[X] My Assistant

[dvandorn]

OK -- before y'all get all het up over the topic title, let me emphasize that this is *my* new idea for asteroid defense. I want to know what people think.

My idea deals with the subset of NEOs that are rubble piles. I'm assuming that a rubble pile is made up of numerous small bodies ranging from sub-micron size up to pieces of solid rock as large as 20 or 30 meters across.

My idea is based on the concept that the Earth's atmosphere can handle the impact, over a period of days and weeks, of thousands of tons of meteorites without generating catastrophic atmospheric heating. The reason the entire mass of an asteroid will cook you whether it comes in intact or in millions of pieces is based on the concept that the entire mass enters the atmosphere within a very short time frame.

So -- if you can bust a rubble pile apart such that the rubble enters the atmosphere over a period of days, or weeks, and if you can push the larger frags away from impacting trajectories, you'd be reducing the overall impact of even a large-ish rubble pile. Depending on how much mass is in the entire pile, you could reduce the overall impact of the event to eliminate any serious threat to life on Earth.

So -- the idea is to choose a point in the asteroid's orbit where you can maximize the spread of the rubble into the largest ellipse possible prior to its impacting the Earth. You use whatever means is most efficient to effect a *relatively slow* disassembly of the rubble pile into this disperse ellipse. And here's the point that I don't think I've read or heard anyone come up with before -- you attach propulsion and attitude control systems to the largest remaining chunks and steer them into trajectories that are designed to 1) disperse the remaining rubble even further, and 2) push them onto trajectories that don't impact Earth.

This is why you want the *relatively* slow initial breakup speed. You use the gravity interactions between the large chunks in their planned traverses of the rubble to spread it all out to your specifications.

If you have a good decade to plan and implement such a defense to a given body, I think it might be one of the few strategies that could be done within our current technologies. It would be expensive -- you'd have to jet around within the initial debris field, attaching propulsion modules to the biggest chunks, and you wouldn't be able to design your large-chunk trajectories until after the breakup was effected. It would take a lot of energy for the maneuvering, and you'd have to have rather massive armor to jet around within the rubble field. But it's do-able with current technologies, if not easily.

-the other Doug

[lyford]

let me emphasize that this is *my* new idea for asteroid defense.

Are you sure you thought this up on your own?

[nprev]

97310 with six guys left? Haven't seen a 'Stroids score like that since high school...

oDoug, it sounds possible but I wonder about practical. Seems like this would require a very large & labor-intensive manned mission to accomplish the fracturing & attach the deorbit packages to the major chunks. I also wonder about the risks to the crew (or even to an armada of extremely sophisticated unmanned vehicles) during the breakup phase.

Shooting from the hip, here, I'd estimate the price tag at about $20 billion with at least five years of development time (and also assuming that a lot of readily adaptable off-the-shelf hardware like the mature Constellation fleet is available). Worth it, of course, if it works & we face a significant impact threat

[JRehling]

It seems to me that given enough lead-time, the best approach is to nudge the thing off course so that it just misses the Earth. If you only have to nudge it by 4,000 km and you have lead-time of a year, then naively you only have to impart a delta-v of 0.13 m/s.

Naturally, for larger bodies, small delta-vs still translate into a lot of kinetic energy, but then, so does blasting an object apart. I'm sure that less kinetic energy is involved in a nudge than an explosion (which would naturally accelerate the individual pieces by much more than 0.13 m/s). A nuclear weapon might end up doing more nudging than blowing-apart regardless of our intentions.

For really long lead time (multiple orbits), I think the desirable situation is to determine on which upcoming pass the object would come nearest the Moon and then just steer it right into Luna, eliminating that particular threat permanently.

[nprev]

For really long lead time (multiple orbits), I think the desirable situation is to determine on which upcoming pass the object would come nearest the Moon and then just steer it right into Luna, eliminating that particular threat permanently.

