Resonant and Co-orbital Asteroids Associated with Earth Martin Connors, Athabasca University Kimmo Innanen, York University AGU Spring Meeting, Montreal 2004

There are presently 19 asteroids known with semimajor axis between 0 There are presently 19 asteroids known with semimajor axis between 0.99 and 1.01 AU and thus potentially affected by resonant interaction with the Earth. Of these, four are in horseshoe orbits, two are currently quasi-satellites, and at least one has a complex high-e high-i interaction (Cruithne).

2002 AA29 and 2003 YN107 move on low-eccentricity orbits very similar to that of the Earth, with moderate i of 11 and 4 degrees. These objects both have horseshoe orbits with respect to Earth and are capable of being captured as quasisatellites (which 2003 YN107 currently is).

2002 AA29 has a horseshoe orbit, approaching Earth and being perturbed to move away.

This results in a characteristic alternation of the semimajor axis around 1.0 AU

2003 YN107 has a similar horseshoe behavior at times but lower inclination. It is currently trapped as a quasi-satellite near Earth.

During QS the semimajor axis stays near 1 During QS the semimajor axis stays near 1.0 and eccentricity changes notably.

Two other objects are known to have horseshoe orbits deviating further from Earth's orbit. 54509 (2000 PH5) and short-arc object 2001 GO2 have eccentricities of about 0.2 and low inclinations. They move ‘in and out’ more and ‘up and down’ less than the preceding objects.

2001 GO2 (a short arc object) and 2000 PH5 have similar horseshoe orbits. 2001 GO2 has regular QS motion in the gap of its horseshoe (insert).

A recently-discovered object has a very regular quasi-satellite state resembling a high-inclination orbit of Earth.

The remaining 14 objects have eccentricity higher than 0 The remaining 14 objects have eccentricity higher than 0.2 and may have high inclinations. At least one such object, 3753 Cruithne, has a complex horseshoe-like orbit with respect to Earth, but most do not show resonant interaction.

We find that some of the co-orbital asteroids are suitable targets for space missions. The known objects on very Earth-like orbits are not energetically the most favorable, largely due to their inclinations. We follow Shoemaker & Helin 1978 for delta-Vs.

Among 2561 NEAs the delta-V needed for rendezvous is minimal at a=1.0. The sample is biased by e needed to discover if a<1.0.

Low-eccentricity target orbits are energetically favored, but the dependence is not strong. For example, Hayabusa target Itokawa has e=0.28 but is only 29th on the list.

These considerations suggest low-e objects with semimajor axis close to 1.0 will be energetically favorable as targets. Earth horseshoe objects have these characteristics. Shoemaker already in 1978 suggested Earth Trojans would be favorable but none are known. We are looking….

Unfortunately, known horseshoe objects have high e or i. If … 2002 AA29 had very low i, it could be one of the most energetically favored.

Intuitively it seems low-e low-i HS orbits should be less stable Intuitively it seems low-e low-i HS orbits should be less stable. A clone study of 2002 AA29 shows a stable band around i=5º and not lower.

We find that despite the interesting dynamics and naïve considerations that HS objects should make good targets, we are unlikely to find low-i HS objects as needed. We have not considered Earth Trojans (ETs) since using only known objects or their clones. ETs could be good for long-duration missions.

If … somewhat better HS targets are found… Relatively long periods spent near Earth when at one end of a horseshoe orbit favor mission operations. Earth-like orbits can favor short missions (manned?). The small size of objects known to date would present a targeting challenge but may mean lack of regolith.

The objects investigated to date have shown relatively short periods of stability in their current orbits and are thus not likely primordial. They likely are recent arrivals from the asteroid belt, which is also the case for most other NEAs.

Acknowledgements and Reference We thank Greg Stacey, Ramon Brasser and Paul Wiegert for assistance with calculations. Rob Whiteley kindly supplied similar results used for comparison purposes. Parameters were taken from the MPC (April 2004) and NeoDys (May 2004 for 2004 GU9). ============================================ Shoemaker, E. M. & E. F. Helin, Earth-Approaching Asteroids as Targets for Exploration, in Asteroids: An Exploration Assessment, NASA, Washington, 1978.