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 are two big ones (according to NASA). First, the mission will help develop technologies that could be used to redirect an asteroid that’s on a collision course with Earth. And, second, snagging an asteroid and dragging it into lunar orbit so a manned spacecraft can poke around it will 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, it’s 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-Earth asteroid with a diameter of about 8 meters and a mass of about 500 metric tons, which, for the record, is not big enough to make it through Earth’s 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 that it’s a one-shot deal: if the capture container fails for some reason, that’s it, you’re 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 a “Tiny Little Piece of an Asteroid Retrevial Mission” (TLPARM). Rather than trying to grab an entire asteroid all at once, NASA's spacecraft will arrive with a 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 they’ll have between three and five
That bit at the end about transitioning to “planetary defense demonstration” means using the spacecraft (with the boulder in tow) as a gravity tug.
A gravity tug is a really nifty way 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 stick a 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 doesn’t.
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 enough—a change in velocity of perhaps one centimeter per second—to make the difference between obliterating the Earth and a near miss that we’d 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 energy directly 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, SEP’s 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 speed of 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, it’s 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 Hall effect thrusters is 3000 seconds.
I’m going digress for just a sec to explain what specific impulse is, since measuring efficiency in seconds is not at all intuitive. Engine efficiency is how much thrust it's able to produce per unit of fuel, and here’s what that equation looks like:
Thrust (kg-m)/s2) = Propellant flow rate (kg/s) x Gravity (usually Earth standard, m/s2) x Some arbitrary measurement of the efficiency if your engine
Since you can measure the thrust of an engine, and you can measure the mass of propellant that it sucks down, and you can measure gravity, you can solve for efficiency without too much trouble. And when you do, all the units cancel out except for one leftover “seconds” unit, and that’s why this measurement of engine efficiency is in seconds.
It's not particularly useful to try and figure out how to turn that “seconds” into something with physical meaning. But if you really want to, setting the thrust of the engine equal to the mass of the engine's starting propellant at one standard gravity turns the specific impulse measurement into the time in seconds that it would take for the engine to run itself out of fuel.
To give you a sense of what specific impulse means in practice, here’s a fun little list of the specific impulse of different space propulsion systems; it’s fun because it reinforces the fact that efficiency has absolutely nothing to do with the amount of thrust an engine is able to generate:
- Space Shuttle Solid Rocket Boosters: 286 s
- Space Shuttle Main Engines: 452 s
- Saturn V: 421 s
- Apollo Lunar Lander Descent Stage: 311 s
- Hall effect thruster: 3,000 s
- VASIMR Magnetoplasma Rocket: 5,000 s
- ESA’s experimental DS4G ion thruster: 19,000 s
Before we finish up with this specific impulse business, it’s worth mentioning that specific impulse is called specific impulse because it's referring to the total impulse of an engine (average thrust x total firing time) divided by the weight.
Anyway, back to NASA’s asteroid mission! Here’s what happens once the rock gets dragged back to lunar orbit:
Just like the animation shows, in the mid-2020s two astronauts will fly an Orion capsule out to rendezvous with the asteroid as part of a 24-25 day mission that will take them farther than humans have yet ventured into space.
So what’s next? NASA still has to identify exactly what asteroid they want to go steal a chunk of, which won’t happen any earlier than 2019. There are several candidates at the moment including Itokawa, Bennu, and 2008 EV5, and NASA expects to add another couple every year. The agency is looking for just the right combination of size, shape, mass, orbit, and rotation, among other criteria. The mission should launch in 2020.