Gravity Tractor discussion ...

[llarian]

Okay, boys and girls, here is a new query, based on an article from JPLs NEO program:

JPL has posited the use of a gravity tractor as a means of deflecting and tracking a possible NEO. How does such a spacecraft work and what is the physics behind the hypothetical gravity pulse to be used for the deflection? Also, how could we simulate this in Orbiter?

Is Orbiter capable of simulating bound orbits between a hypothetical spacecraft and a NEO?

 

[Kaito]

Okay, boys and girls, here is a new query, based on an article from JPLs NEO program:

JPL has posited the use of a gravity tractor as a means of deflecting and tracking a possible NEO. How does such a spacecraft work and what is the physics behind the hypothetical gravity pulse to be used for the deflection? Also, how could we simulate this in Orbiter?

I believe it's a complete waste. You wouldn't be able to get something that has enough gravity to do something. And even if you could, you'd need to launch it SUPER far in advance. I believe it's going to fail miserably. Unless someone can mathematically prove me wrong, I dont support this

Is Orbiter capable of simulating bound orbits between a hypothetical spacecraft and a NEO?

AFAIK, no

 

[Scarecrow]

I think the entire point is that you launch it really far in advance. The way it works is that the ship just maintains it's position on one side of the NEO. The NEO would use it's slight gravity to pull the ship, and the ship would use it's slight gravity to pull the NEO. However, the ship uses it's thrusters to stay a constant distance from the NEO. The total effect is to transfer the momentum from the thrusters to the NEO. As long as you launch it long enough in advance, I see no reason you couldn't change the trajectory enough to do something useful. The only reservation I have about the plan is what happens to the exhaust from the gravity tug's thrusters? It seems they'd have to be oriented at a large angle in order to miss the asteroid, while still staying close enough. This makes thing much more inefficent. The big question I'm unequipped to answer is whether it would be more mass efficent to waste propellent like this, or to include landing gear so the tug can push directly.

 

[ijuin]

There is no gravity "pulse" as such. Rather, a spacecraft will use the gravity caused by its own mass (tens to hundreds of tons) to sloooowly (over a couple of years) deflect the asteroid. Given the amount of lead time planned, the delta-v imparted to the asteroid needs only be about one centimeter per second for it to miss the Earth entirely (i.e. several thousand km difference in where it is when it would have otherwise hit Earth). The gravitational interaction between a spacecraft and an asteroid some one million times as massive is expected to be enough to accomplish this amount of deflection, assuming that the spacecraft uses its ion engines or whatever to hold its distance from the asteroid (thus making the asteroid fall towards it instead of the reverse).

 

[Kaito]

I think the entire point is that you launch it really far in advance. The way it works is that the ship just maintains it's position on one side of the NEO. The NEO would use it's slight gravity to pull the ship, and the ship would use it's slight gravity to pull the NEO. However, the ship uses it's thrusters to stay a constant distance from the NEO. The total effect is to transfer the momentum from the thrusters to the NEO. As long as you launch it long enough in advance, I see no reason you couldn't change the trajectory enough to do something useful. The only reservation I have about the plan is what happens to the exhaust from the gravity tug's thrusters? It seems they'd have to be oriented at a large angle in order to miss the asteroid, while still staying close enough. This makes thing much more inefficent. The big question I'm unequipped to answer is whether it would be more mass efficent to waste propellent like this, or to include landing gear so the tug can push directly.

I know how a gravity tractor works, thats why I think it WONT work.
We wouldn't be able to plan, build, assemble, launch, and rendezvous with the NEO early enough for it to make a difference, unless the tractor has a huge mass, which adds more problems like propellant, cost, etc.

"But wait Kaito! Who cares how much it will cost, the earth wont be destroyed!!!!!!"

Not by a meteor, no, but if the cost is high enough, the world will fall apart due to a depression.

"We have huge telescopes, we can see it coming WAY in advance"

Oh really? Lets assume for practical purposes it takes 1 year to have a gravity tractor ready to launch, and because the stars aligned, we can launch it perfectly in plane with the asteroid. 3, 2, 1, launch....*wait 20 years* oh yay, we rendezvous...oh crap, its 21 years closer...and we all die

"Fine then, just build one in advance!"

And if no NEO requires it? Bye bye several billion dollars in assembly cost. We'll miss ya

As far as I'm concerned, a better way would be either ramming a vehicle into it (Yes, it'd need to be going UBER fast, but thats what Ion engines are for - constant acceleration) or using some sort of laser to change its mass

 

[Andy44]

I tend to agree with Kaito. Ramming at high speed sounds like a simpler and quicker way to impart a small delta-V on a metoeroid.

