Gravity Tractor discussion ...

[the.punk]

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.

Gravitational pull would be better. That's why I was sceptical first until I realized that it was meant that the spacecraft hovers above the asteroid and pulling it with its gravity.

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)?

I also answered me this too. Could we launch such spacecraft?

---------- Post added at 03:53 PM ---------- Previous post was at 03:52 PM ----------

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?

Didn't RisingFury answered something like this the calculation?

 

[Andy44]

Okay, I'm having trouble with the concept of "hovering" near the asteroid using thrusters. Thrusters work by throwing mass out the nozzle and propelling in the opposite direction. When "hovering" you are throwing that mass right at the asteroid. While your gravitational force is pulling it towards you, your jetwash is pushing it away from you. Seems like a net zero to me, minus the assymetric leaking of gasses out the sides of the system. (Unless you are using two thrusters angled out so the jetwash goes around the sides of the object.)

As far as nukes shattering the object, so what. The momentum still transfers, and the fragments are now on different orbits than the object was at first. If some of them are still on a collision course with Earth, well, that's why you carry more than one explosive charge.

Which brings up another point: if you are really paranoid about nukes, you can use conventional explosives. Won't work as well, but will still transfer energy and provide a tiny nudge.

Nuclear explosives are just devices. They can be used as horrible weapons or as tools. This scenario falls under the latter category. They are not going to be un-invented, so stop the moral handwringing, it's unbecoming. As for what protestors will complain about nukes, who cares. The species and the planet are in mortal danger from a big rock. Just push the stinking button, already.

You guys have also brought up the problem of rendezvous. The closer you are to Earth impact, the tougher it's going to be for a gravity tractor mission. Catching it as it passes at 600km requires lots of delta-V, but you have to pay now or pay more later, when it's approaching head-on.

Timed detonation of explosives or ramming, on the other hand, don't require you to match speeds with the object. In fact, the greater the relative velocity, the more it works in your favor. So if a gravity tractor is feasible, it has to be done early on. But as you get later in the game, more violent means are more effective.

 

[the.punk]

Okay, I'm having trouble with the concept of "hovering" near the asteroid using thrusters. Thrusters work by throwing mass out the nozzle and propelling in the opposite direction. When "hovering" you are throwing that mass right at the asteroid. While your gravitational force is pulling it towards you, your jetwash is pushing it away from you. Seems like a net zero to me, minus the assymetric leaking of gasses out the sides of the system. (Unless you are using two thrusters angled out so the jetwash goes around the sides of the object.)

Hmm...:hmm:
That's a good thought.

 

[Hielor]

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.

---------- 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.

Your math doesn't into account that the ejecta from the thrusters will be hitting the asteroid and imparting thrust in the opposite direction.

Moreover, I imagine it would be more efficient to just land some engines on various points of the asteroid and fire them to provide the needed delta-v... what would the numbers look like for that?

 

[RisingFury]

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?

One Ares V launch could put the ship into LEO, another one could bring up a propulsion system.

Your math doesn't into account that the ejecta from the thrusters will be hitting the asteroid and imparting thrust in the opposite direction.

Moreover, I imagine it would be more efficient to just land some engines on various points of the asteroid and fire them to provide the needed delta-v... what would the numbers look like for that?

Read the whole post...

I said angling the thrusters at 45° would make sure that exhaust gets ejected away from the asteroid. Given that the eject velocity from an asteroid is small, that wouldn't put much influence on the final outcome. I mean... the influence would be so small that that all you need to do is park the ship a little closer and fire the thrusters a bit more.


Oh and we're trying to avoid the landing, remember? We have no idea about the geology of the asteroid, making the landing somewhat risky. True, it's a far more efficient approach fuel wise, but at 45° thruster deflection, you just end up burning square root of 2 times more fuel.

 

[Andy44]

Moreover, I imagine it would be more efficient to just land some engines on various points of the asteroid and fire them to provide the needed delta-v... what would the numbers look like for that?

Very simple to compute: a = (force of ion thruster)/(mass of object)

For an ion thruster you could neglect the changing mass of propellant for a first-order approximation, since the mass would be so small compared to the toal mass of the asteroid and ion vehicle combined, and the Isp is very high.

The main problem is that you don't know what kind of surface you're landing on. But if your "landing gear" is a suffuciently wide plastic web or similar device, you should be able to do it.

