APOD: Gravitational Tractor (2013 Feb 21)

[APOD Robot]

Gravitational Tractor

Explanation: How would you change the course of an Earth-threatening asteroid? One possibility - a massive spacecraft that uses gravity as a towline - is illustrated in this artist's vision of a gravitational tractor in action. In the hypothetical scenario worked out in 2005 by Edward Lu and Stanley Love at NASA's Johnson Space Center, a 20 ton nuclear-electric spacecraft tows a 200 meter diameter asteroid by simply hovering near the asteroid. The spacecraft's ion drive thrusters are canted away from the surface. Their slight but steady thrust would gradually and predictably alter the course of the tug and asteroid, coupled by their mutual gravitational attraction. While it sounds like the stuff of science fiction, ion drives do power existing spacecraft. One advantage of using a gravitational tractor is that it would work regardless of the asteroid's structure. Given sufficient warning and time, a gravitational tractor could deflect the path of an asteroid known to be on a collision course enough to miss planet Earth.

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[Guest]

You all are on acid-this is a joke-its not astronomical at all-its Fantasy and NON reality

[metamorphmuses]

^ You're welcome to present a proof to the contrary.

Seriously, do we all not think this idea is hard enough to realize as it stands? Imagine how much money, resources, and potential for error would be involved in this project. If we had the capability to pull this off, we'd also have no trouble sending crewed spacecraft throughout the solar system, which I may remind us all is a technology we do not currently possess - not because we could not possess it, but because there isn't enough of a collective will to realize it.

[neufer]

Special HAL Meeting on Thursday, February 21st

HAL is pleased to have Dr. Jerry Bonnell, one of the founders of APOD, speaking at this month's meeting. The meeting will be held at 7:30PM on the lower level of the Robinson Nature Center. Due to the large anticipated audience we will be in Auditorium adjacent to our usual meeting room. Signs will point the way. Alternately, inquire at the front desk when entering the building.

Click to play embedded YouTube video.

(Open to the Public)

Driving Directions
to HAL Meeting:

The Robinson Nature Center
6692 Cedar Lane
Columbia, MD 21044

http://asterisk.apod.com/viewtopic.php?f=9&t=29032

[Chris Peterson]

^ You're welcome to present a proof to the contrary.

Seriously, do we all not think this idea is hard enough to realize as it stands? Imagine how much money, resources, and potential for error would be involved in this project. If we had the capability to pull this off, we'd also have no trouble sending crewed spacecraft throughout the solar system, which I may remind us all is a technology we do not currently possess - not because we could not possess it, but because there isn't enough of a collective will to realize it.

This is much, much cheaper and easier to manage than manned expeditions around the Solar System.

Sure its expensive, and sure there are technical challenges. But they are well within our financial and engineering means given a bit of incentive... like a massive collision a few years in the future.

[neufer]

-this is a joke-its not astronomical at all-its Fantasy and NON reality

<<Let us suppose that a NEO of size around 100 m, and mass of one million metric tons, threatened to impact Earth. Suppose also that a velocity correction of 1 cm/s would be adequate to place it in a safe and stable orbit, missing Earth that the correction needed to be applied within a period of 10 years.

With these parameters, the required impulse would be: V × M = 0.01 [m/s]×10⁹ [kg] = 10⁷ [N-s], so that the average tractor force on the asteroid for 10 years, = 3.156×10⁸ s, would need to be about 0.032 newtons. An ion-electric spacecraft with a specific impulse of 10,000 N-s per kg, corresponding to an ion beam velocity of 10 km/s (about twenty times that obtained with the best chemical rockets), would require 1,000 kg of reaction mass (Xenon is currently favored) to provide the impulse. The kinetic power of the ion beam would then be approximately 317 W; the input electric power to the power converter and ion drive would of course be substantially higher. The spacecraft would need to have enough mass and remain sufficiently close to the asteroid that the component of the average gravitational force on the asteroid in the desired direction would equal or exceed the required 0.032 N. Assuming the spacecraft is hovering over the asteroid at a distance of 200 m to its centre of mass, that would require it to have a mass of about 20 metric tonnes, because due to the gravitational force we have:



Considering possible hovering positions or orbits of the tractor around the asteroid, note that if two objects are gravitationally bound in a mutual orbit, then if one receives an arbitrary impulse which is less than that needed to free it from orbit around the other, because of the gravitational forces between them, the impulse will alter the momentum of both, together regarded as a composite system. That is, so long as the tractor remains in a bound orbit, any propulsive force applied to it will be effectively transferred to the asteroid it orbits. This permits a wide variety of orbits or hovering strategies for the tractor. One obvious possibility is for the spacecraft to orbit the NEO with the normal to the orbit in the direction of the desired force. The ion beam would then be directed in the opposite direction, also perpendicular to the orbit plane. This would result in the plane of the orbit being shifted somewhat away from the center of the asteroid, "towing" it, while the orbital velocity, normal to the thrust, remains constant. The orbital period would be a few hours, essentially independent of size, but weakly dependent on the density of the target body.>>

• Note that the mass of the tractor goes with the square of the asteroid's diameter:
  • (; e.g., 30 meter 2012 DA₁₄ would require just a 1800 kg tractor
    whereas a 1 km asteroid would require a 2,000 tonne tractor).

