Centre for Science, Athabasca University, Athabasca AB CANADA
Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA USA
Tuorla Observatory, University of Turku FINLAND
Dept. of Physics, The University of Western Ontario, London ON CANADA
Canada-France-Hawaii Telescope, Kamuela HI USA
Dept. of Physics and Astronomy, York University, Toronto ON CANADA
An international team of astronomers has found that an asteroid discovered earlier this year follows Earth's orbit around the Sun and will, in nearly 600 years, appear to orbit the Earth. In the October issue of the journal Meteoritics and Planetary Science, the astronomers announce that the asteroid, named 2002 AA29, follows a "horseshoe orbit" that makes it come near the Earth every 95 years. It will next come close on January 8, 2003, although even then it will be much further away than the Moon and only detectable using large telescopes. The combination of Earth's and Sun's gravity works so that even as Earth pulls in the asteroid, it speeds up and moves away from the Earth. In this way Earth is protected from impact, despite the similarity of the asteroid's orbit to Earth's.
The special horseshoe orbit was pointed out by Paul Chodas of the Jet Propulsion Laboratory in Pasadena to Martin Connors of Athabasca University in Canada, shortly after the object was discovered in January by the LINEAR asteroid survey. Connors, then on leave in California, alerted colleagues at Queen's University in Kingston, Ontario, York University in Toronto, and the Tuorla Observatory in Finland. Rapid calculations confirmed the special nature of the orbit, and followup observations were taken by Christian Veillet with the large Canada-France-Hawaii telescope. "Most of us had worked together to search for objects closely related to this one, so our team was ready to spring into action", states Connors. Besides finding that the object would come close to Earth next year and then move around Earth's orbit to come back in 2098, the team found that in about 600 years, the asteroid will become a "quasi-satellite" of the Earth. While Earth has only one natural satellite, the Moon, for about fifty years this small asteroid will move near the Earth and appear to be in orbit, going around once a year. In fact, explains team member Paul Wiegert of Queen's University, "both the Earth and the asteroid still both go around the Sun, but the relative looping motion of the asteroid in some ways resembles a satellite orbit, with an apparent period of one year". Calculations by Seppo Mikkola in Finland suggest that the asteroid has already been a quasi-satellite, from about 550 to 600 A.D., but since it is such a small object it would not have been seen then.
The vast majority of asteroids are in belt between the planets Mars and Jupiter, thus rather far from the Earth. Even near-Earth asteroids, that sometimes cause a panic when they fly by, usually have orbits reaching out to the asteroid belt. Coorbital asteroids similar in some ways to 2002 AA29 are known to follow Jupiter's orbit and Mars' orbit, but this is the first one known for Earth. Some asteroids are known to have a similar interplay with Earth's gravity, but do not follow our orbit as closely as does 2002 AA29. Wiegert, Mikkola, and Innanen found the first such object, called Cruithne, in the mid-1990s, but its orbit is very stretched out. The orbit of 2002 AA29 is very round like Earth's, but slightly tilted. An orbit similar to Earth's makes a body easy to reach with spacecraft. The scientists hope to use 2002 AA29 as an example showing that asteroids moving along Earth's orbit do exist, so that they can search for other asteroids with orbits even more similar to ours. Such asteroids could be good targets for space missions and could even be sources of raw materials in space. These special asteroids will likely be hard to find but fascinating. As team member Kimmo Innanen says, "Mother Nature keeps showing us that her repertoire is more bountiful than we thought!"
What we call "Earth coorbital asteroids" (or ECAs) here are asteroids in a 1:1 mean-motion resonance with the Earth. That means that they go around the Sun in the same amount of time as the Earth does, that is, one year. Though the Earth and the asteroids don't always move at the same speed at every instant, their average (or mean) motion is the same, hence the name 1:1 mean-motion resonance.
The best known cases of this resonance are the Trojan asteroids of Jupiter. Over 400 hundred asteroids remain stable either approximately 60 degrees ahead of or behind Jupiter as it goes around its orbit; both planet and asteroids take 11.86 years to circle the Sun. The Earth may also have similar bodies (Earth Lagrange point asteroids) though none are yet known.
Earth companion asteroids are bodies in 1:1 mean motion resonance, though are not necessarily confined ahead of or behind our planet the way Trojans are. Nonetheless, they are not just coincidentally in our vicinity in the way most Near-Earth asteroids are. ECAs share a specific though subtle kind of dynamical relationship with the Earth.