Even if the object is a loose collection of pebbles or ice crystals, the momentum will transfer, and you only need a tiny amount with a long enough lead time.

But I haven't done the math, so I may be wrong. And it occurs to me that if the delta-V is less than the error in our trajectory prediction than it doesn't do us much good.

 

[jedidia]

Gravity Pulse? didn't know we're so far advanced with negative energy production allready...

Anyways, I get the concept from Kaitos post, and it doesn't sound very much better. I mean, think of the manouver that is needed to intercept an asteroid. If you spot it early enough, you might be able to slingshot around saturn or jupiter to help you match velocity, IF any of them happens to be at the right place at the right time. That's best case scenario. worst case is you don't get the thing up in time and have to use your fuel to match velocity, which would be a challenge even with a small vehicle. And we're talking about shooting 10 to 100 tons through half of the solar system here. If we can do that, we might as well send an armed ship up and throw it of course with nukes. Or, since the tractor is so darn heavy, just crash it into it at the right angle. Much fuel saved!

 

[Hielor]

There is no gravity "pulse" as such. Rather, a spacecraft will use the gravity caused by its own mass (tens to hundreds of tons) to sloooowly (over a couple of years) deflect the asteroid. Given the amount of lead time planned, the delta-v imparted to the asteroid needs only be about one centimeter per second for it to miss the Earth entirely (i.e. several thousand km difference in where it is when it would have otherwise hit Earth). The gravitational interaction between a spacecraft and an asteroid some one million times as massive is expected to be enough to accomplish this amount of deflection, assuming that the spacecraft uses its ion engines or whatever to hold its distance from the asteroid (thus making the asteroid fall towards it instead of the reverse).

You'd be better off "docking" the spacecraft to the asteroid and using the spacecraft's engines to move the asteroid.

 

[T.Neo]

Not by a meteor, no, but if the cost is high enough, the world will fall apart due to a depression.

I somehow doubt that even at the highest price, such a misson would cause a global recession/depression.

The impact from even a small strike (albeit over a populated area) would probably cause far more damage then a depression.

 

[Sky Captain]

You would need a very timely warning about incoming asteorid for gravity tractor to work, perhaps many years. If an asteorid is spotted only few months before impact gravity tractor would be useless. IMHO the fastest and most efficient way how to push the asteorid away from impact trajectory would be to detonate several nuclear bombs near the surface to create thrust. Nukes are compact have the most energy per unit of mass and are off the shelf technology while gravity tractor would need a lot of R&D and complicated orbital assembly which would take years.

 

[RisingFury]

I think the entire point is that you launch it really far in advance. The way it works is that the ship just maintains it's position on one side of the NEO. The NEO would use it's slight gravity to pull the ship, and the ship would use it's slight gravity to pull the NEO. However, the ship uses it's thrusters to stay a constant distance from the NEO. The total effect is to transfer the momentum from the thrusters to the NEO. As long as you launch it long enough in advance, I see no reason you couldn't change the trajectory enough to do something useful. The only reservation I have about the plan is what happens to the exhaust from the gravity tug's thrusters? It seems they'd have to be oriented at a large angle in order to miss the asteroid, while still staying close enough. This makes thing much more inefficent. The big question I'm unequipped to answer is whether it would be more mass efficent to waste propellent like this, or to include landing gear so the tug can push directly.

Well, the propellant mass isn't really a problem.

You're counting on your ship to be really massive in the first place.



Here's why this approach is much better then ramming or landing on it:


Landing on an asteroid usually demands knowing your asteroid well - you need to know where to land, if it's possible to land in the first place. Sure, we have powerful telescopes, but not nearly powerful enough to resolve the asteroid's surface to identify a landing zone.

Here's why we shouldn't ram it:
You only get one shot.
Again, we don't know much about the asteroid: It's composition and density, which makes mass measurements difficult. If the estimate turns out to be wrong, we won't supply the desired delta-V.

Here's another reason why these two approaches don't work well:
We can't establish the orbit of the thing precise enough far in advance, that we know where it's going to go. As an example I'll provide 99942 Apophis.

