Let's Put An Asteroid In Orbit Around the Moon

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In 2010, President Obama set a sky-high goal for NASA: to send a team of astronauts to visit a near-Earth asteroid (NEO) by 2025.

But rather then propelling humans on a dangerously long journey into interplanetary space, why not bring the asteroid here? This is the ultimate rock collector’s dream — plop a 500-ton space rock into orbit about the moon.

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Bruce Willis couldn’t go walking around the on target asteroid, however. It would be only 20-feet across, small enough to fit in a backyard. NASA would not require something as exotic as a Star Trek tractor beam to tow the space rock back here. A robotic spacecraft would, literally, put the asteroid in a giant shopping bag and tote it home.

This far-out proposal comes from a workshop sponsored by the Keck Institute for Space Studies to investigate the feasibility of identifying, autonomously capturing, and returning a NEO to Earth. They say that this could be done before then end of the next decade. They estimate that the cost would be less than the price tag of sending another Mars Science Lab class rover to the Red Planet.

This is not a new idea. In 1903 the great Russian space visionary, Konstantin Tsiolkovsky, described snagging asteroids in a publication with the understated title: “The Exploration of the Cosmos by Means of Reaction Motors.”

That dream has now become technologically feasible by three key developments: 1) We now have the ability to discover and characterize a number of sufficiently small NEOs for capture and return; 2) We know how to build sufficiently powerful solar electric propulsion systems to transport the asteroid; 3) NASA’s timetable of sending humans beyond low Earth orbit the 2020s will enable astronauts’ exploration of the asteroid.

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The report describes the asteroid mission as a stepping-stone between human presence on the International Space Station in low Earth orbit and deep space exploration. The report emphasizes: “Placing an asteroid in high lunar orbit would provide a unique, meaningful, and affordable destination for astronaut crews in the next decade. This disruptive capability would have a positive impact on a wide range of the nation’s human space exploration interests.”

The house-sized sample would be dissected and provide valuable information for planetary defense strategies that may someday have to be used deflect a much larger NEO headed on a collision course with Earth. Capturing an asteroid would also mark mankind’s first attempt at modifying the solar system to enable the permanent settlement of humans in space.

A long shopping list of space supplies could be extracted from a 20-foot asteroid: 200 tons of silicates, 100 tons of water, 100 tons of carbon compounds, 90 tons of metals: mostly iron, nickel and cobalt.

The material could ultimately be used to build, supply, and radiation-shield a manned outpost at the Earth-moon Langrage point 2, a sort of space “trucker’s stop” on the way to the moon.

What’s enabling about this approach is that the mission would make available 28 times as much mass for building space habitats as the mass required to launch the initial “space tug” for retrieving the asteroid.

This kind of so-called “mass amplification” is unheard of in present day space exploration. Launching material out of Earth’s deep gravitational well is a fraction of the mass of the rocket. Correspondingly, today’s launch costs are a staggering $10,000 per pound of payload weight. Launching the equivalent of the asteroid’s bulk material off of Earth’s surface would blow the budget at $20 billion.

If the asteroid snagging experiment is successful, it’s easy to imagine a freight train of robotic probes continually hauling raw asteroid material back to the Earth-moon system at an estimated cost of $1 billion per mission.

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The launch-to-capture mass ratio could be boosted to 70:1 or greater the study concludes. “The extraction of propellants, bulk shielding and life support fluids from these activities could jump-star an in situ resource utilization industry,” say the authors.

Bagging It

The report envisions using a present day space booster, the Atlas V, to propel a 20-ton asteroid retriever spacecraft into low-Earth orbit. The ship’s solar panels would suck up 40 kilowatts of energy to power a xenon-fueled ion engine (also envisioned by Tsiolkovsky a century ago).

Under the weak but continuous thrust the tug slowly spirals out of Earth’s gravitational field and takes several years to cruise to the NEO. Upon rendezvous the spacecraft’s largely autonomous onboard intelligence has to deal with a tumbling, “non-cooperative,” capture target.

After a series a careful maneuvers the nimble spacecraft matches its rotation with the asteroid’s spin axis. (Hopefully, the asteroid is tumbling at no more than 1 rpm.) A huge capture bag is deployed and cinched around the asteroid. The two objects continue to tumble in a Richard Strauss space waltz until the tug’s thrusters bleed off momentum and de-spin the asteroid.

Several years later, the asteroid/spacecraft bundle enters the Earth-moon system on a trajectory that allows gravitational capture by the moon. The lunar orbit is circularized and human exploration begins with numerous geological and chemical tests of the asteroid’s composition and interior structure. Some samples are brought back to the International Space Station (or its future version) for analysis.

The moon is a safer place to park an asteroid than into Earth orbit (imagine trying to file an environmental impact statement for Earth!). Once the scientific survey and processing of the asteroid is completed it simply could be de-orbited to crash onto the moon. Even if things went haywire on the return trajectory and the asteroid fell toward Earth, it is so small it would disintegrate in the atmosphere — though that assurance might not mollify doomsayers.

The report concludes that, “NASA would be putting human explorers in contact with an ancient, scientifically intriguing, and economically valuable body beyond the moon, an achievement that would compare very favorably to any attempts to repeat the Apollo lunar landings.”

However a recent report by the National Research Council concludes that a manned asteroid mission does not have as much public support as a return mission to the moon would. The asteroid capture scheme might offer the best of both worlds. Add to that the melodrama of towing an extraterrestrial visitor into our celestial backyard.

Image Credit: NASA, KISS

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