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How to move an asteroid

 
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Sir Hamster of Elderberry
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PostPosted: Thu Nov 10, 2005 9:38 am    Post subject: How to move an asteroid Reply with quote

Quote:
Nature 438, 177-178 (10 November 2005) | doi:10.1038/438177a
Gravitational tractor for towing asteroids


[Link may require registration at (or at least cookies from) Nature.com to work. SHoE]

Edward T. Lu1 and Stanley G. Love1

A spacecraft could deflect an Earth-bound asteroid without having to dock to its surface first.

We present a design concept for a spacecraft that can controllably alter the trajectory of an Earth-threatening asteroid by using gravity as a towline. The spacecraft hovers near the asteroid, with its thrusters angled outwards so that the exhaust does not impinge on the surface. This proposed deflection method is insensitive to the structure, surface properties and rotation state of the asteroid.


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Mildew
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PostPosted: Thu Nov 10, 2005 9:56 am    Post subject: Reply with quote

Since I don't have a subscription, I'll do the stupid questions here instead Wink

What kind of mass would such a spacecraft need to have to make any real difference in trajectory of a large object?

If a big mass is needed, how is the spacecraft launched?

How much fuel would the spacecraft consume?

How long time would the spacecraft need to deflect an object?
The time would of course affect both the mass and engine capacity needed for the spacecraft.
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Sir Hamster of Elderberry
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PostPosted: Thu Nov 10, 2005 10:23 am    Post subject: Reply with quote

Some of the figures are not easily copied here, but I did put in images for the formulas. Registration is free though, and site related spam is practically nonexistant.

The answer to Mildew's question is "Not Much", but you might need a considerable head start (like 20 years):

The thrust and total fuel requirements of our mission example would be well within the capability of proposed 100-kilowatt nuclear-electric propulsion systems2, using about 4 tonnes of fuel to accomplish the typical 15 km s-1 rendezvous and about 400 kg for the actual deflection. For a given spacecraft mass, the fuel required for the deflection scales linearly with the asteroid mass.

OK, there was more to Mildew's question, but you can read it for yourself. --- SHoE

Quote:
Gravitational tractor for towing asteroids

Edward T. Lu1 and Stanley G. Love1

A spacecraft could deflect an Earth-bound asteroid without having to dock to its surface first.

We present a design concept for a spacecraft that can controllably alter the trajectory of an Earth-threatening asteroid by using gravity as a towline. The spacecraft hovers near the asteroid, with its thrusters angled outwards so that the exhaust does not impinge on the surface. This proposed deflection method is insensitive to the structure, surface properties and rotation state of the asteroid.

The collision of a small asteroid of about 200 m with the Earth could cause widespread damage and loss of life1. One way to deflect an approaching asteroid is to dock a spacecraft to the surface and push on it directly2. The total impulse needed for rendezvous and deflection is too large for chemical rockets, but would be achievable by spacecraft such as the 20-tonne nuclear-electric propelled vehicles that were proposed as part of NASA's Prometheus programme2.

Regardless of the propulsion scheme, a docked asteroid tug needs an attachment mechanism because the surface gravity is too weak to hold it in place. Asteroids are likely to be rough and unconsolidated, making stable attachment difficult. Furthermore, most asteroids rotate, so an engine anchored to the surface thrusts in a constantly changing direction. Stopping the asteroid's rotation, reorienting its spin axis3, or firing the engine only when it rotates through a certain direction, adds complexity and wastes time and propellant.

Our suggested alternative is to have the spacecraft simply hover above the surface of the asteroid. The spacecraft tows it without physical attachment by using gravity as a towline. The thrusters must be canted outboard to keep them from blasting the surface (which would reduce the net towing force and stir up unwanted dust and ions).

This scheme is insensitive to the poorly understood surface properties, internal structures and rotation states of asteroids. A spacecraft needs only to keep its position in the direction of towing while the target asteroid rotates beneath it. The engines must be actively throttled to control the vertical position as the equilibrium hover point is unstable. The horizontal position is controlled by differential throttling of engines on opposite sides of the spacecraft. The spacecraft can be made stable in attitude by designing it like a pendulum, with the heaviest components hanging closest to the asteroid and the engines farther away.

The thrust required to balance the gravitational attraction is given by



where G is the gravitational constant; see Fig. 1 for definition of other variables. Thus a 20-tonne spacecraft with phi=20° hovering one half-radius above the surface (d/r=1.5) can tow an asteroid of 200 m diameter and density rho=2times103 kg m-3, provided it can maintain a total thrust T of just over 1 newton.

Figure 1: Towing geometry of a gravitational tractor.

The asteroid (assumed to be spherical) has radius r, density rho and mass M. The spacecraft has mass m, total thrust T and an exhaust-plume half-width phi. It hovers at distance d from the asteroid's centre, where its net thrust balances its weight. The thrusters are tilted outwards to prevent exhaust impinging on the asteroid surface.
High resolution image and legend (28K)

The velocity change imparted to the asteroid per second of hovering (Deltav) is given by



So the velocity change imparted to the asteroid in our example in a single year of hovering is 1.9times10-3 m s-1. Because Deltav is largely independent of the asteroid's detailed structure and composition, the effect on the asteroid's orbit is predictable and controllable, as would be required for a practical deflection scheme.

