How can we tell what an asteroid is made of? Until now, we’ve relied on remote spectral observations, though NASA’s recently launched OSIRIS-REx mission may soon change this by landing on an asteroid and returning with a sample.
But what if we could learn more about the asteroids near Earth without needing to land on each one? It turns out that we can — by flying through their dust.
When an airless body is impacted by the meteoroids prevalent throughout our solar system, ejecta from the body are flung into the space around it. In the case of small objects like asteroids, their gravitational pull is so weak that most of the ejected material escapes, forming a surrounding cloud of dust.
By flying a spacecraft through this cloud, we could perform chemical analysis of the dust, thereby determining the asteroid’s composition. We could even capture some of the dust during a flyby (for example, by using an aerogel collector like in the Stardust mission) and bring it back home to analyze.
So what’s the best place to fly a dust-analyzing or -collecting spacecraft? To answer this, we need to know what the typical distribution of dust is around a near-Earth asteroid (NEA) — a problem that scientists Jamey Szalay (Southwest Research Institute) and Mihály Horányi (University of Colorado Boulder) address in a recent study.
Moon as a Laboratory
To determine typical dust distributions around NEAs, Szalay and Horányi first look at the distribution of dust around our own Moon, caused by the same barrage of meteorites we’d expect to impact NEAs. The Moon’s dust cloud was measured in situ in 2013 and 2014 by the Lunar Dust Experiment (LDEX) on board the Lunar Atmosphere and Dust Environment Explorer mission.
From LDEX’s measurements of the dust distribution around the Moon, Szalay and Horányi next calculate how this distribution would change for different grain sizes if the body were instead much smaller — i.e., a 10-km asteroid instead of the 1700-km Moon.
Optimizing the Geometry for an Encounter
The authors find that the dust ejected from asteroids is distributed in an asymmetric shape around the body, with higher dust densities on the side of the asteroid facing its direction of travel. This is because meteoroid impacts aren’t isotropic: meteoroid showers tend to be directional, and a majority of meteoroids impact the asteroid from this “apex” side.Szalay and Horányi therefore conclude that dust-analyzing missions would collect many times more dust impacts by transiting the apex side of the body. The authors evaluate a family of trajectories for a transiting spacecraft to determine the density of dust that the spacecraft will encounter and the impact rates expected from the dust particles.
This information can help optimize the encounter geometry of a future mission to maximize the science return while minimizing the hazard due to dust impacts.
Jamey R. Szalay and Mihály Horányi 2016 ApJL 830 L29. doi:10.3847/2041-8205/830/2/L29