Since 2012, NASA has been trying to figure out how to capture an asteroid and bring it back to Earth. This is a good idea for a bunch of reasons, but there aretwo big ones (according to NASA). First,the mission will help develop technologies that could be used to redirect an asteroid thats on a collision course with Earth.And, second,snagging an asteroidand dragging it into lunar orbit so a manned spacecraft can poke around itwill be a useful way to prepare humans for deep-space travel, eventually, to Mars.

Last week, NASA announced a much more detailed plan of exactly what this asteroid redirect mission will entail. As expected, its a bit more conservative than the original concept for the mission, but with (the agency hopes) a substantially better chance of success.

NASA's original idea was to go out and find a near-Earthasteroid with a diameter of about 8 meters and a mass of about 500 metric tons, which, for the record, isnot big enough to make it through Earths atmosphere intact. Once the spacecraft got to this asteroid, it would capture it inside a giant container of some sort (a net or bag), and then haul it back towards Earth.

The problem with this approach is thatits a one-shot deal: if the capture container fails for some reason, thats it, youre done, and the two year, US $1.25-billion mission amounts to something depressingly close to zilch. Instead, NASA has scaled back the Asteroid Retrevial Mission (ARM) into aTiny Little Piece of an Asteroid Retrevial Mission (TLPARM). Rather than trying to grab an entire asteroid all at once, NASA's spacecraft will arrive witha giant claw. After scouting the asteroid for up to 400 days, NASA will choose a likely looking boulder (3m or so in diameter), and then play the most expensive claw game ever to try and land the spacecraft right on top of it and make the snag. NASA speculates that theyll have between three and five quarters tries.

That bit at the end abouttransitioning toplanetary defense demonstration means using the spacecraft (with the boulder in tow) as a gravity tug.

A gravity tug is a really niftyway of changing the trajectory of something massive (like an asteroid) using something small (like a spacecraft). Everything is effected by the gravity of everything else, and if you sticka spacecraft near an asteroid, the asteroid is going to get pulled a little tiny bit towards the spacecraft. The spacecraft is going to have to deal with a much stronger pull from the asteroid, of course, but the spacecraft has thrusters to compensate for that, and the asteroid doesnt.

The amount of pull that the gravity of a spacecraft that weighs a few tons has on an asteroid that weighs hundreds or thousands of tons is barely noticeable (hundredths or thousandths of a newton), but it's there. Given enough time (like, decades), the spacecraft could nudge the asteroid enougha change in velocity of perhaps one centimeter per secondto make the difference between obliterating the Earth and a near miss that wed probably not even bother to blog about.

To test out this concept, NASA will have its ARM spacecraft orbit the asteroid just ahead of its center of mass, which should ever so slightly pull the rock towards the spacecraft. As a bonus, this will be after the spacecraft picks up the rock, since more mass on spacecraft plus less mass on asteroid equals everything working that much better. Once NASA has determined whether this gravity tug idea works well in practice, the spacraft (with rock in grasp) will make its way into a lunar orbit over the course of about six years.

In order to get the level of propellant efficiency that a mission like this requires, NASA will be relying on Solar Electric Propulsion (SEP), or more specifically, Hall effect thrusters. Until someone figures out how to convert energydirectly into thrust, SEP is one of the most efficient and reliable ways of propelling a spacecraft. Rather than relying on messy chemical reactions, Hall thrusters use electricity (harvested from solar panels) to accelerate xenon ions through a charged grid. The electricity is renewable, and since all (or, almost all) of the propellant gets turned into thrust as opposed to heat or other byproducts, SEPs efficiency is hard to beat.

The downside of SEP is that just tossing xenon out the back of your spacecraft isn't going to generate a huge amount of thrust, even if each xenon ion is reaching the ludicrious speedof 30km/s. A 10 kilowatt Hall thruster (NASA is planning on using four of these on the asteroid redirect spacecraft, plus one spare) can probably produce about 500 mN of thrust, or about the weight of 50 business cards. If you're fighting gravity, this is nothing, but if you're in space, its plenty, as long as you can keep your engines going for a very long time. And this is where SEP shines: the specific impulse of these Halleffect thrusters is3000 seconds.

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NASA Details 2020s Asteroid Capture Mission