In April 2012, a report was published which investigated the feasibility of sending a robotic spacecraft to capture and haul back an entire Near-Earth Asteroid (NEA) to the vicinity of the Earth by the mid-2020s. The expected size of an asteroid that can be retrieved from such a mission depends on the overlap between the smallest NEAs that can be detected and characterized, and the largest NEAs that can be captured and brought back. This overlap appears to centre on NEAs roughly 7 m in diameter. Depending on its bulk density, a NEA with a diameter of roughly 7 m is expected to have a mass of about 500,000 kg. In comparison, the Apollo program brought back 382 kg of Moon rocks from six surface missions.
Capturing a 500 ton asteroid and placing it in high lunar orbit will provide an interesting and affordable destination for future astronauts since NASA expects a human presence in cislunar space to be well established by the mid-2020s. Having an asteroid in high lunar orbit provides a convenient test bed for new technologies and operational experience that will benefit future manned missions to larger NEAs. Returning samples from the captured asteroid will be easier since the flight time to and from high lunar orbit will be a lot shorter than a full-fledged mission into deep space to retrieve samples from even the most accessible NEAs. Finally, the extraction of water and other resources from the captured asteroid allows the asteroid to serve as a source of raw materials for manned deep space missions.
Figure 1: Illustration of an asteroid retrieval spacecraft in the process of capturing a 7 m, 500 ton asteroid. (Image Credit: Rick Sternbach / KISS)
The spacecraft proposed for this asteroid retrieval mission involves a robotic vehicle that is propelled by a 40-kilowatt solar-electric propulsion (SEP) system. At launch, the initial mass of the spacecraft is 18,000 kg and 12,000 kg of the initial mass is in the form of xenon propellant for the SEP system. The SEP system consists of five 10-kilowatt Hall thrusters that are powered by two solar array wings, each with a collecting area of 71 square meters. The spacecraft’s asteroid capture mechanism consists of a large inflatable bag measuring 10 metres by 15 metres that will deploy and envelope the asteroid.
The mission to retrieve an asteroid will be a rather long duration one. An Atlas V rocket will launch the spacecraft to low-Earth orbit (LEO). Once in LEO, the spacecraft will gradually spiral out to the Moon over a period of 2.2 years. Near the Moon, the spacecraft will undergo a lunar gravity assist followed by a 1.7 years cruise to the target asteroid. After rendezvous with the asteroid, the spacecraft will undergo 90 days of asteroid operations where it will capture and de-spin the asteroid. Finally, another 2 to 6 years is required to transport the captured asteroid to high lunar orbit. Hauling back a more massive asteroid will require a longer flight time. As a result, the total mission duration is expected to be between 6 to 10 years.
Figure 2: Asteroid return mission concept. Return flight time of 2 to 6 years depending on the asteroid mass.
NASA is already considering such an asteroid retrieval mission which is estimated to cost $2.6 billion - slightly more than NASA’s Curiosity Mars rover. The potential benefits of using a robotic spacecraft to snag and haul back a NEA to high lunar orbit for further study and exploration are expected to be huge. The team involved in this feasibility study mentioned: “Placing a NEA in lunar orbit would provide a new capability for human exploration not seen since Apollo. Such an achievement has the potential to inspire a nation. It would be mankind’s first attempt at modifying the heavens to enable the permanent settlement of humans in space.”