Satellites of Asteroids and Comets

If asteroids and comets are the products of accretion from a nebula, or even from collisional break-ups, they will invariably be isolated single bodies because their gravitational fields are too weak to affect captures. For example, in a break-up event, most debris escapes, and what does not falls back onto the surface it was ejected from after one orbit. Even if it managed to barely miss the surface, tidal forces would bring it back down in short order. When asteroid satellite captures are simulated on computers, it is done by assuming the catastrophic central breakup of a parent body with debris moving away along radial lines. But that is actually a model for an explosion, not a collision.

By contrast, in the eph, nearby space is filled with debris just after the explosion. Large fragments will find lots of debris inside their gravitational spheres of influence, and these will remain in stable orbits as permanent satellites of these larger fragments. For that reason, I presented papers at the International Astronomical Union meeting in Argentina in 1991, and the Flagstaff meeting of asteroid, comet, and meteorite experts in that same year, pointing out the eph prediction that asteroid satellites should be numerous and commonplace. Specifically, the eph predicted that spacecraft soon to visit asteroids (or comets) should find at least one of the larger debris bodies (satellites) in orbit around the asteroid (or comet) primary nucleus. This prediction, also published in [6] and [[8]], was considered extremely unlikely by mainstream astronomers, one of whom made a public wager with this author that it would not happen.

The Galileo spacecraft flew by asteroid Ida in 1993, and returned images showing a 1-km satellite (now named Dactyl) in a stable orbit around its nucleus. Since that discovery, several dozen additional discoveries of satellites of other asteroids have been made. [[9]] The same has happened for TNO asteroids, where only the largest of well-separated satellites can be detected. Yet estimates are that at least 5% of all TNOs are binary. And this new data supplements occultation and radar evidence of long standing showing probable asteroid satellites. A year before the NEAR spacecraft went into orbit around asteroid Eros in February 2000, I altered the general prediction of satellites to a more specific one: If the gravity field of an asteroid is too irregular for stable orbits to exist near the synchronous orbit (as is the case for Eros), then the debris that once orbited the nucleus would now be found as intact boulders lying on the asteroid surface. [[10]] These would be easy to identify because of their tangential touchdown onto the asteroid, resulting in considerable rolling from their orbital momentum. So “roll marks” were the predicted identifier to show that boulders were former satellites.

Text Box:
Figure 8. NEAR spacecraft photo of a large crater on asteroid Eros with a trail across a crater rim, leading to an interior boulder.
The first image taken by the spacecraft from orbit around Eros is shown in Figure 8. The two rectangles are areas where contrast was stretched for better visibility of the “roll mark”. The image appears to show a track starting in a random location, going up the outside wall of a crater, down the inside wall, and ending in a 50-meter boulder. Many additional examples of boulders, tracks, and boulders at the ends of tracks can be seen in later spacecraft images.

In the meantime, evidence for comet satellites was mounting as well. The Giotto spacecraft was the first to approach a comet, where it found “brightness concentrations” in the inner coma referred to as “dust spikes”. [[11]] Then Hubble Space Telescope observations of Comet Hale-Bopp showed at least one, and probably three secondary nuclei orbiting the primary comet nucleus. [5] Although this finding was controversial, the satellite interpretation was subsequently confirmed as the most reasonable explanation by other investigators. [[12]] The largest of these secondary bodies is a 30-km satellite of an estimated 70-km primary nucleus.

The standard models for asteroids and comets did not predict asteroid satellites. No supporter of those models credited even the reasonable possibility of asteroid or comet satellites, except perhaps by extremely unlikely fluke circumstances, until the first one was photographed. Since then, the mainstream models have struggled to accommodate the finding that asteroid satellites, at least, are numerous and commonplace, just as the eph confidently predicted.