Gravity Tractors to deflect asteroids (43)

1 Name: Sling!XD/uSlingU 2005-11-09 21:01 ID:m2cuQwWi
"A spacecraft could use a gravity "towline" to alter the course of an Earth-bound asteroid, a new study by two NASA astronauts suggests.

Previous schemes to deflect an incoming space rock range from landing a spacecraft on the asteroid and pushing it off course to blowing it up with nuclear weapons.

The new plan takes a gentler approach. A spacecraft would hover above the asteroid and gradually pull it off course using nothing more than the gravitational attraction between the two bodies."

2 Name: !WAHa.06x36 2005-11-10 00:42 ID:GMDnEsDq

Now, I didn't bother looking up the details about this, but the one thing that bothers me is this: The idea is sound, but it involves the spaceship pushing against the asteroid's gravity with its engines.

However, this would most likely entail aiming the engines right at the asteroid, and transferring momentum to it in the opposite direction when the exhaust hits its surface. You'd have to use some very weird configuration or engine to get around that.

3 Name: !WAHa.06x36 2005-11-10 00:43 ID:GMDnEsDq

Well, actually looking at the pictures, they've got several engines aimed outwards in opposing directions. That would work, and fall under "weird configuration".

4 Name: Sling!XD/uSlingU 2005-11-10 04:10 ID:m2cuQwWi

>it involves the spaceship pushing against the asteroid's gravity with its engines.

Huh, no.
It's "A spacecraft would hover above the asteroid and gradually pull it off course using nothing more than the gravitational attraction between the two bodies." (>>1)

In other words: "the gravity tractor would travel alongside the asteroid and gradually pull it off course, using nothing more than the gravitational pull between the two bodies."

The ship is not in contact with the asteroid.

5 Name: !WAHa.06x36 2005-11-10 12:53 ID:7GotplGe

I never said it was.

See, if the ship just tried sitting there, the asteroid's gravity would pull at it, and it's crash into the asteroid. It needs to keep its engines on constantly to counter-act the asteroid's gravity.

If there were no engines involved, the center of mass of the ship-asteroid system would never move. The ship would fall towards the asteroid, and the asteroid towards the ship, until they collided and came to rest. The engines keeps the ship out of reach of the asteroid, and the asteroid slowly falls towards the ship, which constantly moves out of the way. Thus, momentum is transferred to the asteroid.

6 Name: Sling!XD/uSlingU 2005-11-10 18:54 ID:uCrunfsK

Constantly? Yuk, what a waste of energy then.

Okay, slashdot picked it up.
Some points they said that I picked up:

  • it may be less expensive to just throw a bomb at it than having to launch the big mass in orbit, accelerate it to the asteroid, decelerate and then monitor it for 20 years.
  • it should be used only for asteroids that one cannot land on. Mounting a periodically firing rocket on the asteroid is more efficient.
  • if the asteroid is very large then one can forget using this method.

7 Name: !WAHa.06x36 2005-11-10 20:05 ID:GMDnEsDq

There's really no way to move an asteroid without using up a whole lot of energy. Also, I wouldn't go to Slashdot for physics advice.

The bomb idea is generally considered ill-advised. Nuclear weapons would most probably not impart a whole lot of momentum on the asteroid if they are exploded next to it. The vaccuum wouldn't provide any method for momentum transfer except for radiation pressure, which is very, very low. To use a bomb to move an asteroid, you have to get it onto or preferrably below the surface, which is very difficult, and can have unpredictable effects - you might even break up a small asteroid, and that just creates several problems where you initially just had one.

Landing on an asteroid and securely attaching an engine is very hard unless you send actual people along. Bringing people means a much more complicated project. You have to keep the people alive for the duration, and you have to bring them back. That's the whole point of this proposal: Hovering above an asteroid is much easier to do than landing and securely setting up the engine.

The asteroid size, counter-intuitively, doesn't affect anything but the engine size needed. One's first reaction would be that the gravitational pull from a twenty-ton spaceship would be miniscule in relation to the huge mass of a big asteroid. However, gravitational force is proportional to the masses of both objects involved. That means that the asteroid will accelerate just as much if it weighs one kilogram, or a billion tons. However, the asteroid's force on the spaceship will vary. So to move a bigger asteroid, you need a bigger engine to keep from crashing into it.