Wouldn't that be a bit more difficult to accomplish in terms of guidance requirements given that the Moon's orbit is geo- rather than heliocentric? Seems as if the delta-V would have to be extremely precise, and I don't know of a way to make large changes in velocity with great accuracy. (Not so bad with mass drivers or ion thrusters, but of course these take considerable time).

My take on the whole thing is that, generally, any delta-V that takes a threatening asteroid out of an uncomfortably close encounter ellipse (guess "prolate spheroid with semimajor axis roughly aligned along the object's net orbital motion vector" would be more accurate ) is all that's needed. We can get fancy later when it's time to start mining the things.

BTW, that brings up a poser: What do we do if we find a threatening object that turns out to be very rich in metals or volatiles (long-period comets excluded because, uh, they'd exclude us in any case)? I'm thinking that we'd want to do more than just deflect it or destroy it, we should think about putting it in an accessible orbit for later use.

[tty]

If you have time enough I think the "gravity tow" concept is best. It will work on a rubble pile too. And if you don't have time enough, then I would suggest that the least bad alternative would be to use one or more penetrating nuclear charges, and hope to bust it up into pieces small enough to air-burst. That would be nasty, but probably not as nasty as a direct hit.

Also you might want to be a wee bit careful about moon impacts, a lot of the secondaries would probably end up on Eart. Now a Mars impact on the other hand....

[Del Palmer]

BTW, that brings up a poser: What do we do if we find a threatening object that turns out to be very rich in metals or volatiles (long-period comets excluded because, uh, they'd exclude us in any case)? I'm thinking that we'd want to do more than just deflect it or destroy it, we should think about putting it in an accessible orbit for later use.

Good thinking, although it would be nice to study it up-close before we mine the heck out of it...

Regarding disposal a la luna, I don't think that would be compatible with expected future uses of the Moon.

[Guest_PhilCo126_*]

By the Way... This year SpaceGuard will celebrate its 10th anniversary:

http://spaceguard.esa.int/SSystem/SSystem.html

http://impact.arc.nasa.gov/presentations_main.cfm
Philip

[peter59]

Tonight, binary asteroid 2008 BT18 passed 1.4 million miles from Earth.

http://www.universetoday.com/2008/07/14/bi...des-past-earth/

[Greg Hullender]

Also worth mentioning is that NASA's offical word on the subject is that "nuclear stand-off explosions were found to be 10-100 times more effective than the non-nuclear alternatives".

http://neo.jpl.nasa.gov/neo/report2007.html

This is the full report:

http://www.nasa.gov/pdf/171331main_NEO_report_march07.pdf

Rusty Schweickart makes a counter argument that although the other methods aren't as powerful, they're only needed for really big rocks, and by 2020, NASA will already know where all of those are.

http://www.sciam.com/article.cfm?id=nasas-...nuclear-weapons

Oh and everyone seems to agree that fragmenting the asteroid makes things worse, not better. I can guess why, but I haven't seen that clearly spelled out anywhere.

--Greg

[Thucydides]

Most asteroid defense schemes fail because they are too slow or too limited in effect. Many Earth crossing asteroids have been spotted literally "at the last minute" rather than 5 or 10 years in advance, and that leaves very little time to send a mission, much less decide which of the various alternatives will work out best.

The answer has to be very high performance vehicles; high thrust and high ISP; which only leaves the Orion nuclear drive. "Next Big Future" outlines a conceptual high performance interceptor here: http://nextbigfuture.com/2009/02/unmanned-...lear-orion.html, but this paragraph sums it all up:

Get to high velocities with only a few explosives and small shock absorbers or no shocks at all. Launch against a 100 meter chondritic asteroid coming at 25 km/sec. 1000 megatons if it hits. Launch when it is 15 million kilometers away and try to cause 10000km deflection. A minimal Orion weighing 3.3 tons with no warhead would do the job. 115 charges with a total of 288 kiloton yield. Launch to intercept in 5 hours. Ample time to launch a second if the first failed.