[Markus Schwarz]

According to the paper, it would take about 20 years to deflect a 200m asteroid. Since the space craft needs to get there too, which would probably also take a few years, we really need "sufficient warning and time".

[Boomer12k]

I hate to be cryptic....however....

2012 DA14 was discovered on Feb, 23, 2012....it came by almost 1 year later....would it have been enough time to divert in one year?

:---[===] *

[mister T]

Wouldn't it be more efficient to land on the asteroid and use the thrust to push directly on the asteroid??

[Markus Schwarz]

Wouldn't it be more efficient to land on the asteroid and use the thrust to push directly on the asteroid??

According to the authors, a propulsion system would have to be anchored to the asteroids. But this is difficult because asteroids are "rough and unconsolidated". Another problem is that asteroids rotate, and your engine would need to apply the thrust in changing directions.

[metamorphmuses]

This is much, much cheaper and easier to manage than manned expeditions around the Solar System.

Really? If that were the case, then all the better. I suppose it would be nice to see some cost estimates. I assumed the vessel would be manned or at least crew-serviceable.

Sure its expensive, and sure there are technical challenges. But they are well within our financial and engineering means given a bit of incentive... like a massive collision a few years in the future.

Yeah, if that doesn't motivate humankind enough, I don't think anything could.

[CMatisse]

Why didn't we practice on 2012 DA14? (As if I didn't know)

[neufer]

Click to play embedded YouTube video.

Why didn't we practice on 2012 DA14? (As if I didn't know)

You probably don't.
------------------------------------------------------

While the threat of 2012 DA₁₄ provides excellent motivation

and the tumbling of 2012 DA₁₄

provides us an excellent example of how any means OTHER than a simple gravitational tractor "lasso" would prove to be technologically difficult (for either pulling or pushing);

testing such a simple approach is both expensive & unnecessary:
------------------------------------------------------
1) The Japanese have already intercepted
and flown in tandem with an appropriately sized asteroid:

• . http://apod.nasa.gov/apod/ap070422.html

2) NASA has propelled an ion powered spacecraft away from an asteroid:

• . http://apod.nasa.gov/apod/ap120919.html

3) NASA has sent a 23 tonne space craft to the moon:

• . http://en.wikipedia.org/wiki/Apollo_17

4) And we pretty well understand Newtonian gravity.
------------------------------------------------------
The only possible difficulty that I can envision would be the unlikely prospect that the ion rockets (over time) generated a repulsive electrostatic field between the tractor and the asteroid.

In any event, it would be far better now to spend any available funds:

• 1) locating all the dangerous asteroids and
  2) tagging the worst offenders with accurate tracking devices.

[K1NS]

• Note that the mass of the tractor goes with the square of the asteroid's diameter:
  • (; e.g., 30 meter 2012 DA₁₄ would require just a 1800 kg tractor
    whereas a 1 km asteroid would require a 2,000 tonne tractor).

Pardon me, but shouldn't the mass of the tractor be proportional to the cube of the asteroid's diameter. Or does my high school geometry fail me?

[Altair]

Why not use existing supplies of space junk already in orbit for the mass component of a gravitational tractor? The ISS and HST could become tractors following the end of their main missions rather than being deorbited. This could greatly reduce the cost of making tractors. Just attach motors to tractors already in orbit.

[neufer]

• Note that the mass of the tractor goes with the square of the asteroid's diameter:
  • (; e.g., 30 meter 2012 DA₁₄ would require just a 1800 kg tractor
    whereas a 1 km asteroid would require a 2,000 tonne tractor).

Pardon me, but shouldn't the mass of the tractor be proportional to the cube of the asteroid's diameter. Or does my high school geometry fail me?

Well, you are, indeed, half right:

• The mass of the tractor's ion propellent must be proportional to the cube of the asteroid's diameter. Hence, for asteroids much larger than 100m the need for sufficient propellent quickly becomes the dominant issue.

Otherwise, the mass of the asteroid basically cancels out because doubling the asteroid mass also doubles the gravitational pull to the tractor.

All objects (regardless of size or mass) accelerate at the same rate in any gravitational field including that of the tractor.