However, for pretty much any asteroid, the gravitational pull will be miniscule, which is why this is a feasible plan. You can get away with using a very weak engine that lasts a long time, such as an ion engine.

8 Name: Sling!XD/uSlingU 2005-11-10 23:43 ID:uCrunfsK

Ok, 'sounds good.

9 Name: Mad Scientist 2005-11-12 12:27 ID:Heaven

> Landing on an asteroid and securely attaching ... unless you send actual people along...

Yknow, this reminded me of that shitty Bruce Willis flick.. "Armageddon".

10 Name: !WAHa.06x36 2005-11-13 03:07 ID:GMDnEsDq


Indeed, this whole plan is an attempt to avoid sending Bruce Willis into space.

11 Name: Mad Scientist 2005-11-14 13:22 ID:26M3xsSb

Is still agree with using nuclear arms against an asteroid. None of the hollywood bullshit where you land and drill a hole. 4 or 5 Tactical Nuclear Missiles (Minuteman III and Peacekeeper LGM 130 missiles can acheive escape velocity and reach space, since older ballistic rockets were used to launch man into space.) And blast the asteroid on one side. Now, this will not destroy it, but the force of 4 40 Megatonne Nukes will deflect it out of harms way.

12 Name: Mad Scientist 2005-11-14 15:24 ID:GMDnEsDq


So you'd think, because you're used to the idea of nukes being set off in the atmosphere. However, this is the vaccuum of space. There's no atmosphere to heat up and create a shockwave in. The only form of energy transfer you have is the tiny amount of bomb material that is ejected from the explosion, and the gamma radiation from the blast. Neither will impart any significant momentum on the asteroid.

The only chance you have to affect the course of the asteroid is if the blast vaporizes a large amount of material off the surface of the asteroid, and this material is ejected at high enough velocity to nudge the rest of the asteroid in the other direction. But as nobody has tried to do this, we have no idea if it would be enough or not. That's why the idea is a bad one - we have no idea if setting off nukes next to an asteroid would do anything useful at all.

Of course, there's a surefire way to find out for sure.

13 Name: Mad Scientist 2005-11-14 19:58 ID:R1gsHtm9

we also have no surefire way to prove the gravity tractors would work. Nor any of the other plans to avert and asteroid impact

14 Name: !WAHa.06x36 2005-11-15 13:21 ID:7GotplGe


We are pretty sure that gravity works just fine elsewhere in the solar system. The maths to calculate the amount of course change is easy enough to do. It's a simple, well-understood problem.

Nukes are a whole different thing, and their effects are massively complex, and only well studied in the atmosphere.

Equating the difficulty of the two for the reason that we haven't actually tried either is just plain silly.

15 Name: Mad Scientist 2005-11-15 21:26 ID:R1gsHtm9

Now, building a craft large enough to effect the gravity of a extremly heavy iron based space rock, and then the expenses as well as building a launch vehical to get that craft into space. Taking the gamble of the systems functioning, and if one system goes down on this, well... its the end.

16 Name: Mad Scientist 2005-11-16 01:52 ID:Heaven

The plan calls for a craft weighing 20 tons. That's within the carrying capacity of several different lifters we have at the moment, I'm pretty sure.

With nukes, you're taking a gamble with much more complicated equipment. A nuke isn't just a lump of explosive and fuses, it needs highly sophisticated triggering electronics. And getting it there requires just as much sophistication as getting the gravity tractor there.

You know, people did think this through before publishing the idea.

17 Name: !WAHa.06x36 2005-11-16 01:56 ID:GMDnEsDq

Also, >>12 and >>16 were me, orz.

18 Name: Mad Scientist 2005-11-16 14:23 ID:FVNfFQIr

idea seems plausible enough but what are the odds of us spotting this asteroid before it's too late?

'sides this takes a pretty insane amount of time right?

(would check my facts but apparently the internet is down, aside from 4-ch for some reason and i can't really be bothered to do it later because i'm a sociopath)

19 Name: Mad Scientist 2005-11-16 21:06 ID:R1gsHtm9

Ok, but if a 20 ton craft isnt large enough... What about a really heavy massed asteroid. If the Craft is too light, it aint gonna do anything except hit earth with the rock. There are too many things. A nuclear launch system is much less complex than a modern space launch system.
I understand there was much thought, and it is rather plausible. but you must also look at what could go wrong. Say the engines fail before it gets off the ground, mid air explosion, system failure on way to asteroid. Anything can happen, and if thats the case, we cant just go, oh... lets try again. in this situation we get one shot at it.