Asteroid 2002 AA29 was discovered by the LINEAR asteroid search program in January 2001. Shortly after this, the coorbital nature of the orbit of 2002 AA29 was pointed out by Martin Connors (Athabasca U) and Paul Chodas (JPL). A follow-up image, taken by Kyle Smalley with the 0.75 meter telescope at the Astronomical Society of Kansas City's Powell Observatory, is shown on the left. The image is composed of sequential exposures which were "stacked" so that asteroid is in the same spot in them all. Since the asteroid moves against the background stars from exposure to exposure, the stacking results in the stars getting smeared out into long tracks across the image. The bars indicate the position of the faint asteroid 2002 AA29. It was 0.065 AU (astronomical units, equal to the distance between the Earth and the Sun) or about 10 million km away from Earth when this was taken. Note: other faint dots on the image are not asteroids, but glitches due to cosmic rays hitting the detector. These can be differentiated from real objects by examining the image at higher resolution than seen here. To view the time-lapse path of 2002 AA29 gliding past distant galaxies, check out this animation of images taken by Christian Veillet at the Canada-France-Hawaii Telescope. 2002 AA29 is the fuzzy dot that can be seen drifting along against the background stars (the line it moves along was added in post-processing). The two brightest blobs are galaxies that happen to be in the same field of view. What looks like another asteroid passing through the upper left corner of some of the frames is not real, just a by-product of the way the frames were processed.
Most of the time, the path of asteroid 2002 AA29 looks as shown to the right. Note that we are looking at the asteroid from above the Sun, and in a frame which moves along with the Earth, which remains stationary at the bottom of the frame (for an animated view, see the MPEG movie clip).
When seen in this frame, the reason for the name "horseshoe orbit" becomes apparent. The asteroid loops along the Earth's orbit, but reverses direction when it approaches our planet from either side. This phenomena is a result of the 1:1 mean-motion resonance. Each loop takes one year to complete; and it takes 95 loops (years) for 2002 AA29 to go from one end of the horseshoe to the other. A fuller explanation of horseshoe orbits can be found on the 3753 Cruithne page.
However, for part of the time, 2002 AA29 goes into quasi-satellite mode. At this time, instead of avoiding the vicinity of the Earth, the asteroid's motion is restricted there. Instead of covering the whole horseshoe orbit, the asteroid becomes temporarily "trapped" in the horseshoe's gap. To the left, we see a three-quarters perspective view of the motion of 2002 AA29 at this time. Note the Earth, rather faint in this image, in the middle of the asteroid's loops, and the Sun in the background. The blue dots outline the Earth's orbit.
During this time, the asteroid remains, properly speaking, in orbit around the Sun. Asteroid 2002 AA29 travels once around the Sun each year, as does the Earth. However, since the planet and asteroid orbit in the same amount of time, they remain close to each other throughout their journey. An analogy would be two cars (the planet and the asteroid) traveling with near-equal speeds around a circular race track (their orbit around the Sun). Both cars will remain near each other because of their similar velocities without there necessarily being any physical connection (eg. strong gravity) between them. An MPEG clip shows the motion in more detail.
To the right is another view, this time looking along the Earth's orbit. The Sun is off the image to the right here. We see that, though trapped in the neighbourhood of our planet, 2002 AA29 loops safely around us. Because there are slight differences between the velocities of Earth and 2002 AA29, they do not remain completely stationary with respect to one another. Rather, the asteroid drifts slightly and as a result, loops around our planet over the course of a year. Hence the name "quasi-satellite". You can learn more about this kind of behaviour at our quasi-satellite page.
After a while, 2002 AA29 will escape from quasi-satellite mode and go back to horseshoe mode. The last period of quasi-satellite motion was in about 550 A.D.; the next will be in 2600 A.D. or so. This ability to make the transition from horseshoe to quasi-satellite motion is part of what makes 2002 AA29 so interesting: no other asteroids are known to do this. It also has a very low eccentricity e (only 0.012 at discovery, less even than the Earth's very small 0.0167) compared to the average of 0.29 for near-Earth asteroids in general. As a result, its orbit is almost circular, and matches the shape of the Earth's closely. It is the tilt or inclination i of 2002 AA29, a modest 10 degrees, that keeps the two objects orbits apart; otherwise they would lie almost on top of each other. The effect of the small eccentricity and more substantial inclination can be seen in the images above. They result in the asteroid's loops being elongated in the up-down direction (due to i) but narrow across the middle (due to the small e).
Note: the following clips (and the previous images) are only illustrative of the orbits involved. In particular, the number of "loops" has been reduced for easier visibility, and the sizes of the Sun, Earth and the asteroid have been exaggerated for the same reason. As a result, the asteroid appears to pass much closer to the Earth than it does in reality.
The view from Earth
Other miscellaneous images:
One A view of 2002 AA29 approaching Earth along its horseshoe orbit
Two A similar view, but with a realistic density of loops. It takes 95 years(=loops) to go from one end of the horseshoe to the other.
These images and animations were put together with the help of the free-ware Persistence of Vision Ray tracer (POV-Ray). Many thanks to the POV-Team for their work.