Apophis is a rock about 600 m wide. Not big enough to destroy the Earth, but certainly big enough to do some damage. We know that on April 13th 2027 it will make a very close approach of Earth. Close enough that it will fly below geostationary altitude. It's orbit is uncertain enough, that we can't tell exactly where it will fly. If it hits a 600 km (in altitude - but not 600 km from Earth's surface) area, it will hit the Earth 7 years later.

All we need to do to deflect it, is to drag it away from that 600 km area.

If we ram the thing, there is a large chance that the orbit will again be unpredictable enough that we can't tell for certain where it will go. And, because we just spent years building and launching the thing, we won't have the time to do it again.



But if we park a massive spaceship near the asteroid and have it maintain altitude using Ion based jets (which can run for years - Dawn will clock 2000 days of operation), we can control the trajectory of the asteroid along it's entire path. Granted, the orbit would again be uncertain, but it would give us the ability to correct it.

Afterall, this would provide the same delta-V as landing on it and buring a rocket, it would just take more time.


Edit:

Uncertainty of the orbit:

http://en.wikipedia.org/wiki/File:Apophis_pass.svg

 

[the.punk]

Gravity is a field caused by mass. And I don't think that it is possible to have a gravity "pulse". The mass the ship needs to do such would be veeeery high.

---------- Post added at 09:10 AM ---------- Previous post was at 09:05 AM ----------

And to transport this mass to the asteroid would be much more energy as the result.

 

[jedidia]

Well, the propellant mass isn't really a problem.

I can see your logic, but I have to disagree. You need an insurance that the thing has a certain mass when it arrives at target. So, a certain mass of the vehicle can be labeled "payload", as it is indespensible if the mission objective is to be achieved.

Then you need the fuel to get that mass there. Even if the "payload"-mass would be in actual fuel, you'd still not be allowed to tap it too much. You'll need that mass when you get there. So the problem is not different than any other flight: "have payload, need fuel". And as we all know, you need more fuel per ton payload the more fuel you need.

Of course, if the whole mass would be provided in fuel, you'd have a reasonable error-margin. If the thing weights 100 tons, one ton less wouldn't really matter. use 5 to 10 tons too much, and you might get in trouble. Of course, fuel savings would work in favour, since you'd had more mass on the site than you actually planned. But you can't really say that fuel mass isn't a problem, especially if you have to pull of a high DV manouver to catch the bastard.

All we need to do to deflect it, is to drag it away from that 600 km area.

Or, let it pass within 600 km and jump it when it flies by, then correct it within the seven years remaining. I could imagine that, because you'd only have to match velocity at flyby, and not first spend DV to intercept, and then some more to match.

Here's why we shouldn't ram it:
You only get one shot.

That, of course, is true. But if we don't just ram it but try for some "nuclear propulsion" (i.e. nuking it out of orbit), we might get a hell of a lot of shots for that mass. More than we'd need, so maybe we could reduce the mass to reasonable amounts and still have enough shots left.
I'd say the gravity tractor is not a very feasible option unless we really conquer and hold LEO first. With sufficiently cheap surface to LEO ( or to GSO) transport, it might become an option.

But as long as nobody does the math, we're merely speculating.

Anyways, wheather feasible or not, it would sure be fun to pull off in Orbiter.

 

[the.punk]

Maybe this with the gravity could be possible in future, because we don't know what the future brings.
But it would be fun in orbiter.

---------- Post added at 11:24 AM ---------- Previous post was at 11:19 AM ----------

All we need to do to defect it, is to drag it away from that 600 km area.

If we ram the thing, there is a large chance that the orbit will again be unpredictable enough that we can't tell for certain where it will go. And, because we just spent years building and launching the thing, we won't have the time to do it again.

That is a good point.

 

[RisingFury]

I can see your logic, but I have to disagree. You need an insurance that the thing has a certain mass when it arrives at target. So, a certain mass of the vehicle can be labeled "payload", as it is indespensible if the mission objective is to be achieved.

Then you need the fuel to get that mass there. Even if the "payload"-mass would be in actual fuel, you'd still not be allowed to tap it too much. You'll need that mass when you get there. So the problem is not different than any other flight: "have payload, need fuel". And as we all know, you need more fuel per ton payload the more fuel you need.

Even if the thruster was conventional and used conventional fuel, you'd still arrive at your destination with enough fuel to complete the mission. If I planned the mission, I'd make sure of that.

Of course, if the whole mass would be provided in fuel, you'd have a reasonable error-margin. If the thing weights 100 tons, one ton less wouldn't really matter. use 5 to 10 tons too much, and you might get in trouble. Of course, fuel savings would work in favour, since you'd had more mass on the site than you actually planned. But you can't really say that fuel mass isn't a problem, especially if you have to pull of a high DV manouver to catch the bastard.

