OSIRIS-REx to Hunt for Earth ‘Trojans’

by | Dec 14, 2016 | Asteroid and Comet Deflection | 18 comments

The so-called ‘trojan’ asteroids that cluster at 60° ahead and behind the planet Jupiter make up a surprisingly populous category. Consider that thus far we have found only one trojan at Earth’s Lagrangian points, while over 6000 have been discovered in Jupiter’s orbit. The total number of trojans larger than 1 km in diameter associated with Jupiter has been estimated to be about 1 million, which matches up well with objects of equivalent size in the main asteroid belt. These days ‘trojans’ can also refer to similar bodies associated with other planets. We know, for example, of about 20 trojans involved with Neptune.

That solitary Earth trojan, 2010 TK7, was discovered oscillating around Earth’s L4 Lagrangian point in 2010 by the NEOWISE team using NASA’s Wide-field Infrared Survey Explorer spacecraft. The object has a diameter of about 300 meters; its oscillations take it back and forth on a nearly 400 year cycle that at times puts it close to opposite the Sun with respect to the Earth. On the other side of its elongated loop of the L4 point, it can close to within about 20 million kilometers of Earth.

What catches the eye here is that 2010 TK7‘s orbit may have, within the past 2000 years, oscillated about the L5 Lagrangian point instead of L4. This seems to be an unstable libration, a fact that makes predicting its future course problematic. That’s why I’m interested in what scientists at the University of Arizona are doing with the OSIRIS-REx mission. Between February 9 and 20, the spacecraft’s onboard camera suite will be activated to search for other Earth trojans.

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Image: In February 2017, the OSIRIS-REx spacecraft will undertake a search for Earth-Trojan asteroids while on its outbound journey to the asteroid Bennu. Earth trojans are asteroids that share an orbit with Earth while remaining near a stable point 60 degrees in front of or behind the planet. (Illustration: OSIRIS-REx/UA).

The problem with Earth’s trojans, assuming there is more than one, is that they are hard to find because from the Earth’s vantage point, they appear close to the Sun. OSIRIS-REx will deploy its MapCam imager to scan the regions where we might expect trojans to exist. The observations have a tactical purpose as well — consider them a warm-up for the encounter with the asteroid Bennu (1999 RQ36), the target of OSIRIS-REx. Checking for rocky material near Bennu will be critical as the spacecraft closes in for surface mapping in 2018. The mission will also attempt to return a sample of the asteroid to the Earth.

From a University of Arizona news release:

“The Earth-Trojan asteroid search provides a substantial advantage to the OSIRIS-REx mission,” said Dante Lauretta, OSIRIS-REx principal investigator and professor of planetary science at the Lunar and Planetary Laboratory. “Not only do we have the opportunity to discover new members of an asteroid class, but more importantly, we are practicing critical mission operations in advance of our arrival at Bennu, which ultimately reduces mission risk.”

So we may learn soon whether more Earth trojans exist, perhaps choosing one day to mine them for rare elements, although 2010 TK7 is not itself a good candidate for such operations (at least early on) because of its highly inclined orbit. Other near-Earth asteroids present better options.

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Image: The inner Solar System, from the Sun to Jupiter. Also includes the asteroid belt (the white donut-shaped cloud), the Hildas (the orange “triangle” just inside the orbit of Jupiter), the Jupiter trojans (green), and the near-Earth asteroids. The group that leads Jupiter are called the “Greeks” and the trailing group are called the “Trojans” (Murray and Dermott, Solar System Dynamics, pg. 107). Credit: Wikimedia Commons.

And on the matter of nearby objects, you’ll recall 3753 Cruithne, a 5 kilometer wide asteroid whose orbit, an overlapping horseshoe, is not related to the kind of oscillations around the Lagrangian points associated with trojans. Another non-trojan near-Earth companion is (469219) 2016 HO3, considered by scientists to be a ‘quasi-satellite’ rather than a trojan. These asteroids are on curious and interesting orbits, and the suspicion is that we’ll find a good many more such objects in coming days.

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18 Comments

  1. DJ Kaplan on December 14, 2016 at 13:54

    You start out by comparing the number of Jupiter’s trojans to those of Earth’s that we have discovered. It’s not clear; should we expect the same number of every planet, or some ratio based on planet mass? Naturally, Jupiter’s proximity the asteroid field might have some influence on jovian mass quantities…

    Anyway, I think it’s very important to research Near-Earth Asteroids!

  2. Alex Tolley on December 14, 2016 at 14:24

    What catches the eye here is that 2010 TK7’s orbit may have, within the past 2000 years, oscillated about the L5 Lagrangian point instead of L4. This seems to be an unstable libration,

    Can someone explain how this happens?

    • Paul Gilster on December 14, 2016 at 17:30

      Stand by while I float this past the Arizona team.

      • Paul Gilster on December 14, 2016 at 21:21

        Alex, here you go, from Renu Malhotra at University of Arizona:

        —

        “The minor planet 2010 TK7 is currently oscillating about the Earth-Sun L4. However, unlike a small amplitude oscillator (which would be stable), TK7 has a very large amplitude of oscillation and it is unstable. Over a long time, the cumulative effects of all the planets make this amplitude grow enough that it breaks out of oscillating around L4 and flips over to oscillating about L5. (Think of a pendulum which is oscillating with a very large amplitude; a small extra force could carry such a pendulum over the top.) A numerical simulation of the long term orbital motion of 2010 TK7 was published by Martin Connors and his collaborators in 2011 (Nature, v. 475, p. 481).

