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Radar image of asteroid 2005 YU55. At 400 meters in diameter, this one's too big to retrieve. (NASA/JPL-Caltech)

Finding NEO

Experts disagree on whether NASA’s asteroid capture mission is doable, or even worthwhile. And first they have to find a suitable target.

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Some see it as a bold, outside-the-box proposal to send American explorers into deep space for the first time in 50 years. Others call it a detour from Mars and the Moon. Either way, NASA’s idea of capturing a small asteroid and delivering it to a high lunar orbit for astronaut inspection will have to overcome daunting engineering and cost challenges before it can fly. That may have been the only consensus view among scientists and mission design experts who vigorously debated the Asteroid Retrieval and Redirection Mission (ARRM) at the Target NEO 2 workshop held Tuesday at the National Academy of Sciences in Washington.

The ARRM would launch a robotic spacecraft around 2018 that would use solar electric thrust to rendezvous with and snare a 500-ton, 7-meter-wide asteroid, and return it to translunar space in 2023 (NASA had originally said 2021, but the date is now flexible). Astronauts on their first trip beyond Earth orbit since 1972 would visit the retrieved rock in their Orion spacecraft and collect tens of kilograms of asteroid material for analysis on Earth.

Since it was announced in April as part of the administration’s new human spaceflight budget proposal, the mission has met with skepticism in Congress and criticism over its feasibility. Workshop attendees from inside and outside NASA wrestled with the many technical hurdles to be overcome to enable a launch in just five years. Some openly called the mission ill-advised, and likely to fail. Other experts, however, argued that the mission is worth serious study, even while admitting that its design and execution will stretch NASA’s abilities to the limit.

One immediate challenge is “finding NEO”—a near-Earth object suitable for capture. NASA wants one with a mass between 500 and 1,000 metric tons in an Earth-like orbit, where a 40-kilowatt solar electric spacecraft could nudge it into a stable retrograde orbit high above the Moon. Ideally, the chosen asteroid should resemble carbonaceous chondrite meteorites, harboring water-rich minerals that might someday furnish propellant to lower the cost of expeditions to Mars.

But such targets—small, distant, and dim—are at the limit of our detection capabilities. Even when discovered and tracked, a “good” target mustn’t be too massive, should be roughly spherical, should rotate no faster than twice a minute, and should not tumble chaotically. Experts agreed at the workshop that at least several thousand targets are likely to be out there in reachable orbits. But finding several with all the right characteristics is a very tough objective. 

JPL asteroid dynamicist Paul Chodas said 14 such small objects in reasonable orbits are already known; with NASA’s proposed, expanded search program, perhaps another 15-20 might be found by the 2018 ARRM launch date. For each target discovered, NASA will need to coordinate rapid follow-up with telescopes to get radar and infrared data that reveal the asteroids’ composition and physical characteristics. 

Southwest Research Institute scientist Bill Bottke suggested that a better capture target might be a small “mini-moon”—an asteroid or impact fragment blown off the Moon that has become trapped temporarily in Earth’s gravity field. These objects, wandering in the Earth-Moon system, should be easier to find with existing telescopes than asteroids. Their years-long loiter near Earth might enable astronauts to reach and retrieve them directly, without sending out an elaborate robotic capture spacecraft. 

The Applied Physics Lab’s Andy Rivkin warned that most small asteroids are “fast rotators,” spinning several times an hour or faster, or perhaps tumbling chaotically, which would complicate the capture maneuver. JPL’s Brian Wilcox expressed confidence that existing control systems and mechanisms should enable the ARRM spacecraft to match spin rates, engulf, and then control 500 tons of rotating asteroid. Analysis shows that just 300 kilograms of rocket fuel would be enough to stop a typical target from spinning.

Experts also cited observational and theoretical evidence that some of these small, very-low-gravity asteroids might be rubble piles, rotating clumps of dust, pebbles, and rocks held together by weak Van der Waals forces, acting at the molecular level. The very act of trying to corral the “flying sandbar,” stuff it into a bag, then stop it from spinning might cause it to disintegrate—leaving the visiting astronauts with a few hundred tons of weightless dust and gravel. 

The workshop closed with a wide-ranging discussion of potential benefits from the mission, weighed against possible costs. Many of the asteroid researchers present were upset that NASA’s April ARRM announcement was not vetted by the scientific review processes applied to other planetary missions. But NASA’s chief of exploration and human spaceflight, Bill Gerstenmaier, defended ARRM’s heritage, calling it a technology demonstration and a way to advance key areas like solar electric propulsion, which will be needed for human expeditions to Mars. Science would be a spin-off, not a primary objective; other objectives include learning spacewalking skills for astronauts in deep space. 

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