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Astronomical Observatory: Cool Images


Asteroid 188847 Rhipeus: Discovery of a Trojan Asteroid


  • The Discovery: During Spring Break in March 2006, Calvin College student Josh Vanderhill and Professor Larry Molnar discovered a new asteroid. This in itself was not particularly unusual – at the time, over 60 asteroids had been discovered by students and faculty at Calvin. But upon closer inspection, it seemed like this asteroid was behaving strangely. It was moving through the images with less than half the speed of other asteroids in the same field. We realized that this is a Trojan asteroid – the first discovered at Calvin – a rare type of asteroid that is roughly twice as far from the sun as asteroids in the Main Belt. See a java interactive graphic from NASA of the orbit. (Only 1 out of every 150 known asteroids fall in this category; see the list of known Trojans from the Minor Planet Center.) It was given the provisional designation 2006 FT9 and has ushered in a new stage in observing at Calvin College.

    Establishing the orbit: The newly discovered asteroid was tracked over the course of 25 days, after which it was too far and too faint to detect. To firmly establish the orbit, more observations would be needed the next time the Earth passed closer to it (Spring 2007). The job of recovery was assigned to the Physics 134 class that semester. Using the data from 2006, a predicted location could be computed. But since the first set of observations did not span a long time period, we needed to know how far from the predicted location the asteroid might be. Using the freeware program findorb, random noise was added to the data a hundred different times, and a hundred additional predictions were made. The result was a very long (several times the field of view of our camera), but narrow ellipse of possibilities. In order to search the full ellipse, a series of images were made. Each field of view was assigned to a team of two students to search. The asteroid was found in one of these image, right on the ellipse and moving slowly as before. With this connection made, it was possible to make accurate predictions backward (where the object was found on one night of images from 2003) and forward (where the Spring 2008 Physics 134 class easily found it again). With good measurements from four different years, the Minor Planet Center was able to establish a high quality orbit, and assigned the asteroid a permanent identifying number: 2006 FT9 became asteroid 188847 on June 18, 2008.

    So we have a Trojan asteroid for sure, but what is a Trojan, anyway?

    What are Trojans? Asteroids are lumps of rock that orbit our sun. See our earlier web pages for more on what asteroids are and on how Calvin students discover them. Most of them orbit in a broad region between the orbits of Mars and Jupiter known as the Main Belt. But that is not the only place asteroids can be found. The Trojans are one example of non-Main Belt asteroids.

    Trojans are asteroids that orbit at about the same distance from the sun as Jupiter, and are locked into a one-to-one orbital resonance with it. This means that they go around the sun once each time Jupiter does – a year for a Trojan is exactly one Jupiter year (which is 11.9 Earth years). The Trojans are actually made up of two groups that lie ahead and behind Jupiter on its orbit. The positions 60° ahead and behind Jupiter (known as the L4 and L5 Lagrange points) are positions where gravitational forces balance, making the orbits of Trojans stable. Our new Trojan, 2006 FT9, is near the L5 point. See Figure below.

    The Trojans move around these points much as a marble oscillates around the bottom of a smooth bowl. This movie shows the Trojans as they orbit around the Sun with Jupiter. The Trojans are in green, and the Hildas (another asteroid group) are shown in red. This movie is the same as the previous one, but with the frame rotating so that Jupiter stays in place. This shows how the Trojans move about the Lagrange points.

    How big are Trojans? As the Trojan asteroids are more distant from both the Earth and the Sun than main belt asteroids, a Trojan observed to have a given brightness must in fact be much larger than main belt asteroids seen to have the same brightness. This is compounded by the observation that Trojans are generally dark, reflecting back only 4% of the light they receive. We estimate Asteroid 188847 to be about 10 km across, about the size of Grand Rapids, and by far the largest asteroid discovered at Calvin.

    Why are they called Trojans? Following the precedent set 100 years ago (1906) by Max Wolf upon the first discovery of a Trojan asteroid (588 Achilles), Trojan asteroids are named after participants in the ancient war between the Greeks and the Trojans as described in Homer's Iliad, and Vergil's Aeneid. Specifically, asteroids in the L4 group are named for Greek heroes and those in the L5 group for Trojan heroes. (As of 19 August, 2008, asteroid (188847) 2006 FT9 was named Rhipeus.)

    Where did the Trojans come from? Although we now understand what is special about the Trojans' present orbits, scientists are not sure how they got there in the first place. There are two major theories, however, and the latest one may have startling implications for our solar system.