The only problem is that an asteroid twice the diameter must lie twice as far away from the tractor (on average) and therefore is only subject to a quarter of the gravitational field by the inverse square law. Hence, ignoring propellent needs, the mass of the tractor goes with the square of the asteroid's diameter (and the tractor can always be refueled or replaced if necessary).

[neufer]

Why not use existing supplies of space junk already in orbit for the mass component of a gravitational tractor? The ISS and HST could become tractors following the end of their main missions rather than being deorbited. This could greatly reduce the cost of making tractors. Just attach motors to tractors already in orbit.

Alternatively, since we all talk about mining asteroids, what would be better than to mine the asteroid, itself, for the needed mass. (Note, however, that for the larger asteroids the required mass of the propellent will prove to be more than sufficient.)

Alternatively, one could mine the asteroid and simply "catapult" the material away (at greater than escape velocity and along the asteroid's orbital path) and thus skip the tractor altogether.

[FloridaMike]

I would add that "a velocity correction of 1 cm/s ...to place it in a safe and stable orbit" is sort of a "Gold Standard". All we need is enough time to slightly alter the speed of the object to miss us, the " safe and stable orbit" part can come later.

[henrystar]

I've seen this illustration before, but an idea occurred to me on looking at it again: depending on how a solid asteroid was rotating, we could simply paint one side black and one side white, so that the changed differential strength of solar light pressure would alter the path so as to miss Earth. (If no one has thought of that, I'd like to patent it.)

[Chris Peterson]

Why didn't we practice on 2012 DA14? (As if I didn't know)

Because there's not much need to practice. There's no doubt that the method would work, and there's no doubt about our ability to design and launch such a device. So why expend the considerable resources for practice? Furthermore, a body like DA14 is probably the last sort we'd want to practice on. It's no threat, but could easily be made into one with the wrong nudge. If you wanted to test a gravitational tug, it would make more sense to do so with an asteroid in the main belt, or one that didn't have an Earth-crossing orbit.

[Chris Peterson]

2012 DA14 was discovered on Feb, 23, 2012....it came by almost 1 year later....would it have been enough time to divert in one year?

No. But DA14 wouldn't be enough of a threat to justify something like a major diversion project. A body the size of DA14 probably wouldn't make it to the ground, and at worst would produce only very localized damage. I'd expect a body would need to be at least 100 meters in diameter before a project like this would be seriously considered, and the larger the object, the more likely that we will be aware of it decades before any possible collision.

[Chris Peterson]

This is much, much cheaper and easier to manage than manned expeditions around the Solar System.

Really? If that were the case, then all the better. I suppose it would be nice to see some cost estimates. I assumed the vessel would be manned or at least crew-serviceable.

The whole point is that it isn't manned, and isn't serviceable- both of which make for a prohibitively difficult and expensive mission. Manned would be absurd, since we're talking about a mission likely to be in deep space for a decade or more. Serviceable with robots might be considered, depending on the orbit, but probably isn't necessary. If the HST has taught us anything, it is that making serviceable probes is probably a waste of money.

[neufer]

I've seen this illustration before, but an idea occurred to me on looking at it again: depending on how a solid asteroid was rotating, we could simply paint one side black and one side white, so that the changed differential strength of solar light pressure would alter the path so as to miss Earth. (If no one has thought of that, I'd like to patent it.)

This might work IF the asteroid was NOT rotating and
one painted the front side black and the trailing side white (or vice versa).

(Since {Power = F ⋅ v}, it is critical to always point the force along the direction of the velocity.)

However, asteroids are almost always rotating (Catch 22)

Hence, a similar (but much more logical) approach would be
to paint the whole thing white or black and make use of the Yarkovsky effect:

Asteroid impact avoidance comprises a number of methods by which near-Earth objects could be diverted, preventing potentially catastrophic impact events. Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, operations, and technology readiness. There are various methods for changing the course of an asteroid/comet. These can be differentiated by various types of attributes such as the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy (interception, rendezvous, or remote station). Strategies fall into two basic sets: destruction and delay.

"Painting" or dusting the object with titanium dioxide (white) or soot (black) to alter its trajectory via the Yarkovsky effect.

[aldenrw]

Wuh-oh. Both the earth and moon show visible discs in that image. Just a gut feel, but I'm betting by that point it would be too late to inflict a significant course change on the asteroid. So... DUCK!

[TEB]

Moving all that mass around to intercept an asteroid would be very inefficient. Also unnecessary. If the object is a puffball, like the one that just arrived over Russia, then interception is not really necessary. It will break up in the atmosphere just like that one did. Only the solid ones are really dangerous. So a contact propulsion system would work fine and be much faster an more efficient.