A Gigaton of kinetic energy will ring anyone's bells, and even a rubble pile asteroid will have a significant fraction of its mass converted to vapor or plasma, and the rest diverted into many non Earth crossing orbits. The nose of the spacecraft can open up like an umbrella once clear of the atmosphere to ensure the energy is transferred to the asteroid and not lost by punching through the mass of the target.

A more versatile and capable Orion vehicle would include shock absorbers that are unlocked after clearing the atmosphere and the ability to go at a much more leisurely pace. If the asteroid was a rich source of metals and volatiles, a soft "landing" could be arranged and the Orion used to gently nudge the asteroid into a safe orbit for exploitation.

The physics is easy, managing the political considerations and deflecting the "junk science" objections would make the project very hard......

[Vultur]

Yeah, I don't think the public opinion would ever let NASA build an Orion...

Unfortunate, because the fallout would be insignificant compared to the effects of being hit with an asteroid. (Some things I've seen make it sound like it would be insignificant, period, with modern technology; apparently the right kind of launch plate would help a lot.)

[SpaceListener]

I think that the most feasible solution would put a pair surveillance satellites at two Lagrangian points: L1 (near to Sun) and L2 (shadow from Moon). On the other hand, I think that the selection of these points L1 and L2 is due to the fact the most asteroids travel to and away from Sun. I might be wrong.

[nprev]

Re NEO detection, the existing ground-based programs (Spaceguard, LINEAR, etc.) are doing an outstanding job. I don't think that a space-based asteroid survey system of any sort would provide any significant gain in the discovery rate at this point, and certainly not enough to justify its cost.

[SpaceListener]

Re NEO detection, the existing ground-based programs (Spaceguard, LINEAR, etc.) are doing an outstanding job. I don't think that a space-based asteroid survey system of any sort would provide any significant gain in the discovery rate at this point, and certainly not enough to justify its cost.

Why not significant gain?

There, the view has much better vision so the time of anticipation is sooner.

About the cost, don't mind it since any hit of asteroid on Earth might out-weight to the mission cost.

[nprev]

I'll hunt around for real citations & numbers later when I have some time, but from what I gather they've already detected something like 95% or more of all NEOs >1km in "diameter" using existing searches based on statistical modeling. So, the rate of discovery has already leveled off for the largest & most threatening population of objects, and in fact I'm sure that it's declining already. Plus, they're already finding plenty of little rocks that don't pose a threat so the detection threshold is dropping as technology improves & experience is gained.

Based on that, it would be hard indeed to justify a space-based search system since it almost certainly wouldn't find anything truly new & the price tag would be high.

[SpaceListener]

Nprev, good Insight with another perspective. Let see what would be the best solution after a time.

[PDP8E]

For the case of long lead times (5-50yrs) then the Gravity Tractor (love that name) seems to fit the bill.

http://en.wikipedia.org/wiki/Gravity_tractor

As you know, the B612 Foundation is focusing on this issue...

http://www.b612foundation.org/about/welcome.html

On the other hand, those close in, find them at the last minute types...thats a tough one.

I hope to see more ideas on how to deal with them

[Greg Hullender]

Biggest hole that space-based detectors might fill is rocks for which Earth is near aphelion. They spend so much time close to the sun, and the phase angle is usually so bad, that they're hard to detect from Earth. A Venus-orbit (even Sun-Venus L2) telescope has been proposed to make it easier to catch those. Earth-based systems ought to eventually catch everything else.

There are lots of details ini the report to Congress:

http://neo.jpl.nasa.gov/neo/report2007.html

--Greg

[nprev]

Thanks for adding that link, Greg; good to have real detection numbers, and I was in error.

I could see an NEO detector as a ride-along on a Venus orbiter (maybe a high-res/highly targeted radar mapper as a follow-up to Magellan?) It's a much easier sell for any mission if it's able to satisfy multiple scientific objectives, though a line has to be drawn, of course.