    It is well-accepted in the scientific community that the planets were not always where they are now. Jupiter, for example, was once farther out and has migrated inwards towards the sun since the time of the early Solar System. The first theory, therefore, simply says that as Jupiter moved in, it “collected” rocky objects that were caught in its path. The Trojans are some of those that were caught. Originally, the Trojans would have been fewer and larger. Collisions with each other and with other objects would have broken them down into the sizes we see today.

    The second theory is newer and more radical. First, one must ask how Jupiter moved inwards. It did it by pushing on Saturn via small objects in between the two planets. Jupiter pushed on them from one way and Saturn from the other, causing Jupiter to move inwards and Saturn out. As they did this, the ratio of their orbital periods increased. The current ratio is 2.48. Long ago, as it passed the 2.00 mark, they would have been in a two-to-one orbital resonance, with Jupiter completing two orbits for every one Saturn orbit. In this case, the gravitational influences of the two biggest planets in the Solar System would be lined up. This combination could wreak havoc on the Solar System. One important consequence is that objects in the Kuiper Belt – an extended zone of small bodies past Pluto – would be stirred up, sending many of these bodies careening into the inner solar system. Many of these would have been trapped by Jupiter and locked into its Lagrange points. These would be what we now call Trojans. Others may have struck our Moon at that time, forming the enormous craters we still see as the dark Mare. If this were the case, it would allow us to study the icy bodies of the Kuiper belt without traveling all the way to the fringes of the Solar System – we would only need to go as far as Jupiter.

    The fun part of science comes when two competing theories can be found to make different predictions. Then the appropriate followup measurements can be made to decide between them. Calvin student Kathy Hoogeboom realized that since the second theory predicted the Trojans would be more icy than rocky, they should be less dense on average, and hence would have different rates of spin. As a honors thesis project, she began systematically measuring the spin rates of Trojan asteroids with the Calvin-Rehoboth telescope. She found a distinct trend towards slow rotation, just what is expected for a lower density. But she did not have a large enough sample to be sure. As a summer research project, Calvin student Melissa Haegert picked up where Kathy left off and measured more Trojan spin rates. The trend held up, and the combined results were published in April 2008.

    A Greek Trojan : [added 10/11/2008]

    On 7 October 2008, while following up on an asteroid recently discovered by students in the Astronomy 110 class (asteroid 2008 SH11), Prof. Molnar discovered Calvin College's second Trojan asteroid: 2008 TC9. Whereas asteroid 188847 Rhipeus is near Jupiter's L5 point (lagging behind Jupiter in its orbit), 2008 TC9 is near Jupiter's L4 point (leading Jupiter in its orbit). So while the name Rhipeus was selected from among the Trojan heroes of the war, 2008 TC9 will need to have a name selected from among their Greek opponents once its orbit is established.


    Fernandez, Y. R., Sheppard, S. S., and Jewitt, D. C. 2003, Astrnomical Journal, volume 126, pages 1563-1574. "The Albedo Distribution of Jovian Trojan Asteroids".

    Fleming, Heather J., and Douglas P. Hamilton. Icarus, vol 148, pp. 479-493, December 2000, “On the origin of the Trojan asteroids: Effects of Jupiter's mass accretion and radial migration.” (link to abstract)

    Groshong, Kimm, NewScientistSpace, 1 February 2006, “Icy Trojan asteroids boost planet-forming theory.”

    W. M. Keck Observatory News Release, Spaceflight Now, 1 February 2006, “Trojan asteroid Patroclus: Really a comet in disguise?”

    Kortenkamp, S.J., and D.P. Hamilton. “Asteroid Formation: Origin of the Trojans.” American Astronomical Society. 27 October 2000. (link to abstract)

    Molnar, L. A., Melissa J. Haegert, and Kathleen M. Hoogeboom, The Minor Planet Bulletin, volume 35, pages 82-84 (2008 April). "Lightcurve Analysis of an Unbiased Sample of Trojan Asteroids"

    Morbidelli, A., H. F. Levison, K. Tsiganis, and R. Gomes. Nature, 26 May 2005, Vol. 435, pp. 462-465. “Chaotic capture of Jupiter's Trojan asteroids in the early Solar System.” (link to abstract)

    Scheirich, Petr, 1 May 2005.“Asteroid (and Comet) Groups, Release 2.0”

    Wikipedia, “Trojan Asteroid”

    [Content by J. Vanderhill and L. Molnar 7/20/06; updated by L. Molnar 10/11/08.]


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