Using Gravity To Tow Asteroids

cryptocom writes "Space.com is reporting that two scientists at NASA are proposing using a 20-ton spacecraft to pull asteroids off a possible collision course with Earth, using the spacecraft's own gravity as an attractor. This idea would not only be cheaper, but have a much higher chance of success, due to not having to actually land on the asteroid's surface."

mir

[elinden]

mir weighed 135 tons and it burned up just fine on its way down. 20 tons, relatively speaking, isn't really all that much.

Re:It's been awhile since I've taken physics...

[twiddlingbits]

Wouldn't that would be 10^8 Newtons of force pulling the spacecraft toward the asteriod? The asteriod is much more massive and would have a gravity well of it's own. Wouldn't that attractice force have to be overcome for 20 yrs, plus a slight acceleration in the direction the asteroid needed to move? The 20 ton spacecraft would have a higher force of gravity on the 'roid than that of the Sun for 20 yrs (or however long the tractor lasts) so it could gradually change to orbit, How do we make things that can stay in space for 20 yrs w/o repair? How do you get that much fuel on-board? Solar Cells are not an option that far from the Sun. A nuclear reactor maybe but they would have all sorts of issues there, even if the "tractor" was not launched from Earth, the fission elements would have to be launched as I don't think you find Uranium just floating in space. And heaven forbid someone mis-calculates and they push it onto a collission course..they it takes another 20 yrs to fix that! This article sure makes a LOT of assumptions and figure on new ideas/technologies we don't have. It is a neat idea but IMHO it belongs under the topic of Science Fiction not in a journal like Nature.

Re:that's what i was thinking

[Anonymous Coward]

But from the article, they'd really only need about 1 pound of force over a decently long time to shift the orbit significantly. So, bulk is what you need. Attach some thrusters to a big rock or space junk, and you've got what you need. Incredibly light but high powered wouldn't work, as attempting to tether to asteroid would likely just rip the asteroid apart.

Again, the expense in getting the JIMO up is launching all that weight. Ion drive would work just fine if we can figure a way to tether the drives to the counterweight mass.

Re:It's been awhile since I've taken physics...

[n0dalus]

When dealing with approximately spherical objects (such as this asteroid), the entire thing can be treated as though it is a point mass in the center (As long as the second mass is not INSIDE the first one, then you get problems). As for the ship, it's too small to matter how it's shaped in this scenario.

Re:Relative position of spacecraft to asteroid

[Alderin1]

Sounds to me like it will be travelling in parallel with the asteroid, but being that it is using gravity, it will lose "altitute" relative to the object and need to regain it by thrusting away from the object without pushing the object or damaging it, thus bending the orbital arc toward the craft ever so slightly.

bah

[toiletmonster]

...if I simply ask you what you think your life is worth, you may be tempted to exaggerate, so economists judge people based on their actions. Dangerous jobs tend to earn higher pay, while activities such as buying smoke alarms or buckling seat belts carry their own costs as well as bringing safety benefits.

A typical calculation: you might pay up to $6,000 on a safer car that reduced your risk of dying by one in a thousand. Six thousand divided by one in a thousand is six million, so you are valuing your own life at about $6m. This is a typical result for residents of the US.

http://www.timharford.com/deareconomist/2005/10/va lue-of-iraqi-life.html [timharford.com]

Re:that's what i was thinking

[SteveAyre]

Launching the craft. How much fuel would it take to get escape velocity on something this massive? Probably not a small amount.

20 ton spaceship. That's actually smaller than the Space Shuttle.

I can't remember the source now, but the Shuttle can lift about a 30ton payload. The boosters themselves can lift far more but of course have to carry the Shuttle too (which is something like 120tons).

The Saturn V rocket [wikipedia.org] was capable of lifting 118 tonnes (with the 3 stage versions).

The Shuttle Derived Launch Vehicle [wikipedia.org] will have a capacity of 125 tonnes.

All seem plenty to lift a 20ton spaceship if it's the only thing being launched. Even with a Shuttle it should be doable, or we can have another up there waiting to deploy it or use the ISS team.

Since we're able to use existing launchers to get the spaceship into orbit, it shouldn't cost any more to launch than any existing mission. All your left with is the pricetag for building it and giving it enough fuel to reach the asteroid.

The crew. The time the crew would be away from earth would be how long? 10 years? 20 years? Managing and provisioning crews for such a long amount of time is probably among the major challenges facing the extension of our space travel abilities.

Coming home. What happens when a ship this large is re-entering Earth's atmosphere? That sucker will have a lot of force coming down.