Next, and this is the harder problem, is that your thrust vector probably won't be centered on the asteroid's center of mass. This means you will induce a rotation in the object as well as linear thrust. On top of this, the asteroid will already be rotating when you touch down on it, as all objects floating in space have some rotation or other in the inertial frame.

To solve this problem, your thruster must be gimbaled, and it needs star trackers to know its attitude and rotation rate. Then you turn the thruster on only when the object is facing the right direction to give you your delta-V. You have to turn it off during those times when the object rolls over to the wrong attitude.

Not a simple engineering problem at all, but not impossible. Nukes are much simpler, especially if you're in a hurry.

 

[Hielor]

Read the whole post...

I said angling the thrusters at 45° would make sure that exhaust gets ejected away from the asteroid. Given that the eject velocity from an asteroid is small, that wouldn't put much influence on the final outcome. I mean... the influence would be so small that that all you need to do is park the ship a little closer and fire the thrusters a bit more.


Oh and we're trying to avoid the landing, remember? We have no idea about the geology of the asteroid, making the landing somewhat risky. True, it's a far more efficient approach fuel wise, but at 45° thruster deflection, you just end up burning square root of 2 times more fuel.

I fail, I saw a radius of 270, not diameter.

I think, when talking about an asteroid it's really more of a "docking" than a landing, since the acceleration due to gravity is so low. You just need a way to latch onto the surface, and mountain climbers have been doing that in all sorts of unfriendly surfaces for decades.

You could have multiple smaller vessels dock at several points on the surface, allowing them to vector/throttle their thrusters to compensate for not being evenly spaced about the asteroid's CoG.

It also seems to me that this method would be much safer in terms of potential failures onboard the craft(s).

 

[RisingFury]

As for what protestors will complain about nukes, who cares. The species and the planet are in mortal danger from a big rock. Just push the stinking button, already.

"We're gonna nuke the bastard!" just doesn't make for a very appealing campaign slogan...

You guys have also brought up the problem of rendezvous. The closer you are to Earth impact, the tougher it's going to be for a gravity tractor mission. Catching it as it passes at 600km requires lots of delta-V, but you have to pay now or pay more later, when it's approaching head-on.

Timed detonation of explosives or ramming, on the other hand, don't require you to match speeds with the object. In fact, the greater the relative velocity, the more it works in your favor. So if a gravity tractor is feasible, it has to be done early on. But as you get later in the game, more violent means are more effective.

Well, building and launching the thing in 6 or 7 years time is not exactly a short deadline and leaves 20 years for deflection, 13 years before it hits the keyhole - as the 600 km area is called...

This is the safest approach in terms of mission success. There's no risk with landing, it's independent of geology (if the asteroid was liquid, it'd still work), doesn't require tremendous amounts of explosives and eliminates the unpredictability of blowing things up and it doesn't require high delta-v.

This is a high risk mission so to maximize your chance of success, you eliminate as many of the unpredictable variables as you can. And as far as micrometeorite damage.... well, let's just say you've got 200 tons of armor plating you can put on.

 

[Andy44]

Good points.

Assuming we use your mission as the primary, I still say we plan more than one mission as back up. Maybe 2 gravity tractor missions, or 1 tractor and one impactor, and a nuke mission just in case. We are talking about the Big One, after all.

BTW, this one scenario alone justifies a space program in my mind. The rest is questionable, but species survival is no joke.

 

[Hielor]

Good points.

Assuming we use your mission as the primary, I still say we plan more than one mission as back up. Maybe 2 gravity tractor missions, or 1 tractor and one impactor, and a nuke mission just in case. We are talking about the Big One, after all.

BTW, this one scenario alone justifies a space program in my mind. The rest is questionable, but species survival is no joke.

Well, with 200 tons of necessary payload, you could easily have multiple nukes along for the ride, as well as several landable thrusters (if possible).

Arrive at the asteroid, begin hovering and start the "gravity tug" process. Meanwhile, deploy some science instruments and investigate what other possibilities would be effective.

 

[ijuin]

One other issue with "landing" your engines on the asteroid in order to move it is that said asteroid is, like every other solid object in the universe, rotating about its own axis. That means that you can't just have your engine pointing straight up out of the ground--even if you could gimbal it almost 90 degrees in all directions, you'll still have to wait for the right part of the local "day" for your thrust to be in the right direction.