Again, I highly doubt that such a mission would use conventional thrusters. I think Ion based thrusters are more likely because they can provide high efficiency at long firing durations. You'd need closer to 100 kg of fuel then 100 tons. That means that the mass of the fuel would be rather negligible compared to the payload mass.

Or, let it pass within 600 km and jump it when it flies by, then correct it within the seven years remaining. I could imagine that, because you'd only have to match velocity at flyby, and not first spend DV to intercept, and then some more to match.

That is doubtful at best.

The asteroid will accelerate to insane speeds, while passing by Earth at slightly lower then geostationary orbits... delta-V for that would be quite large.

That, of course, is true. But if we don't just ram it but try for some "nuclear propulsion" (i.e. nuking it out of orbit), we might get a hell of a lot of shots for that mass. More than we'd need, so maybe we could reduce the mass to reasonable amounts and still have enough shots left.

We can't even get a 200 RTG into space without huge public fear. Nuclear warheads wouldn't just spark public protests. I fear international distrust could cause a war.

Besides, once we break the ice, what's there to stop us? Do we really wanna be a civilization that nukes every threat to our existence, despite having alternative means to deal with the problem?

I'd say the gravity tractor is not a very feasible option unless we really conquer and hold LEO first. With sufficiently cheap surface to LEO ( or to GSO) transport, it might become an option.

And you still want to catch up with the thing at GSO altitude?




Ok, so assuming we won't nuke the thing, we have two options. Ram it or tow it.

So here are some numbers:

99942 Apophis:
Mass M = 2.7 * 10^10 kg
Diameter d = 270 m

The ship:
Mass m = 2 * 10^5 kg (200 tons)
Impact velocity RelV = 10 km/s
Construction time + getting to the asteroid = 6 years.


Assuming a huge impact velocity:

m * RelV = M * delta-v (we can assume that the mass of the asteroid won't change much)

delta-v = (m * RelV) / M = 0.07 m/s on impact.

The diameter of the asteroid is some 270 m, meaning we can hover around 200 m from the surface.

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

a = m * G / r^2 = 3.3 * 10^-10 m/s^2

With a hover time of 20 years:

delta-v = 0.2 m/s with 0.01 m/s change per year.

The force required to keep the ship from falling onto the asteroid is around 0.5 N. At around 45° angle, that would mean around 0.7 N. I would say that would be split up between at least 4 thrusters, giving a workload of less then 0.2 N per thruster. I think this is entirely doable.


Granted, there's still the trouble of launching the thing, but I think we can all agree that the most fuel efficient approach is to eject it from Earth's orbit at like 4 km/s or something (once in LEO) and rendezvous with the asteroid in outer space and not trying to catch it at flyby of Earth.

I also think that a relative velocity of 10 km/s will be difficult to achieve, however, maybe a sling around Jupiter into a retrograde orbit and then hitting the asteroid when it's mid way from it's perihelion to aphelion could do the trick.

Still, the total delta-v of the asteroid resulting from parking the ship next to the asteroid will be greater then that of an impact. It would take an impact of more then 25 km/s relative velocity.

The advantage of the impact is that it changes velocity of the asteroid right away, while it takes some years before the parked ship can do the same... and the earlier you change the velocity the better, but the ship can provide a far larger delta v after that and I think it might compensate for the time it took.

I know these numbers are just rough estimates but they clearly show that we'd get more bang for the buck from gravitational tether.

---------- Post added at 02:00 PM ---------- Previous post was at 01:22 PM ----------

One more thing...

Assuming an impact delta-v of 0.07 m/s and an acceleration of 3.3 * 10^-10 m/s^2 for 20 years, the impact moves the asteroid a total of 44 000 km and the ship moves it a total of 65 000 km. Of course, that doesn't directly translate to the flyby, but it does give a measure of which is more powerful.

 

[jedidia]

Nice job, can't argue with math.

The asteroid will accelerate to insane speeds, while passing by Earth at slightly lower then geostationary orbits... delta-V for that would be quite large.

Yes, you are right. I didn't take that into consideration.

We can't even get a 200 RTG into space without huge public fear. Nuclear warheads wouldn't just spark public protests. I fear international distrust could cause a war.

I didn't take that into consideration either, allthough I think that the atomic powers might come to some sort of agreement, after all it's the end of the world knocking at the door. Still, the point is well put.

