On November 20th, 2016 we learned about

ARM spacecraft aims to retrieve rocks, realign asteroids and rendezvous with astronauts

Only in the world of rocket science does sending a robot out to retrieve a boulder off an asteroid, divert that asteroid’s path, go into the Moon’s orbit and then rendezvous with astronauts count as “scaling back.” This incredible to-do list, bundled together as the Asteroid Redirect Mission (ARM), isn’t fully fleshed out at this point, but planning is on track for a 2020 launch date. If successful, the whole project would be a test run for various concepts and technologies, including a way to save our planet from possible extinction-grade impacts in the future.

Bagging a boulder

Once a suitable asteroid is selected, the ARM spacecraft will fly out to meet it in its natural orbit. The surface will be scanned to find a conveniently-sized boulder, at which point the spacecraft will slowly and carefully lower itself down over the boulder, resting on three legs that double as a huge, crane-game claw. Now, NASA’s not sending a spacecraft to an asteroid only to have it frustratingly drop its prize over and over, as with carnival crane-games, so a smaller set of gripping arms will lower down like a moray eel’s pharyngeal jaw, helping secure the boulder before lifting off.

Adjust an asteroid

As neat as this boulder retrieval is, the next step may eventually prove to be the most important part of the mission. With the captured boulder providing a bit of extra mass, the spacecraft will enter a small orbit near the asteroid, staying just close enough to create a gravitational influence on the larger rock. As long as the thrust of the spacecraft’s ion thrust don’t push directly on the asteroid, the spacecraft should act as a gravity tractor. Rather than try to push the asteroid into a new path, the gravity of the spacecraft and boulder will try to pull it to a slightly altered course. It’s a slow, gradual process, but theoretically it could save life on Earth from the fate of the dinosaurs. A small craft would only be an effective gravity tractor if it gets to an asteroid early enough, and if that asteroid isn’t too gigantic (say, under 1640 feet). On the plus side, it should work regardless of the structure of the asteroid, since even a pile of rubble would be pulled by the tugging tractor’s gravitational influence.

Sample some sediment

Once the ARM spacecraft finishes this potentially-Earth-saving proof-of-concept portion of its mission, it will carry its boulder into an orbit around the Moon. There it will be met by a crewed Orion spacecraft, delivering astronauts for a short spacewalk around the boulder. Samples will be taken, and then brought back to Earth with the crew once they’re done examining the captured piece of asteroid. Once the astronauts return home, the boulder will be placed in a wide orbit around the Moon, ensuring it doesn’t get caught in any kind of collision course with Earth.

Aside from being a complicated way to snag a boulder in space, this mission is actually paving the way for variety of future endeavors. Redirecting an asteroid via gravity tractor is currently thought to be one of our best strategies for heading off dangerous collisions, but we need to verify that before relying on it. This mission would allow rigorous testing of new Solar Electric Propulsion (SEP) thrusters, which are expected to play a bigger role in future spacecraft. And while the astronauts may only interact with the boulder in its final phase of the mission, putting the new Orion spacecraft, launched by the Space Launch System rockets, into the Moon’s orbit will be an excellent model for eventual crewed missions to Mars. Oh, and in case that wasn’t enough, the boulder itself may also tell us more about the formation of our solar system. The only compromise was that we’re no longer dragging the entire asteroid to the Moon.

My second grader asked: How big is the asteroid? Could it hit the Earth? Like the one that got the dinosaurs?

Obviously, the folks at NASA live here too, and really don’t want to send big rocks crashing down on us when figuring out how to prevent exactly that. They also realize that nobody’s perfect, so they’re purposely targeting asteroids that aren’t big enough to wipe out life on Earth. So while the Chicxulub asteroid that wiped out most of the dinosaurs was thought to be nearly 6.2 miles in diameter, the leading asteroid being considered thus far are less than 1,300 feet across, and is over eight-times the Moon’s distance from Earth. The boulder, which will of course be brought much closer to home, can only be 13 feet across, which isn’t big enough to survive a trip through our atmosphere, even if it did shake loose of the Moon’s gravity. This is one of the advantages of only grabbing a boulder, and leaving the bulk of the asteroid further out in space.

Source: NASA's Asteroid Redirect Mission Emerges from First Planning Stages by Sarah Lewin, Space.com

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