Transterrestrial Musings

Comments

Rand

A science fiction story that I enjoyed "A World out of time" by L. Niven had the future Earth moved out to Jupiter by pulling it behind Uranus. I believe the story's engineers used a similar system.

Posted by Rod at April 4, 2006 08:02 AM

Re: World Out of Time

It's important to mention that they had to move Earth because the Sun grew hotter and larger (the story takes place millions of years in the future). Not just for fun. :)

Posted by V-Man at April 4, 2006 08:23 AM

"..pulling it behind Uranus."

There's a joke there, but it's just too easy. ;-)

Posted by Cecil Trotter at April 4, 2006 08:41 AM

In order to have a gravity field large enough to influence an asteroid wouldn't our gravity ship need to be pretty massive (either dense or large)? Wouldn't that create a whole host of additional problems worse than simply using a nuke airburst to boil off the surface layer of the target and shift its course slightly?

Personally I like the idea of capturing an asteroid now, putting a mass driver on it and parking it in cislunar orbit. Than if we spot an asteroid coming our way we get the best pool player in the business to line up the shot (via some kind of Xbox interface) and fire away.

Posted by rjschwarz at April 4, 2006 08:51 AM

It depends on the size of the asteroid, but you don't need to nudge it much to change its orbit enough to render it harmless.

Posted by Rand Simberg at April 4, 2006 08:56 AM

It depends on the size of the asteroid, but you don't need to nudge it much to change its orbit enough to render it harmless.

And the earlier we know about a potential hit, the less of a nudge we'd need to sufficiently modify the orbit. That, of course, is a whole different issue.

-S

Posted by Stephen Kohls at April 4, 2006 09:16 AM

To make any signficant difference in the orbit the body that "tows" the second body must have its own source of thrust to enable the "towing". It is much easier just to move the first object!

Dennis

Posted by Dennis Wingo at April 4, 2006 10:04 AM

It is much easier just to move the first object!

Not necessarily. If it's a comet, or carbonaceous, it could be too fragile to grapple, or apply thrust to, without breaking it up. Gravitational forces would be gentle and completely distributed, by definition, rather than having force concentrations.

Plus, the tow truck could be outfitted in cis-lunar space, rather than having to attempt to attach a thruster to an asteroid millions of miles from earth. This actually makes a lot of sense to me.

Posted by Rand Simberg at April 4, 2006 10:15 AM

The problem is not just that the object to be towed is too fragile, but rather that it is likely to be rotating or tumbling. It would be difficult to deploy a solar sail on a rotating asteroid.

Posted by Paul Dietz at April 4, 2006 10:52 AM

The problem is not just that the object to be towed is too fragile, but rather that it is likely to be rotating or tumbling.

Yes, another good point.

Posted by Rand Simberg at April 4, 2006 10:55 AM

Shortly after sunset Friday, April 13, 2029, if the sky is clear enough, people across Europe and North Africa will see an asteroid appear as a bright point of light flying 19,400 miles overhead before it disappears silently below the western horizon. A short time later, if astronomers' worst fears are realized, the asteroid will pass through a region of space less than 2,000 feet across. At that place, the gravitational pull of Earth will yank the asteroid into a new orbit around the sun - and on a collision course with Earth seven years later.

Here is an idea to raise money to respond to this threat, without depending on tax dollars. Sam Dinkin's business model is the inspiration, of course.

Contact an on-line or internet c@sino for sponsorship and create a paramutual style betting system to predict exactly where this asteriod passes in 2029. If it passes within the 2000 foot "keyhole" ALL proceeds are made available and added to a prize fund to deflect the puppy. (This couldn't be the sole souce of funding as 2029 would be far far too late to start a defensive mission from scratch but it could provide supplemental funding as the critical moments approached.)

If it passes outside the keyhole, the accumulated money is distributed among those who gave the best or more accurate predictions of how far out the keyhole it passes.

= = =

Are there fly-bys sooner than 2029? Set up some prediction grids for those passes to generate publicity and some revenue.

Golden Palace (for example) might take this for its own pubicity purposes.

Posted by Bill White at April 4, 2006 10:55 AM

Reading the article more carefully, maybe predicting the asteroids observed locations in 2013 (when the asteroid re-emerges and becomes visible to radar) could raise money and awareness sooner.

Posted by Bill White at April 4, 2006 10:59 AM

As much as some of us (me included) have been bashing science articles, this one is very well written.

I'm not sold on the gravity tractor thing, though. The quote from the article that a 1-ton craft could move Apophis out of a keyhole just doesn't sit right in my gut. Anyone seen (or done) any of the math?

Posted by Tom at April 4, 2006 12:36 PM

I can see that working in theory - but we don't live there! 1 ton force over 1 month on a 1,000,000 ton object (rough guess) gives a delta v of about 26 m/s - probably enough to make a difference.

The real problem is that "hovering" action. What is making it hover? Any propellant expelled in the propper direction to cause the hover will hit the planet, exactly eliminating the pulling effect. In order to avoid that, you would need to have two engines firing at angles to each other - in other words, wasting fuel - so you're probably better off putting a bunch of ion thrusters on the surface.

Posted by David Summers at April 4, 2006 12:44 PM

I was envisioning it flying alongside, in formation, gradually turning and pulling its trajectory sideways. It's not efficient, but it doesn't necessarily need to be.

Posted by Rand Simberg at April 4, 2006 12:48 PM

A one-ton object doesn't exert one ton of force. The force goes down by the square of the radius. That's the rub. Based on a quick calculation I did using the gravitational equation (search Wikipedia for Gravitational Constant), assuming point masses of 1,000,000,000 kg and 1,000 kg, 1 newton of force is exerted between the two when they're 8 meters from each other. That would be inside the asteroid.