Besides, once we break the ice, what's there to stop us? Do we really wanna be a civilization that nukes every threat to our existence, despite having alternative means to deal with the problem?

As long as the threat isn't anything living, and there's no environmental damage, I don't really have a problem with it. Allthough, in a millenia or two the environmentalists will probably start to protest against blowing up asteroids... :lol:
but, I wasn't actually thinking of blowing it up, merely giving it a push in the right direction. After all, NASA even proposed to nuke humans to mars (Orion drive is practically that), so it should work on an asteroid too.

And you still want to catch up with the thing at GSO altitude?

ahhh no. That was meant like "if we had cheap LEO access, we might try jumping it with the gravity tractor when it passes earth. If we don't, nuke it in any case". But yeah, slingshoting around a major body and catching it up from behind is certainly a cheaper way. What I meant was if the schedule is too tight, and you have unfavourable constelations, you might have to intercept it frontal or at best from the sides, and matching velocities under these circumstances might prove quite devastating on the fuel budget.

Anyways, conclusions so far: Tractor beats Ram.

 

[TSPenguin]

Ramming or pushing by nuke might, and most likely will, result in breaking up the asteroid. And if all goes wrong, you punch a chunk out and the majority of the mass is still on target for us.
Gravitationaly pulling can deal with an infinite number of objects. Which is preferably anyway, as asteroids, especialy those with a lot of ice, tend to break up from time to time. And without inspecting it closely beforehand, we wouldn't even know if the asteroid is one solid piece anyway.

 

[the.punk]

Ok I understand that gravitational pulling would be an option.
But can launch something heavy from earth?
Or how would we get it to the asteroid?

 

[RisingFury]

Thanks!

I didn't take that into consideration either, allthough I think that the atomic powers might come to some sort of agreement, after all it's the end of the world knocking at the door. Still, the point is well put.

There would have to be some hard negotiations and there are still rouge nations like NK and Iran who don't trust anything the others say...

As long as the threat isn't anything living, and there's no environmental damage, I don't really have a problem with it. Allthough, in a millenia or two the environmentalists will probably start to protest against blowing up asteroids... :lol:
but, I wasn't actually thinking of blowing it up, merely giving it a push in the right direction. After all, NASA even proposed to nuke humans to mars (Orion drive is practically that), so it should work on an asteroid too.

It's true that we wouldn't be nuking any humans and deflecting the asteroid won't cause any effects for life on Earth, other then maybe a flash in the distance... but we are still walking a fine line. I don't think we wanna be dependent on nuclear detonations for our space exploration and defense.


Although the asteroid is like 600 by 300 m or something, which isn't enough to cause an extinction level event, it would do some serious damage and would definitely cause lasting effects on Earth.

We don't have to worry about changing the orbital parameters of Earth or blowing away it's atmosphere... but it would be comparable to a large volcano...

Oh and the problem with nuking it is that there is a chance that the asteroid might shatter. We've had few probes fly by and land on asteroids and we've seen that they are heavily cratered. So it's reasonable to assume that this one is as well. If there was a large enough impact in the past that the asteroid barely survived, there is a chance that it would shatter if we try to blow it up. That would result in not just one piece flying towards Earth, but many. Even if you managed to crush it completely into dust and let all the dust burn up in the atmosphere... well, the kinetic energy is still there and instead of going into an impact, it would all go into heat at reentry. It would turn the sky as bright as the Sun and generate enough heat to set forests on fire...

 

[llarian]

Thanks, RisingFury, finally an answer not catering to opinion.

Perhaps "gravity pulse" as a term was misused, but it is used in the article I read. It seemed somewhat confusing unless someone somewhere has a practical gravity physics application that hasn't been publicized widely.

More specific is the concept of bound orbits by a spacecraft. Obviously we can achieve them, as NEAR became bound in the reverse manner to what a gravity tractor is. What are the practical limits of such a spacecraft, masswise. What minimum mass is required, can we launch that much mass from this deep in the gravity well, or is the craft going to have to be constructed in orbit (or in lunar orbit)? Have two satellites in earth orbit already exhibited orbital bounding and has propulsion on one, exhibited an effect on the other?

I interpret the term "pulse" to be more correct as applied to a thrust impulse from a bound spacecraft. Presuming a type of ion thruster, how big (massive) would the required thruster be to impart a measurable effect on a bound asteroid (for sake of argument let's use Apophis as in RisingFury's example), and can it be practicably built and flown?