20 Name: Mad Scientist 2005-11-16 21:39 ID:oTAVv+WE

>What about a really heavy massed asteroid.

Elementary physics: a heavy asteroid will be attracted just as much as a light one.

>A nuclear launch system is much less complex than a modern space launch system

A nuclear launch system is first and foremost less capable than a modern space launcher. Try leaving LEO with a stock ICBM.

Nukes are a very poor way to deflect asteroids. It's not so much about the bomb's complexity, but the result of the explosion is hard to predict. You don't know enough about the asteroids composition.
Even if you can get a significant impulse out of the explosion in the first place (and that requires getting close, probably even landing - see Hayabusa/Minerva if you think that's so easy), as long as you don't hit the center of gravity you just might send it spinning.

21 Name: !WAHa.06x36 2005-11-17 12:57 ID:7GotplGe


>>20 answered most of your questions, some of which I had already answered earlier in the thread, if you had actually read it. I'll just address the final part:

> but you must also look at what could go wrong. Say the engines fail before it gets off the ground, mid air explosion, system failure on way to asteroid. Anything can happen, and if thats the case, we cant just go, oh... lets try again. in this situation we get one shot at it.

Your arguments there are against nukes! Do you somehow think it's better to have a mid-air explosion of a nuke than a twenty-ton lump of metal? Also, the whole point of the plan is that is launched long in advance - we'd have a lots of time to launch another, and a third, if there was an accident.

22 Name: Mad Scientist 2005-11-18 03:33 ID:R1gsHtm9

No, the nuclear weapon would not detonate before the trigger point, the engines could explode, but that would cause no problem with the Nuclear payload. And if the Nuke fails to get off the ground, within moments later, another one can be launched from a different silo. If the Craft fails to get off the ground, its the single shot if we miss we're fucked.

23 Name: !WAHa.06x36 2005-11-18 12:52 ID:7GotplGe

You think that pulverizing many kilograms of plutonium in the atmosphere is "no problem"?

You also completely ignored the arguments we already placed against you: The gravity tractor would not, I repeat, not have only a "single shot". There would be many years to build and launch any number of them.

Furhtermore, a ballistic missile can not even reach orbit - they're called "ballistic" for a reason, being that they fall back down. They most definitely can not reach escape velocity. You need pretty much the exact same equipment to get a nuke out to an asteroid as you need to get the gravity tractor there.

24 Name: Mad Scientist 2005-11-18 15:39 ID:Heaven


>[...]within moments later[...]

Why would "moments" matter?
As >>23 said, one would have years or decades for another try.

>If the Craft fails to get off the ground, its the single shot if we miss we're fucked.

You are probably watching too many Hollywood movies.

25 Name: Mad Scientist 2005-11-19 07:01 ID:R1gsHtm9

>>23 You are wrong in this effect. Ballistic Missiles are in fact capible of reaching orbit. The only modifications made to the Titan Missiles in the 1960's was the fact that a warhead was removed and that section of the rocket was modified to fit a cockpit and enviromental system to accomidate a human. We used early ICBMs to launch astronauts into orbit. A Peacekeeper-3 Missile could be launched from Washington D.C. and hit Akron Ohio after circling the globe once. Now, launch that in a vertical trajectory. It needs a high velocity to be able to fire that range.

Would you rather spend several billions of dollars on building each individual craft, which must be rolled out to a launch pad and prepared before launch. or press a button and fire somthing from a silo. And as I also stated, if we make a miscalculation on the mass of the asteroid, the space craft will stand no chance at diverting it. If its too light, you just add onto the debris heading for earth. And how much resources would be used to make these craft?

26 Name: Mad Scientist 2005-11-19 09:56 ID:Du29sZ+G

I'll give it a last try.

>Ballistic Missiles are in fact capible of reaching orbit.

"Orbit" being of course a ~300 km low earth orbit.

