Status of "The NEAR Challenge"
We reported in our last issue that it is apparent that Eros has an
extremely irregular shape. It looks like a bent icicle at some orientations.
That raises the question whether any stable orbits exist for such an
irregular gravity field. That question has been answered in the negative
for the most likely satellite orbits, those with orbital motion near
synchronous with the rotation of the primary asteroid. See, e.g., D.J.
Scheeres, “Analysis of orbital motion around 433 Eros, J.Astronaut.Sci. 43
#4, 427-452 (1995); and D.J. Scheeres, S.J. Ostro, R.S. Hudson, R.A. Werner,
“Orbits close to asteroid 4769 Castalia”, Icarus 121, 67-87 (1996). The most
common fate of objects in unstable orbits is to impact gently on the
surface, usually at a grazing angle, followed by rolling until the orbital
angular momentum (from orbital speeds of typically a few meters per second)
is dissipated, then coming to rest on the surface. [See graphic by Boris
The chances of intact objects coming to rest on the surface are nil except
for satellites because the typical relative speeds between field asteroids
are of order 5 km/s. Such speeds would result in highly destructive,
crater-forming impacts. So finding surface "satellites", especially with
tell-tale roll marks, when the NEAR spacecraft goes into orbit around Eros
next year is still a good way to distinguish between the standard model and
the exploded planet hypothesis.
Stable orbits do exist beyond about 40 km from the primary, and for some
retrograde orbits. So actual satellites in orbit may yet be found, although
tidal forces would evolve such orbits rapidly, so that only small masses
might be expected to survive to the present.