Astronomical Institute AS CR, Ondřejov Observatory, Czech Republic Spin and satellite constraints for asteroid mining targets Petr Pravec Astronomical Institute AS CR, Ondřejov Observatory, Czech Republic “Asteroid Science Intersection with In-Space Mine Engineering” (ASIME 2016) Luxembourg 2016 September 22

Asteroid spin and satellite properties Their knowledge is (likely) needed for target selection Operations at an asteroid may be constrained or complicated by its dynamical properties: Rotation rate and state Satellite(s) For a mission target selection, you will probably want to know them. However, these properties are known (measured) for only a small fraction of near-Earth asteroids. And the small NEAs typically come to favorable observing conditions for Earth-based instruments only infrequently (once in several years or tens of years). The best observability conditions for a given NEA are typically offered around the discovery time. Need to run observations to characterize NEAs quickly after discovery; best possible with dedicated telescope(s).

Rotation rates Some NEAs rotate very slow Rotation periods of many days observed. There are long nights at those asteroids. Example: 2013 NJ Radar got a very narrow echo (not resolved the object in frequency) – could be either a long period, or a near pole-on aspect. Photometry (covered 37 days) revealed a period of 250 hr. An observational bias against long periods present – there may exist many small slow rotators we don’t know yet.

Rotation rates Some NEAs rotate very fast Rotation periods < 2 hours frequent for NEAs smaller than ~200 meters. The spin rate is above the critical frequency – negative surface acceleration.

Rotation states Some NEAs are in free precession Asteroids in non-principal axis rotation (“tumbling”). Varying illumination and visibility conditions at any spot on the surface. Centrifugal force varies. Example: 2008 TC3 (Scheirich et al. 2010) Observational bias against tumblers – they seem to be actually very frequent: ~30% of small NEAs are in NPA rotation states (recent result).

Binary asteroids Some NEAs have satellites Binary asteroids are frequent among NEAs larger than about 100-200 m: The fraction of binaries estimated 15 ± 4% (Pravec et al. 2006). There appears to be a cutoff at D ~ 100 m; the smallest binaries detected recently (by radar) have D1 = 120-150 m (e.g., Benner 2015, Naidu et al. 2015). Binarity doesn’t seem to be an issue for smaller NEAs; larger NEAs need to be searched for satellites.

We will want to know the dynamical properties for asteroid mission target selections Together with spectral characterization, we should run photometric (and radar, where possible) observations. Resources required: A photometric telescope of a 2-3m class (to reach V ~ 21 with good S/N) available on short notice (for that the observations can be best taken right after discovery). To characterize one NEA, a few observing nights are typically needed; the telescope(s) will have to be dedicated.

“Questions from the Asteroid Miners” A response to one of the “Questions from the Asteroid Miners”

Q: What observations can be made from Earth or Earth orbit that can ascertain the internal structure of an asteroid (rubble pile, fractured shard, etc)? A: If we find that the asteroid has or had a satellite (i.e., it’s a binary system or an asteroid pair, respectively), the models of asteroid spin fission suggest that it is a rubble pile. Certainly at least there was a negligible cohesion of (near-)surface material from which the satellite formed. Moreover, the equatorial ridge observed on about half of asteroid binary primaries suggests a migration of loose regolith under the tidal forces from the secondary. (Ostro et al. 2006) (Pravec et al. 2010)

Conclusions A knowledge of the dynamical properties of asteroid targets will be needed for space mission planning. The needed observations can be usually best taken right after asteroid discovery, requiring a prompt response (with a dedicated telescope).