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In the 2020s, NASA’s human spaceflight program will revolve around sending astronauts to high lunar orbit to study a small boulder robotically plucked from the surface of a large asteroid, agency officials announced yesterday. The announcement is a crucial milestone for the agency’s nascent Asteroid Redirect Mission (ARM), which is intended to set the stage for future missions sending humans to Mars and other deep-space destinations.
NASA’s decision comes after months of delays as two separate teams investigated how to best achieve ARM’s objectives. The original ARM proposal, dubbed Option A, called for a “grab and bag” approach, in which a robotic space tug captures a small asteroid whole and wraps it in a protective sheath before guiding it into a stable lunar orbit. Though the boulder-snatching concept, Option B, is projected to cost $100 million more than Option A, it won out because it offers more operational flexibility, said NASA associate administrator Robert Lightfoot.
During a conference call with reporters, Lightfoot noted that telescopic surveys had yet to locate asteroids small and slow-moving enough for Option A, and that any suitable targets would have been very difficult to characterize from Earth with present capabilities. That meant Option A would have been “a one-shot deal” with a lot of uncertainty compared to the greater predictability and number of targets available for Option B, according to Lightfoot.
“From what we know of the asteroids we’ve been to, they have boulders on the surface,” he said, which means mission controllers would have many choices for which one to grab. “I’m going to have multiple targets when I get there, is what it boils down to.” Option B would allow ARM to retrieve and deliver something as large as a 4-meter-wide boulder to high lunar orbit, Lightfoot added.
NASA has already identified three possible targets for ARM’s boulder-plucking phase: the asteroids Itokawa, Bennu and 2008 EV5. Japan’s Hayabusa spacecraft visited Itokawa in 2005, and Bennu is the destination of NASA’s OSIRIS-REx sample-return mission, slated to reach the asteroid in 2019. No spacecraft has ever visited 2008 EV5, but NASA presently considers it the top ARM candidate. The final target selection will occur no earlier than 2019, Lightfoot said.
“[2008 EV5] has been extensively observed” using infrared and radio telescopes, said Lindley Johnson, program executive for NASA’s Near-Earth Object Program. Scientists have used those observations to pin down the asteroid’s orbit, as well as its size, shape, spin rate and composition. 2008 EV5 resembles a slowly spinning 400-meter-wide walnut, with a prominent ridge wrapped around its middle. It’s a carbonaceous asteroid, meaning it’s made of a mixture of rock, organic compounds and water-rich minerals thought to mirror the make-up of the primordial nebula from which our solar system first condensed. Scientists would love to get their hands on more of that stuff, but they don’t need ARM to do that, particularly since OSIRIS-REx is planning to return samples from Bennu, another primitive carbonaceous asteroid.
Then again, science is secondary for ARM. Its stated purpose is to test and develop new technologies for spaceflight, such as NASA’s Space Launch System heavy-lift rocket, its Orion deep-space crew capsule and an advanced solar-electric propulsion engine suitable for long-haul cargo trips. NASA is also pitching the missions as a step forward in demonstrating how a spacecraft can alter the orbits of potentially Earth-threatening asteroids—that’s the “Redirect” part of the ARM moniker.
According to Lightfoot, the current plan calls for a late 2020 launch of the robotic tug, then a 2-year cruise to reach its targeted asteroid. The robotic tug could linger at the asteroid for up to 400 days, carefully selecting which boulder to take. Once retrieved the tug would use the boulder’s extra mass to act as a “gravity tractor,” orbiting the asteroid in such a way as to subtly change the asteroid’s trajectory. The orbital shift would be slight, Lightfoot says, but measurable with ground-based instruments, and would be meant to demonstrate NASA’s ability to make more robust orbital shifts for future, Earth-threatening objects. Then, with the boulder in its clutches, the robotic tug would return to the vicinity of the Moon, to wait for the arrival of two astronauts in an Orion capsule as early as late 2025. The astronauts would dock with the robotic tug and conduct spacewalks to investigate the boulder before returning to Earth, spending a total of almost a month in space.
As exciting as this mission may seem, it is a far cry from the precursor proposals initially used to justify ARM, and many scientists and policymakers view its present form with lukewarm enthusiasm if not outright disdain.
ARM's genesis dates to 2010, when President Obama canceled plans to return to the moon and pledged to instead send astronauts to an asteroid by 2025. That approach, some experts thought, could rapidly take humans to Mars via near-term visits to the asteroid-like Martian moons Phobos and Deimos. The trouble was, NASA’s budget didn’t include enough funding to build the new heavy-lift rockets and deep-space crew capsules in time to meet that deadline. Sending astronauts to an asteroid in its native orbit would not be possible by 2025. But there was a loophole left by Obama’s vague language—what if NASA could instead send an asteroid to the astronauts? Thus, ARM was born. Now, with its latest iteration, even that watered-down objective is diminished. Instead of sending an entire small asteroid to high-lunar orbit for an astronaut rendezvous, a boulder from an asteroid’s surface is meant to suffice.
To critics, this steady shifting of goalposts bodes poorly for the mission’s future and suggests that ARM is an awkward kluge created primarily to fulfill arbitrary political deadlines rather than to bring NASA closer to human missions to Mars. In a January meeting of NASA’s Advisory Council, council members expressed skepticism that ARM was the right path for the agency to take in pursuit of capabilities many agree it needs, such as the advanced solar-electric engine. “If you’re going to spend $1.25 billion plus launch vehicle costs to do something,” said council chair Steve Squyres, a Cornell planetary scientist, “and you get the most important things by not going after the rock, don’t go after the rock.”
According to Mark Sykes, the director of the Planetary Science Institute and an outspoken ARM critic, plucking a boulder from an asteroid may be the most pragmatic and low-risk choice, but in pursuit of what?
“It is not at all clear how this mission is necessary to advance the stated objective of sending humans to Mars,” Sykes says. “Or even its vicinity.”
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Nasa's Plan to Visit an Asteroid Faces a Rocky Start
[https://www.scientificamerican.com/article/nasa-s-plan-to-visit-an-asteroid-faces-a-rocky-start/]