The mean change in velocity required to deflect an asteroid from an Earth impact trajectory is about 3.5times10-2/t m s-1, where t is the lead time in years4. So a 20-tonne gravitational tractor hovering for one year can deflect a typical asteroid of about 200 m diameter given a lead time of roughly 20 years.

The thrust and total fuel requirements of our mission example would be well within the capability of proposed 100-kilowatt nuclear-electric propulsion systems2, using about 4 tonnes of fuel to accomplish the typical 15 km s-1 rendezvous and about 400 kg for the actual deflection. For a given spacecraft mass, the fuel required for the deflection scales linearly with the asteroid mass.

Deflecting a larger asteroid would require a heavier spacecraft, more time spent hovering, or more lead time. However, in the special case in which an asteroid has a close Earth approach, followed by a later return and impact, the change in velocity needed to prevent the impact can be many orders of magnitude smaller if applied before the close approach5. For example, the asteroid 99942 Apophis (2004 MN4), a 320-m asteroid that will swing by the Earth at a distance of about 30,000 km in 2029, has a small probability (10-4) of returning to strike the Earth in 2035 or 2036 (ref. 6). If it is indeed on a return impact trajectory, a deflection of only about 10-6 m s-1 a few years before the close approach in 2029 would prevent a later impact (A. Carusi, personal communication). In this case, a 1-tonne gravitational tractor with conventional chemical thrusters could accomplish this deflection mission as only 0.1 newtons of thrust would be required for a duration of about a month. Should such a deflection mission prove necessary, a gravitational tractor offers a viable method of controllably steering asteroid 99942 Apophis away from an Earth impact.
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PostPosted: Thu Nov 10, 2005 12:26 pm    Post subject: Reply with quote

Yes, I am now registered, but no, I can't access the article. Registration is free, but you must either pay for a subscription to Nature or pay for the individual articles to see them.

Thanks for posting the article's text so that we can read it. #ni-1
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Sir Hamster of Elderberry
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PostPosted: Thu Nov 10, 2005 12:55 pm    Post subject: Reply with quote

Millennium Knight wrote:
Yes, I am now registered, but no, I can't access the article. Registration is free, but you must either pay for a subscription to Nature or pay for the individual articles to see them.


After much puzzlement, I think I figured this out. I am subscribed to the Nature Alert (free), but not to the magazine itself (not free).

Go to "My Account", select the "E-alerts" tab, Scroll down and select the "Nature" checkbox. This should sign you up for the weekly newsletter with direct links.

Hmmm. that still doesn't explain why I can see the pages but some others can't. Sad

Here are some alternate links to try:
http://www.nature.com/doifinder/10.1038%2F438177a
http://www.nature.com/nature/journal/v438/n7065/abs/438177a.html
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PostPosted: Thu Nov 10, 2005 3:48 pm    Post subject: Reply with quote

Sir Hamster of Elderberry wrote:


Hmmm. that still doesn't explain why I can see the pages but some others can't. Sad
l

I have access to the file because the institute I work for has an institutional subscription. Maybe you have the same?
So from home I can't access the article. (Unless I login to a server at work)
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Sir Hamster of Elderberry
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PostPosted: Thu Nov 10, 2005 6:11 pm    Post subject: Reply with quote

Ercewee wrote:
Sir Hamster of Elderberry wrote:


Hmmm. that still doesn't explain why I can see the pages but some others can't. Sad
l

I have access to the file because the institute I work for has an institutional subscription. Maybe you have the same?
So from home I can't access the article. (Unless I login to a server at work)


Hmmm ... could be.

This isn't the first time this problem has occurred, and some people are able to access the site anyway(Sir Loin, for example).
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Mildew
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PostPosted: Fri Nov 11, 2005 1:39 am    Post subject: Reply with quote

That was a very interesting article. Thank you Sir Hamster! Very Happy
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PostPosted: Sun Nov 13, 2005 4:28 am    Post subject: Reply with quote

I Fart in the General Direction of your silly asteroid Razz . Strap my ass to a Titan rocket, get me positioned in front of the offending space rock, and I'll use my anus to blast it to Uranus Shocked Smile Confused Embarassed .

All I require in payment is a couple of kegs of beer which I assume will stay cold in deep space and a couple of wild and wolly sheep that I can shag rotten while on my space journey Twisted Evil .

Oh yea, I'm gonna need some good space brakes on my spacecraft cuz I'm gonna hit Warp 11 after I let 'er rip . You wouldn't want me to shoot by earth and suffer a fiery demise in the sun, now would you Cool ?

#ni-1
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PostPosted: Sun Nov 13, 2005 4:35 pm    Post subject: Reply with quote

Quote:
wouldn't want me to shoot by earth and suffer a fiery demise in the sun, now would you ?


Nope. I'm not ready yet for it to go nova Wink
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