To paraphrase an old saying, if the astronaut can’t go to the asteroid, then the asteroid must come to the astronaut. In a study released by the Keck Institute for Space Studies, researchers outlined a mission to tow an asteroid into lunar orbit by 2025 using ion propulsion and a really big bag. The idea is to bring an asteroid close to Earth for easy study and visits by astronauts without the hazards and expense of a deep space mission.

Developed in conjunction with NASA’s Jet Propulsion Laboratory and others, the study takes on a problem with the new NASA initiative to send manned missions to the area around the Earth known as cislunar space that extends out to just beyond the Moon's orbit. It’s a great technological challenge, but there isn’t anything actually there for the astronauts to visit. The Keck study's solution is to find a near-Earth asteroid and tow it back for study.

The idea of exploiting asteroids has been around for a very long time. In 1903, Russian space pioneer Konstantin Tsiolkovsky wrote about it in his book The Exploration of Cosmic Space by Means of Reaction Motors. Since then, it’s been a staple of science fiction stories and in 1969 Hammer Films made a feature about moving asteroids called Moon Zero Two. Now, according to the Keck study, the technology exists to make asteroid towing feasible.

The Keck study describes a number of asteroid retrieval missions, but the basic one is an unmanned mission of six to ten years duration, with an estimated cost of US$2.6 billion and a completion date of 2025. The first step is earthbound with a ground-based survey using a variety of instruments to identify and classify at least five candidate asteroids per year. What they’re looking for is a 7-meter (23 ft), 500-tonne (551 ton) carbonaceous asteroid in an accessible orbit.

The next step is to launch an unmanned tug. This spacecraft would be surprisingly small for such a big job. It will weigh only 18,000 kilograms (39,683 lb), which makes it light enough to be delivered to low-Earth orbit by one Atlas V rocket. The key to this craft is its ion propulsion consisting of five 10-kW Hall thrusters mounted on two-axis gimbals for steering. Running on xenon propellant, only four thrusters would be used at any one time with the fifth held as a back up.

Ion thrusters work by using solar panels to electrically charge the xenon atoms. A cathode accelerates the xenon ions, producing thrust. Although the thrust is very slight, ion thrusters can operate for months on end, so that tiny thrust can build up to a lot of speed. Even then, it would take the tug four years to reach the target asteroid.

Once on station, the tug would spend 90 days studying, bagging, capturing and de-spinning the asteroid. The bagging and capture phase involves a mechanism consisting of a 10-meter long, 15-meter wide (32.8 x 49.2 ft) bag with inflatable arms and hoops sewn in to spread it open. When deployed, this bag would be maneuvered around the asteroid and then deflated and closed tight using cinching cables that draw the tug up against the asteroid. Then the tug would use rockets to despin the asteroid to make it more manageable.

Once secured, the tug would return to Earth in two to six years using a combination of ion propulsion and gravity assist maneuvers to bring the asteroid into a high lunar orbit – preferably at Lagrange point 1 or 2. Lagrange points are where the gravitational forces of the Earth and Moon cancel each other out, so anything set there will simply maintain its position. These points were chosen to avoid danger of the asteroid hitting Earth, yet keeping it in close enough proximity for astronauts to visit it.

Simulations indicate that an asteroid can remain in such a position for 10 to 50 years before it begins to drift and eventually fall towards the Moon. Safety is also the reason why a relatively small carbonaceous asteroid was chosen. Such asteroids rarely survive entry into the Earth’s atmosphere, so even in the unlikely event of an accident the threat would be small.

According to the Keck study, the advantages of having an asteroid on Earth’s doorstep are considerable. One is that laboratories on Earth would have access to tons of asteroid samples instead of micrograms. Also, it would be a high-value target in cislunar space for human visits at an affordable price. There would be no need to expose astronauts to long, costly missions with dangerous radiation for the first manned asteroid mission and it would take weeks instead of months.

Additionally, it would provide operational experience for astronauts working on asteroids that would be valuable for future deep space missions. Such an approach would also provide a new working relationship between robots and astronauts as the machines fetch home the asteroids and the humans study them.

The study also points out that a carbonaceous asteroid is an excellent source of minerals such as water and raw materials for radiation shielding for deep space missions. A single 500-tonne (551 ton) asteroid may hold 200 tonnes (220 tons) of volatiles (including about 100 tonnes (110 tons) of water), 100 tonnes (110 tons) of carbon-rich compounds, 90 tonnes (99 tons) of metals (including 83 tonnes (91.5 tons) of iron and 6 tonnes (6.6 tons) of nickel and 1 tonne (1.1 ton) of cobalt) and 200 tonnes (220 tons) of silicate residue. These could be used as raw materials for permanent space settlements or to develop technology to allow explorers to exploit local resources.

Another spinoff that the study sees is that the refinement of ion propulsion could be used to support manned deep space missions by pre-positioning supplies and equipment along their route, and asteroid moving technology could also be used to build space defenses against rogue asteroids headed for Earth.

On the more intangible side, the Keck study says that asteroid retrieval would be an opportunity to promote international cooperation and build public awareness with a mission that harks back to the glory days of Apollo.