>or press a button and fire somthing from a silo

You don't have a clue about ICBMs either, do you?
Flight control for an ICBM is laid out for a ballistic curve, not for interplanetary space. As soon as the booster rocket runs out the warhead is unpowered. No course corrections by a control center, no cameras, no other sensors of any kind. And designed to split up into a dozen MIRVs.
So one would need a new warhead, a new flight control system, and a vastly more powerful booster. There's your custom build.

>if we make a miscalculation on the mass of the asteroid

(M = mass of the spacecraft, m = mass of the asteroid, a = acceleration imparted on the asteroid, r = distance between spacecraft and asteroid, G = gravitational constant)

G*M*m/r^2 = m*a
=> a = G*M/r^2

The mass of the asteroid is irrelevant for the tractor.

A nuke on the other hand gives you a fixed impulse p. The imparted change in velocity would be:

v = p/m

A miscalculation of the asteroids mass is only a problem if you use a nuke.

27 Name: !WAHa.06x36 2005-11-19 19:12 ID:GMDnEsDq

Yes, ICBMs can be modified to reach LEO - now, pray tell, how are you going to get from LEO to the asteroid? Just because you're in space doesn't mean you can magically go anywhere you like.

As >>26 pointed out, you are severly confused about even the most basic physics involved. I should also point out this line:

> Now, launch that in a vertical trajectory.

Do you really think that escaping Earth is done by flying straight upwards? And do you really think an ICBM would have enough thrust to do that?

To recapitulate, here are the arguments against nukes:

  • ICBMs can't reach an asteroid, and you need a spacecraft just as advanced as the gravity tractor.
  • There is very little energy or momentum transfer through the vaccuum of space, so a nuke has to be landed on the asteroid to do anything. Landing is very, very hard.
  • We do not know how much momentum a nuclear explosion would impart on the asteroid, so predicting the result is impossible.
  • You do not know the mass or composition of the asteroid very well. If your nuke is too powerful, you might break it up, and the problem will be much, much harder to deal with.
  • If the impulse of the nuke is not centered properly on the center of mass of the asteroid (which you do not know exactly where it is), the explosion will send the asteroid spinning. This makes future landings harder, if not impossible.

Whereas the gravity tractor has the following benefits:

  • Doesn't need to land.
  • Will affect any asteroid of any mass just as much, due to the nature of the force of gravity.
  • Can move a spinning asteroid just as well as a stationary one, because it does not need to land.
  • Will not crack the asteroid into parts.
  • The effect is easily calculated and predicted, as opposed to the effect of a nuclear explosion.

28 Name: Mad Scientist 2005-11-19 20:32 ID:R1gsHtm9

You are wrong on this part: "Will affect any asteroid of any mass just as much, due to the nature of the force of gravity."

The nature of the force of gravity is what you say, but a larger massed asteroid will have more mass than the gravity tractor. Thus making the gravity tractor just a satillite of the asteroid, and now a man made peice of debris hurtling at earth as well. There is no certain way to measure the mass of an Asteroid, only estimations. The gravity Tractor will use alot of resources just to build. Resources we are now running low on to begin with. Time and money constraints to somthing never even tested. I dont trust it. Frankly, I see it as the earth is screwed none the less.

29 Name: !WAHa.06x36 2005-11-19 22:04 ID:GMDnEsDq

Obviously the thrust of the tractor would have to be adapted to the asteroid size, but asteroids of pretty much any size have a very, very weak gravitaitonal fields, and are not likely to overwhelm the thrust of the ship. You're right that we wouldn't know the exact mass, but we'd know it within an order of magnitude easily, which is enough by far for planning the mission.

We're not gonna go out there and suddenly notice the whole asteroid is made of neturonium.

30 Name: Mad Scientist 2005-11-19 23:21 ID:PEet4s0c

>Thus making the gravity tractor just a satillite of the asteroid[...]

A few numbers:
The asteroid mentioned in the article has a mass of ~5e10 kg. A spacecraft hovering in 100 m distance will have to counter 5.3e-5 m/s^2 of acceleration and needs another ~2e-11 m/s^2 to "pull" the asteroid. Assuming a 20000 kg spacecraft this requires ~1.2 N of thrust (firing at an angle of ~30° in this scenario). This is achievable with ~15 NSTAR ion engines (as used on Deep Space 1). For redundancy and safety reasons let's double or triple the number of engines, the things are small enough.

>Time and money constraints to somthing never even tested.