Although the upper atmosphere of Venus is pretty chilly if infrared was selected as the band of choice, the sheer albedo of the dayside combined with greater proximity to the Sun makes me wonder how effective an NEO instrument would be from there unless the spacecraft could dwell in the planet's shadow; an equatorial Molinya orbit, perhaps?

[Greg Hullender]

I think the idea was just for the telescope to be in a "Venus-like" orbit. Sun-Venus L2 would work (as would L4 and L5) but simpler would probably just be an orbit a few million miles larger than Venus'. Having it near Venus itself wouldn't buy you anything *I* can think of. Using Venus for gravity assist (or even aerobraking) might work, but I haven't seen a concrete proposal.

That seems to rule out trying to link it to another mission, though. A big telescope would seem like a big ask for such a thing anyway.

--Greg

[nprev]

That seems to rule out trying to link it to another mission, though. A big telescope would seem like a big ask for such a thing anyway.

--Greg

I agree, esp. considering the almost ridiculously low cost of extant ground-based searches to say nothing of low/no risks compared to a launch. As is, they're projecting crossing the 95% detection threshold in a bit more than 10 years, and I suspect that they'll get there sooner than that. A dedicated mission would likely be obsolete by the time it flew!

[dvandorn]

I'll hunt around for real citations & numbers later when I have some time, but from what I gather they've already detected something like 95% or more of all NEOs >1km in "diameter" using existing searches based on statistical modeling. So, the rate of discovery has already leveled off for the largest & most threatening population of objects, and in fact I'm sure that it's declining already.

If all you're worried about are NEOs (insert Matrix joke here), then you're absolutely right. Why do I get the feeling, though, that a supremely smug human race, having cataloged every NEO which could possibly threaten, will expire with absolute shock and surprise when that 10-km comet comes around from the opposite side of the Sun and smacks us at solar near-parabolic velocity mere days or weeks after we first detect it.

In other words, it's not good enough to know about all the NEOs around, since it's an observed fact that such things only hit us every 50 to 100 million years. It's the body that sneaks in and provides us only days or weeks of warning that is more likely to kill us. (Not more likely to come in on an impact trajectory, just more likely to kill us since we will have insufficient time to respond.)

I think we need to have Solar System monitors that can scan on the far side of the Sun from us, as well as on the side we can easily scan from our own planetary neighborhood.

-the other Doug

[nprev]

Can't argue with an ultimate goal of knowing about every object in the Solar System that we can, of course!

Problem is that even if we knew years in advance about a long-period comet likely to strike us there really isn't a thing we could do about it. Those things pack tremendous kinetic energy while in the inner Solar System due to their elongated orbits & Kepler's Second Law.

The world's combined nuclear arsenal probably wouldn't be enough to stop nor deflect/disintegrate one on a direct collision course, even if there was some way to simultaneously deliver it all to the target.

[Greg Hullender]

The better question is how soon something like PANSTARRS or LSST could detect such a thing. I found references to an interesting article:

Edward Tagliaferri ... [et al.] -- Warning times and impact probabilities for long-period comets

I'll see if I can catch a look at it in the UW Library. Until then, some back-of-the-envelope calculations suggest we'd have decades of notice.

PANSTARRS says they'll be able to spot things down to almost magnitude 30. Halley's comet is a bit brighter than that, out near aphelion. Comets from the Oort cloud tend to have eccentricities close to 1, meaning little or no hyperbolic excess velocity.

That means it'll be going a lot slower than Voyager or New Horizons, so it ought to take at least a decade, and probably twice that, between the time we detected something the size of Halley and the time it reached us.

