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.”