in RA and DEC coordinates) that will be searched by OSIRIS-REx for Earth-Trojan asteroids. Jupiter is in our field of view and we will use it to test our Optical Navigation system.

Scientists predict that there should be more Trojan asteroids sharing Earth’s orbit. These asteroids are difficult to detect from Earth because they appear close to the sun from Earth’s point of view. In mid-February 2017, however, the OSIRIS-REx spacecraft will be positioned in an ideal spot to undertake a survey. We have decided to take advantage of this favorable position to perform a search for Trojan asteroids in the Earth-Sun L4 region. This activity will occur between February 9 and 20, 2017.  Before February 8, the spacecraft will be too far from the Earth Trojans. After February 22, the spacecraft will be at a heliocentric range of >1 AU, leaving the Earth Trojans behind us. We will use the OCAMS MapCam imager to image a portion of the volume of space where we expect the Earth Trojan asteroids to reside.


Inclined view of the OSIRIS-REx trajectory passing through the Earth-Sun L4 point.

While the potential science return from this activity is exciting, the real reason I approved this search is to exercise the operations team. The observations required to search for the Earth-Trojan asteroids closely resemble those planned for the 10-cm Bennu Natural Satellite Search during our Approach Phase. Ten centimeters is the smallest moon we need to detect around Bennu to ensure spacecraft safety. Thus, this activity is a real-time operational readiness test of a mission-critical activity. It will require us to use our observation planning software. Also, since Jupiter is in our field of view, we will practice our Optical Navigation procedures using Jupiter as a substitute for Bennu. We will achieve a complete end-to-end test of Science Operations, Downlink, Processing, Analysis, Publication, and Public Affairs. The fact that we may make an exciting science discovery is secondary to these risk-reduction activities. Stay tuned for the results of our search!


  1. […] to detect from Earth because they appear close to the sun from Earth’s point of view,” writes OSIRIS-REx principal investigator Dante Lauretta at the University of Arizona. “In […]

  2. […] also Lauretta’s latest blog entry for more information about the Trojans. The OSIRIS-REx mission is managed by NASA’s Goddard […]

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