It's all tested technology and a relativley simple design to boot. A few dozen ion engines plus reaction mass, enough solar panels/RTGs to power them (power supply would actually be the biggest problem), and a few tons of scrap metal to make up the bulk of the spacecraft.
Time constraints: the most "dangerous" asteroid discovered to date (99942 Apophis from the article) had its possible impact predicted for 2029. Considering the US space program went from barely reaching orbit to landing on the moon in 10 years, this should be quite enough time to react.
Money constraints: we are talking about a few billions for the whole program. That's hardly unaffordable.

31 Name: Mad Scientist 2005-11-20 08:04 ID:R1gsHtm9

Have we successfully altered the course of a major body in space with this technology? Thus, it has never been tested. From computerized simulation to real life situation it is very different, many factors. Yes, and we put lives on the line, and nearly lost three Men in space for the fact of faulty equipment during the lunar missions. As well as losing 14 good people in two accidents involving Faulty equipment in the last 20 years. I dont know if any of these will successfully work.

And im not just talking the metal to build the craft. The fuel required to launch it, waiting for the appropriate launch window, hoping for a fair weathered day for that window... Also the fact that Ion engines are not yet fully endorsed by nasa, and the launch of a craft that will test out the Ion engine for the first time in space, has been indefinately suspended.

32 Name: Mad Scientist 2005-11-20 11:58 ID:NANMovs/

>Have we successfully altered the course of a major body in space with this technology? Thus, it has never been tested.

The "technology" in question is gravitational attraction. I would say it is successfully tested.

>I dont know if any of these will successfully work.

Sure. Let's just ignore the 8000 satellites in earth orbit and the 400+ planetary and deep space probes.

>The fuel required to launch it,

Titan IV launcher. 25 t to LEO. 350 million USD.
~90 kg of Xenon per engine per year. For a 30 engine configuration and a 5 year mission this will cost ~16 million USD.

>waiting for the appropriate launch window,

Launch window could be tricky, but one would still know it years in advance. And as soon as a spacecraft can travel under continuous acceleration it is no longer bound to only a handfull of low energy transfer orbits anyway.

>[..]test out the Ion engine for the first time in space[...]

I specifically chose an existing ion engine that has already been put to use.

33 Name: !WAHa.06x36 2005-11-20 12:02 ID:Heaven

You're not making any kind of sense at all any longer.

34 Name: Mad Scientist 2005-11-21 13:25 ID:26M3xsSb

Yes, its been prooven in the sense that the planet has gravity. I dont remember any of these 8000 or so satellites managing to move the earth. We have never built an object to Manipulate the course of a preexisting natural body in space.

Ok... Its been tested once. Can we be absolutly certain that it will work every single time?

35 Name: !WAHa.06x36 2005-11-21 17:40 ID:Heaven

36 Name: Mad Scientist 2005-11-21 20:49 ID:Heaven

37 Name: Alexander!DxY0NCwFJg 2005-11-21 22:18 ID:Heaven

38 Name: Sling!XD/uSlingU 2005-11-21 23:49 ID:Heaven

39 Name: Mad Scientist 2005-11-30 00:12 ID:Heaven

This thread has peacefully ended!

40 Name: Mad Scientist 2005-12-02 15:01 ID:Heaven

41 Name: Mad Scientist 2005-12-04 21:09 ID:Heaven

This thread has ended with much confusion.

42 Name: Mad Scientist 2005-12-09 16:10 ID:MwVSCQKf

I'm with >>32. What is there to prove about gravity? I think the most mindboggling thing is that even a flea jumping about alters the course of the earth through gravity. Only minimally.

When talking about an asteroid the size of NewYork, it is just as difficult to imagine that a bus-sized probe could actually alter its course. It can: it's just a matter of forces. But it will happen excruciatingly slowly. So, If we are talking about sending asteriods, scheduled to crash with earth in 10 years or so, and we start with the project now, then I think it can work. All that is needed is to make it veer of just so much that it doesn't graze the earth.

Of course, we'd need to know in advance every dangerous rock in space. And we need to know their path and whereabouts very early. I'm afraid this is the annoying part, because there is always the chance that some 'surprise' rock will just swoop down on us out of nowhere.

hope that made some sense

43 Name: Anonymous Scientist : 2006-05-11 18:06 ID:vml5sjlt

Beam me up Scotty.

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