Asteroid 2002 AA29

Time Lapse view of 2002 AA29 moving among distant galaxies

Paul Wiegert, Astronomy Dept, University of Western Ontario



 

Much like 3753 Cruithne, 2002 AA29 is an Aten asteroid with a semi-major axis of about one AU and an orbital period of approximately one year. However, the eccentrictiy is much lower (0.012 - almost circular), even lower than Earth's eccentricity.  Its perihelion is only 0.988 AU and its aphelion is 1.012 AU, keeping it very close to Earth's orbit.  But this object is never in danger of colliding with Earth. It is locked into a 1:1 resonance with Earth, and this resonance is very stable. Its inclination, orbital tilt with respect to the ecliptic, is 10.739°.

What makes this asteroid unique is its orbit with respect to the Earth. When it is just inside Earth's orbit, it is travelling faster than Earth and so will get farther and farther ahead of Earth until it will almost lap Earth. At this point, Earth's gravity will slow down 2002 AA29 which will cause it to move to a higher orbit. This is a consequence of the conservation of angular momentum. When an object is moving slower in a circular (or elliptical) orbit, it has to move farther away. This is just like Kepler's Second Law of Planetary Motion. As a planet moves closer to the Sun, it is moving faster. Farther away, it moves slower.

 

Now, since the Earth is moving faster in a lower orbit, 2002 AA29 lags farther and farther behind Earth until eventually Earth catches up from behind and almost laps the asteroid. Earth's gravity now accelerates the asteroid, causing it to move to a lower orbit, and the orbital dance continues. Each part of this dance (2002 AA29 moving farther and farther ahead of Earth and almost lapping Earth; lagging farther and farther behind till Earth almost laps it) takes approximately 95 years. Because the Earth and 2002 AA29 only approach each other and the orbits never cross, there is not danger of the asteroid hitting us.



Overhead view of the orbit of 2002 AA29 with repect to the Earth's (Erde) orbit

"2002aa29-orbit-3" by JPL/NEO, Daniel Arnold - NASA - Jet Propulsion Laboratory - Near Earth Object Program, labeling and size of bodies modified by Daniel Arnold

Licensed under Public domain via Wikimedia Commons -

Inclination of 2002 AA29 to Earth's orbit

"2002aa29-orbit-2" by JPL/NEO, Daniel Arnold - NASA - Jet Propulsion Laboratory - Near Earth Object Program, labeling and size of bodies modified by Daniel Arnold

Licensed under Public domain via Wikimedia Commons -

 



This speeding up and slowing down create a unique shape. Each of the loops in the image below is the position of AA29 with respect to the Earth's orbit every year. The whole series of loops has a shape like a horseshoe with the Earth being in the gap at the two ends of the horseshoe.

 



 

"2002aa29-orbit" by JPL/NEO, Daniel Arnold - NASA - Jet Propulsion Laboratory - Near Earth Object Program, labeling and size of bodies modified by Daniel Arnold

Licensed under Public domain via Wikimedia Commons -