Due to the distance it would have to travel a robotic mission remote controlled from Earth would make the most sense. Just in case anything breaks which isn't workaroundable/fixable it would probably make sense to send more than one.

In this case it'd be best to leave it out there - without enough fuel to return it'd be cheaper and without a crew to bring home there's no real reason to.

Shelf life. So we make a ginormous space tractor. Maybe we don't face an asteroid threat for 15,000 years. That's a lot of upkeep.

The launchers are already around, and it wouldn't take long to build a ship which is essentially a remote controlled engine with a lot of metal attached.

Assuming that we'd know of the threat in enough time to send this to the asteroid, as long as we still have the launchers to get it into space in the first place it shouldn't be unreasonable that we can build them as we need them.

If we don't have that kind of timescale then we're probably in trouble even if we could send it straight away. Since the launchers seem capable of lifting more than 20 tonnes though, we could just build a 40 tonne version and half the time we'd need (disclaimer: not linear, i think it'd be more like 3/4?).

Re:that's what i was thinking

[Impy the Impiuos Imp]

Wouldn't the ions shot out the ass end smack into the asteroid, counteracting the gravitational pull?

You'd have to have at least two, and arguably more, ion jets pointed away at opposing angles such that, combined, they push the craft in the correct direction, but individually, they shoot the ions past the edge of the asteroid. Like legs on a tripod, say.

Re:How about simply attaching stuff on the asteroi

[phritz]

If you just attached a mass of stuff to the asteroid, wouldn't that change its course over time because the gravitational effects from all the heavenly bodies would then be different?

Nope. All gravitational force exerted on an asteroid is directly proportional to its mass. Since Force=Mass*acceleration, the mass cancels out of the equation - the acceleration (and hence the trajectory) is independent of its mass. This is the exact same reason why objects of different weights fall at the same rate - both acceleration AND gravitational force are proportional to mass, so mass cancels out of the equation.

Of course, I'm assuming that gravitational interactions with objects of comparable size (i.e. other asteroids) are negligible. My argument only holds if everything the asteroid interacts with is much more massive than it (and thus not affected much by the asteroid's gravitational pull). But I would bet that's a pretty good assumption.

Re:that's what i was thinking

[Rei]

You're not getting it:

F=G*m1*m2/d^2

Cutting the distance in half quadruples the force of gravity; distance is far more relevant than mass. You can pick how much gravitational attraction there is between your craft and the asteroid (within limits, but those limits aren't closed to being reached)

Lets say that your craft has the new HiPEP engine under testing (JIMO's engine). I believe that's a 0.5 newton engine (very strong for an ion engine). It takes the entire 20 tonnes of craft mass to run. Lets assume that the 200 meter diameter asteroid is porous rock with a density 50% more than water - 4/3*3.14159*(200/2)^3*1500kg=6.28e9kg. Thus, the balanced equation becomes:

0.5=6.67e-11 * 20000 * 6.28e9 / d^2

Solving for d, we get 130 meters, or 30 meters over the surface. However, it gets better: asteroids are almost never uniform spheres, and are usually somewhat oblate. Thus you can hover closer to its center of gravity if need be. So, even if we can improve the mass of the engines, radiators, or engines, we can simply fly closer to the asteroid. The larger the asteroid, the more this holds true: double the diameter of the asteroid, the hovering distance increases fourfold from the center (and 5.5x the distance from the surface). You'd have to make your craft more efficient by orders of magnitude in order to justify adding dummy weight to it.

*You Do Not Need Extra Bulk*. With the limits of modern technology, you inherently have more than enough bulk just in the functional mass of the spacecraft. And even if you needed extra bulk, scientists would *love* to supply it in the form of a science payload.

Sorry, but this won't work.

[Physics Dude]

They do the force calculations, but don't think this through far enough.

Assuming a spherical iron asteroid with a 100m radius (the article mentions two football fields across) and a 20 ton ship you can provide a maximum gravitational force of about 1 pound. This is find and dandy and could provide a deflection of nearly the diameter of the earth over a decade period.

But...

The problem is how to produce that required force on your ship without impacting the asteroid. Conventional rockets or ion thrusters would necessarily be directed in the direction of the asteroid which would nullify any net force on the system (ship+asteroid). If you get enough distance between the asteroid and the ship so yout thrust can miss the asteroid and provide a net force, the force you can provide on the asteroid due to gravity drops as the square of the distance and becomes unusably low. You'd need litterally centuries or millenia of advance warning!

If anyone has ideas how to avoid this problem, I'm all ears. :)