If you're 1000 meters from the center (just a number I pulled out of my head) the force is .00006672 N, leading to an acceleration of 6.672e-14 m/sec2 on the asteroid. Applied over 1 month (the timeframe given in the article), that's 1.73e-7 m/sec2.

Again, this is all possible,especially since we're only preventing keyhole passage, but it would need to be done decades in advance, or for a much longer period of time, I would think.

Maybe the numbers are just too big/small for me to wrap my puny brain around.

Posted by Tom at April 4, 2006 01:17 PM

Yes, I was assuming that they meant "a rocket large enough that it weighed 1 ton when next to the asteriod". You are assuming a 1000 kg rocket. You're assumption is more defensible, but the way I did it works, so I assumed that was what they were thinking of.

Posted by David Summers at April 4, 2006 01:21 PM

The keyhole is merely 600 meters meaning the amount of deflection is miniscule. That is why deflection before 2029 is so important.

Posted by Bill White at April 4, 2006 01:27 PM

I still think blowing up the asteroid (nuke or collision with another asteroid) would be better. Whomever did it could film the event and sell it on pay-per-view to help pay for it. If WWF can make a bizzilion off of pay per view what would nuking an asteroid get you?

And its a lot better then diggin up the world's best deep core drillers and sending them up.

Posted by rjschwarz at April 4, 2006 01:48 PM

A short time later, if astronomers' worst fears are realized, the asteroid will pass through a region of space less than 2,000 feet across.

We could alter the orbit of the ISS and arrange for it to fill that 2000' gap at just the right time. This would simultaneously eliminate A) the greatest threat to Man's future in space, and B) the asteroid.

Posted by lmg at April 4, 2006 03:36 PM

"It is much easier just to move the first object!"

I'd have to agree. Anything large enough to generate a significant gravitational force is also going to be way too massive for us to move, much less launch into space. A far more efficient use of the mass than mere dead weight producing gravity would be just in terms of chemical thrust, or nuclear propulsion of some sort. A nuclear thermal thruster could push something at whatever acceleration needed far more effectively than it could gravitate it.

Posted by at April 4, 2006 05:45 PM

Is it just me, or are people desperate not to solve this problem in the obvious manner?

Posted by Aaron at April 4, 2006 05:48 PM

Is it just me, or are people desperate not to solve this problem in the obvious manner?

No kidding. Just light the damned thing up from whatever the appropriate standoff distance is. Feel to take as many test shots as you want, too... it's not like we're low on ammo.

Posted by at April 4, 2006 06:02 PM

"Feel free to take," that is, obviously!

Posted by at April 4, 2006 06:04 PM

Anything large enough to generate a significant gravitational force is also going to be way too massive for us to move, much less launch into space.

Not really. You could get most of the mass from the asteroid itself. The ratio of masses of the attractor mass to the asteroid needn't be large at all -- maybe 1/10,000th the mass of the asteroid, if that, operated over a period of maybe a decade.

The difficult part would be the low thrust rocket or sail.

Posted by Paul Dietz at April 4, 2006 06:23 PM

Anything large enough to generate a significant gravitational force is also going to be way too massive for us to move, much less launch into space.

Who would be foolish enough to launch all of the needed mass into space? There's lots of mass already in space.

Posted by Rand Simberg at April 4, 2006 06:42 PM

[i]Who would be foolish enough to launch all of the needed mass into space? There's lots of mass already in space.[/i]

In a similar orbit to said asteroid? Applying dv to a 100,000 ton object such that it will be brought alongside another 100,000 ton object would require expenditures of fuel on the order of 100,000 tons for most inner solar system orbital adjustments. (Furthermore, assuming you could move such a massive object, why wait for gravity to do the job? Why not just slam the one into the other and send them both off flying in different directions?)

Posted by at April 4, 2006 08:42 PM

It is a great idea and one that I came "close but no cigar" to inventing. When I was thinking about moving double asteroids( they're more common than you might think) with mass drivers a couple of years ago. Dennis and I and a few other guys collaborated on a paper about that.

I realized that you only had to push one of the two asteroids; the other would follow, dragged along by gravity, so long as you didn't exceed relative escape velocity. That's not too hard, particularly for a large asteroid. It just didn't occur to me to use a heavy ship as a surrogate asteroid satellite. This was partly, I suppose, that with a high thrust energy efficient momentum transfer device like a mass driver, you need to use a very massive ship, hundreds or thousands of times more massive than the very low thrust and energetically inefficient thrusters Ed Lu has proposed.Of course, now we can see a reason to build such ships. They might even be largely constructed of in situ asteroidal material.

The beauty of the idea from the point of view of a mass driver proponent is that you no longer have to do the messy and costly despin operation that we previously thought necessary before beginning to alter the asteroid's trajectory.(O'Leary, 1979) This trick should allow much more rapid change in velocity for a given propulsion system.

Another benefit is that it offers a neat way to use solar sails.

Posted by Lee Valentine at April 4, 2006 09:53 PM

if it's just a 600 m diameter fragile asteroid, why not net it and attach solar sails or mass drivers then? The net shouldn't be a thousand ton job, since the acceleration is going to be wery low. Certainly it would be much lighter than a gravity attractor.
(And if you are going to use mass from the asteroid for the attractor, then you have to net it anyway.)

Because of the funcamental weakness of the gravitational force in relation to mass, the intial idea seems crazy to me. Maybe only if you can somehow get away with extremely minuscule deltavees, you can send a satellite to orbit the asteroid and thrust gently in the transverse direction or something, if that helps.

Posted by meiza at April 5, 2006 06:08 AM

*fundamental

Posted by meiza at April 5, 2006 06:11 AM

In a similar orbit to said asteroid?

A very similar orbit. :)

Posted by Paul Dietz at April 5, 2006 09:17 AM

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