        What we are seeking in our upcoming survey with OSIRIS-REx are the small amplitude librators about L4. Such small amplitude librators, if they exist, could be the relatively stable reservoir which feeds the large amplitude librators like 2010 TK7 (and possibly other asteroids with Earth-like orbits). This is somewhat analogous to how the main asteroid belt feeds the unstable and transient population of near-Earth objects.”

        • Michael Fidler on December 15, 2016 at 1:39

          If I remember correctly, Lagrange points (L1,L2,L3,L4,L5) around any celestial body can be used as a super highway. An object such as a space craft can move between them with much less energy then it would normally take to go that distance. That is probably why this asteroid is moving from the L5 to L4 Lagrange points. (Chaos Theory and Strange Attractors)! These places are also good locations for extraterrestrial observation spacecraft since they are also hard to observe from earth!

          • Michael Fidler on December 15, 2016 at 4:43

            This reminds me of an unusual object seen passing in front of the Sun in 1762. This was two years after 18th century mathematicians Leonhard Euler had come up with the theory for the L1,L2, and L3 points and 10 years before the French mathematician Joseph-Louis Lagrange, who wrote an “Essay on the Three Body Problem” in 1772. He was actually extending the mathematics of Leonhard Euler.

            August 1762, Basel and Solothurn, Switzerland
            Slow-flying spindle in the Sky

            Two witnesses at separate observatories (Rostan in Basel and Croste in Solothurn) reported a vast spindle-shaped cigar in slow flight in front of the Sun.

            Monsieur de Rostan, an astronomer and member of the Medicophysical Society of Basel, Switzerland, observed the object with the aid of a telescope as it eclipsed the sun. This object could be observed daily for almost a month from Lausanne and also by a
            second astronomer in Sole, near Basel. Monsieur de Rostan traced its outline with a camera obscura and sent the image to the Royal Academy of Sciences in Paris. The drawing has not been preserved, unfortunately, but there is no doubt that it once existed and was regarded with some amazement. As this is historically an important incident the original report is produced in full below:

            An account of a very singular phenomenon seen in the disk of the sun, in different parts of Europe, and not in others.

            “The 9th of August, 1762, M. de Rostan, of the economic society at Berne, of the medico-physical society at Basle, while he was taking the sun’s altitudes with a quadrant, at Lausanne, to verify a meridian, observed that the sun gave but a faint pale light, which
            he attributed to the vapours of the Leman lake; however, happening to direct a fourteen foot telescope, armed with a micrometer, to the sun, he was surprised to see the eastern
            side of the sun, as it were, eclipsed about three digits, taking in a kind of nebulosity, which environed the opaque body, by which the sun was eclipsed.

            “In the space of about two hours and a half, the fourth side of the said body, whatever it was, appeared detached from the limb of the sun; but the limb, or, more properly, the northern extremity of this body, which had the shape of a spindle, in breadth about three
            of the sun’s digits, and nine in length, did not quit the sun’s northern limb. This spindle kept continually advancing on the sun’s body, from east towards west, with no more than about half the velocity with which the ordinary solar spots move; for it did not disappear till the 7th of September, after having reached the sun’s western limb.

            “M. Rostan, during that time, observed it almost every day; that is to say, for near a month; and, by means of a camera obscura, he delineated the figure of it, which he sent to the royal academy of sciences at Paris.
            “The same phenomenon was observed at Sole, in the bishopric of Basle, situated about five and forty German leagues northward of Lausanne. M. Coste, a friend of M. de Rostan, observed it there, with a telescope of eleven feet, and found it of the same spindle-
            like form, as M. de Rostan, only it was not quite so broad; which, probably, might be owing to this, that growing near the end of its apparition, the body began to turn about, and present its edge.
            “A more remarkable circumstance is, that at Sole it did not answer to the same point of the sun as it did at Lausanne: it therefore had a considerable parallax: but what so very extraordinary a body, placed between the sun and us, should be, is not easy to divine. It
            was no spot, since its motion was greatly too slow; nor was it a planet or comet, its figure seemingly proving the contrary. In a word, we know of nothing to have recourse to in the heavens, whereby to explain this phenomenon; and, what adds to the oddness of it, M. Messier, who, constantly observed the sun at Paris during the same time, saw nothing of such an appearance.”

            Source: “Natural History: An Account of a Very Singular Phenomena Seen in the Disk of the Sun, in Different Parts of Europe, and Not in Others,” Annual Register 9 (1766):
            120 – 121 .

        • DJ Kaplan on December 15, 2016 at 16:23

          How much do we know about its shape? Could its tendency to oscillate be due to an non-spherical shape?

  3. Paul Gilster on December 14, 2016 at 17:29

    I don’t think we know how many trojans to expect per planet — have not seen a suggested ratio in terms of mass. So we have much to learn.

  4. James Scanlon on December 14, 2016 at 18:25

    Paul ..

    Would any of the earth trojans ( assuming we find more of them )
    be in suitable orbits or be candidate bodies to be placed in suitable
    orbits to facilitate keeping earth in the habitable zone going forward ??
    Your post on 12/12/2016 “Glimpsing our solar system’s future” and
    “comments” section prompted my question ..

  5. Paul Gilster on December 14, 2016 at 21:18

    James, I assume you’re talking about gravitational maneuvers to gradually draw Earth’s orbit outwards. I’m afraid I don’t know the answer to this one because we just don’t know what the population of Earth trojans is, or how a future technology might manipulate them. A fascinating topic, though, and I’ll run the question past Greg Laughlin, who has written before on the matter of adjusting planetary orbits.