PROCYON update: Asteroid 2000 DP107 target selected, ion engine stopped

PROCYON (PRoximate Object Close flYby with Optical Navigation) is a microsatellite that launched on December 3 as a secondary payload with Hayabusa2. The mission has now selected their asteroid flyby target -- a binary asteroid named 2000 DP107 -- but is reporting a problem with their ion engines.

Reports came out in various news media, including this article on the Japanese website Space Elevator News. Another site, Monoist, has photos from the April 6 press briefing. User "pandaneko" at unmannedspaceflight.com helpfully translated the main points of these stories:

The target of PROCYON is now set to "2000 DP107" which is a [binary asteroid]. 800m and 300m. [After the encounter], PROCYON will not be able to return to near Earth orbit to perform a [second] swingby. So [200 DP107 is the] only chance of asteroid approach. The Earth swingby is December 3, 2015 and asteroid passby is May 12, 2016.

The nominal mission has been accomplished. Solar panels are OK, temps are OK, triaxial control is OK. It is very much alive.

The ion engine stopped in mid-March 2015, trying to restart, but no indication yet of recovery at the moment. The cause of this has not been identified yet, but the most likely reason is thought to be that a minute metal piece has migrated into the gap between the 2 ion grids of the ion source.

Therefore, several options have been identified, as follows.

1. Give this grid a thermal cycle of expansion and contraction.
2. Give the probe an acceleration and rotate, stop, rotate, stop etc.
3. Apply high voltage and burn out the meal piece.

These options will be tried carefully. However, the deadline for recovery is currently end of April 2015 in time for the Earth swingby in December 2015. This deadline is also being re-evaluated for accuracy.

In the meantime, geo-corona image acquisition has been successful. The image has not been made available as [papers] are being written at the moment.

If PROCYON manages to reach it, 2000 DP107 is an exciting target. It was the first asteroid that radar astronomers discovered to be a binary. Since then, lots more binaries have been found; it's now estimated that about 1 in 6 near-Earth asteroids has two or more components.

In these delay-Doppler radar images, the primary appears to be much larger than the satellite, but actually the satellite is more than a third as large as the primary. The reason that the primary appears so large in the radar image is that spread along the horizontal axis in a radar image is a function of the spin rate, and the primary is spinning very fast, once every 2.8 hours. The satellite is probably locked in spin-orbit resonance, spinning once with every 1.77-day revolution, so it appears quite small.

The high-quality observations in 2008 permitted Shantanu Naidu and coworkers to develop a shape model for both components of 2000 DP107. Here is the shape model for the primary:

You might notice that this asteroid is pointy at its poles and bulgy at the equator. That's a common feature of fast-spinning near-Earth asteroids. Other asteroids known to have this shape include 2008 EV5 (the current leading target for the asteroid retrieval mission) and Bennu (the target for OSIRIS-REx).

The shape has a simple physical cause: it's spinning so fast that the force of gravity at its equator is almost perfectly canceled by the centrifugal acceleration caused by its spin. Objects on its equator could float into space with the tiniest kick. So it's considered quite likely that a chance encounter with a passing asteroid delivered just such a kick that launched a bunch of material (at extremely slow speed) off of the primary's surface, which eventually coalesced into a satellite. Spin-up followed by rotational fission is the likely cause of all those near-Earth asteroid binaries.

You can read all about the shape in Naidu et al. 2015, available on the arXiv.