Earth’s many minimoons
Asteroids captured by Earth could be future test sites for the mining of space minerals
“TCOs are barely moving in relation to Earth, and so would be relatively easy to reach”
How many natural satellites does the Earth have? There is the Moon, of course, but actually at any one time there are also a number of small asteroids in orbit around the Earth, albeit only for a short time. The first of these temporarily captured orbiters (TCOs) or ‘minimoons’ was discovered in 2006. Each typically only sticks around for about nine months, before it is perturbed by gravitational interactions from the rest of the Solar System and drifts away again. Around one per cent, however, will end up on a collision course with the Earth and streak into our atmosphere as meteors.
As Bryce Bolin at the University of Hawaii and his colleagues argue, these temporary moons are extremely important: this population of nearby objects offers a perfect opportunity for studying asteroids to better understand the Solar System. And, since some of them end up as meteors, studying them would offer an unparalleled opportunity to measure the physical properties of meteoroids before their fiery entry into our atmosphere. TCOs are barely moving in relation to Earth and so would be relatively easy to reach – a space mission would require only a very low ‘Delta-v’ (a measure of the amount of energy required for orbital manoeuvres).
But despite their proximity, TCOs are actually very challenging to spot. The asteroids drift in and out of their captured orbit around the Earth and there aren’t a great number of them at any one time. They mainly orbit about four times farther away than the Moon, and they’re also pretty small – most are only a metre or two across, though it’s calculated that about every 100,000 years, something up to 100m wide is temporarily captured by the Earth.
So what’s the most effective way to find these minimoons in the first place? Bolin and his co-authors discuss how TCOs cluster mainly into the L1 and L2 Lagrangian points on the Earth-Sun line, where the gravitational tug of the Sun and Earth precisely balance so as to allow stable orbits with them. So a large, ground-based optical telescope could scan the L2 region opposite the Sun and catch minimoons in opposition, when they would appear brightest. Bolin picks out the US Department of Defense’s Space Surveillance Telescope, which was built to detect and track satellites and orbital debris, as being ideally suited for surveying for TCOs. The Arecibo dish could also be used for radar sweeps. But perhaps the best approach would be to carry out a space-based infrared survey – spotting the soft glow of Sunwarmed asteroids. Bolin recommends an infrared observatory with a mirror around 1m across, to be located near the Earth-Sun L1 Lagrangian point.
One of the biggest hopes for the future is to mine asteroids for metals and other valuable commodities, and TCOs would make perfect targets for missions to practise navigation and space rendezvous, or even retrieval missions. In this way, our temporary minimoons could serve as stepping stones to learning how to exploit space’s resources.