 

[RisingFury]

One other issue with "landing" your engines on the asteroid in order to move it is that said asteroid is, like every other solid object in the universe, rotating about its own axis. That means that you can't just have your engine pointing straight up out of the ground--even if you could gimbal it almost 90 degrees in all directions, you'll still have to wait for the right part of the local "day" for your thrust to be in the right direction.

Good point.


And I've also been thinking about how to do the math if we nuke the thing...


The main thing that comes to mind is conservation of momentum. We all know that if you eject mass in one direction, you get propulsion in the other, so in order to achieve a large delta-v, you'd need to eject a significant amount of asteroid's mass. Granted, you might deflect the asteroid, but you'll create loads of fragments in the process - ranging from stuff as small as a grain of sand to as large as cars or houses... and some are bound to hit Earth. Granted, the effects of such impactors would be limited, but you don't want a hail storm of rocks at several km/s over your head, do you?

 

[Andy44]

You don't have to eject any of the asteroid's mass. The nuke ejects part of its own mass towards the asteroid and the other half of its mass away, providing propulsion. That's how the Orion drive works; it doesn't eject any part of the vehicle's mass except the spring-loaded nuke charge itself, which provides almost no propulsion until it detonates..

 

[dougkeenan]

Great thread guys.

 

[2552]

Would this idea work? Drill a hole in the asteroid, put a 20mt nuke in it, and detonate it. The blast will be channeled out the hole, creating a kind of thruster. If that can work without blowing up the asteroid, how much delta v could that give the asteroid?

 

[Kaito]

Would this idea work? Drill a hole in the asteroid, put a 20mt nuke in it, and detonate it. The blast will be channeled out the hole, creating a kind of thruster. If that can work without blowing up the asteroid, how much delta v could that give the asteroid?

If there was such a material that could survive a point-blank nuclear explosion, We just found a new source of propulsion.

The asteroid would be destroyed. But the problem is drilling the hole

 

[Andy44]

The asteroid would not necessarily be destroyed. Depends on how big it is and what it's made of.

But how are you going to drill a hole? Remember, you've never sent a probe to this rock before, you have no idea what the surface is made of or how dense it is. You've never had robotic hole-drillers in space, before, either. And, of course, before you can drill a hole, you've got to match velocities with the rock, soft-land, and anchor yourself to the surrounding surface, which you know nothing about.

If you're going to use a nuke for a propulsive maneuver, just get it close to the rock and set it off. No need for all that complicate hole-drilling nonsense. Just another reason why Armageddon was a dumb movie...

 

[Zatnikitelman]

Nuking it wouldn't be a bad thing. Remember my "Shotgun Effect...SO WHAT?" topic back on M6? The gravity-sim guy ran a simulation that showed most of the fragments would miss Earth and those that hit were small. Perhaps I should revive that topic...

 

[TSPenguin]

BTW, this one scenario alone justifies a space program in my mind. The rest is questionable, but species survival is no joke.

Really really good point. One that I have been using quite regularely too.
Especialy people who are against space programs but tend to throw around phrases like "Think of the kids" and "Children are our future" have to think about that one pretty hard.

Nuking it wouldn't be a bad thing. Remember my "Shotgun Effect...SO WHAT?" topic back on M6? The gravity-sim guy ran a simulation that showed most of the fragments would miss Earth and those that hit were small. Perhaps I should revive that topic...

Nuking is by itself a very nice thing. And as a solution when there is no time for anything else, sure, fire away.
If you have the time, using a tractor to pull/push that things leaves you with less space hazard. We are talking about space junk in LEO and GSO all the time, let's not start with the solar system the same way we did here, m'kay?!

And I totaly agree, if the "Big One" comes, we should launch as many missions as possible. Coordinated of course, but this is an all or nothing situation. What would the aliens think of us, if we send one mission and it fails?!?

 

[the.punk]

One other issue with "landing" your engines on the asteroid in order to move it is that said asteroid is, like every other solid object in the universe, rotating about its own axis. That means that you can't just have your engine pointing straight up out of the ground--even if you could gimbal it almost 90 degrees in all directions, you'll still have to wait for the right part of the local "day" for your thrust to be in the right direction.

I think I have something. We could put two engines on this asteroid. But in the the opposite direction. One is burnong until it rotates in the wrong direction and then the other is igniting. But that would mean that we need two fuel tanks and thereby more payload.