--Greg

[Greg Hullender]

Okay, I worked out how long it takes an object to fall straight into the Sun, given it's falling from infinity and we first observed it at distance R from the Sun. It's an easy calculus problem to get the velocity at distance R of a body falling from infinity:

V = sqrt(2GM/R)

Where G is the gravitational constant and M is the mass of the Sun. Obviously the velocity inicreases as it approaches the Sun. If it didn't, then we'd have T = R/V, but that'd be too easy.

So to get time-to-impact out of this, I used v = -dr/dt to set up a separable differential equation, from which I got

T = 2R/3V

Which is a kind of surprisingly clean result (so someone ought to check my math). I played with some real data, though, and it does look to be correct.

So when our hypothetical Killer from the Oort Cloud reached Earth's orbit, it was moving at 42 kps (relative to the Sun; 51 relative to the Earth) and from here it would have only taken 27.5 more days to hit the Sun if it hadn't hit Earth first.

If we had spotted it when it crossed Neptune's orbit (30 AU), we'd have had 12 years and a few months warning.

If we'd spotted it at the edge of the Kuiper Belt (50 AU), we'd have had over 26 years of advance notice.

And if we'd spotted it at 100 AU, we'd have had 75 years to figure out what to do about it.

Again, this models something with no hyperbolic excess velocity, but that's consistent with observation. The killer comet might be going slightly faster than this, but not very much.

Conclusion: Once the new telescopes are operating, nothing as big and bright as comet Halley (~10 km) can sneak up on us from the other side of the Sun.

--Greg

[tty]

A difficulty would be how early it would be possible to calculate the orbit with enough precision to know whether an incoming comet would be a threat. If the comet makes a tight turn around the sun a very minor error might become very large, so you might have to consider quite a few comets as threatening initially, though the number would decrease as they come closer in. Also comets can break up and spread out as we have all seen.

[dvandorn]

That's exactly the point I was about to make, tty. My concern is more for getting extremely accurate predictions of the trajectories of bodies on parabolic (and heck, even hyperbolic) trajectories. The ability to predict a trajectory after perihelion is affected not only by even minute errors in our knowledge of the pre-perihelion trajectory, but also by myriad factors including gravitational perturbations from Mercury, Venus, and even Earth, as well as the current state of solar weather. An active Sun blows more mass out into near-solar space than a quiet Sun, thereby increasing (minutely) the drag such an object will encounter "coming 'round the horn".

It's also possible (if not likely) that a rather dim object, a km or more in longest dimension, could even now be wandering in on a relatively steep fall to Sol that is targeted for a pretty wide perihelion, a fairly minimal solar deflection, and a trajectory that would cross Earth's orbit (where Earth might happen to be located at the time) on its outbound leg. Such a body could be dim enough that it wouldn't be easily spotted if it sweeps in from a vector roughly opposite that of the Sun from Earth (i.e., spending a lot, though not all, of its of time hidden in the glare), and/or, even if it's spotted, not being noticed until it's too late to get a great hack on its trajectory prior to its disappearing completely behind the Sun.

Yes, Greg seems to be quite correct, that once we get some of these telescopes up and running we'll have enough observing time/power to find most outlying threats with lots of time to spare. There's a short window, though, in which an object could announce itself with only days or weeks to spare. (Of course, if something did happen in that window, it's not like we have the deployable technology at present to do anything about an object of any size, even with a few years of warning... *sigh*... )

-the other Doug

[nprev]

One more thing: Don't forget that comets have built-in thrusters that can fire (or not) anytime they want, any direction, esp. near perihelion. Probably not much delta-V is ever imparted, but it does complicate the orbital projection problem.

Earth is after all a pretty small target considering the volume of the inner Solar System. I don't think we'd have even 80% confidence of a direct hit from a comet more than a few weeks in advance.

[Greg Hullender]

As is often the case, this is a problem in probability. Given a few observations of an object, there's more uncertainty than after a long series of them. Given the volatility of a comet, there's some inherent uncertainty. This does not, however, mean that the comet can just go anywhere. Instead, we can think about an imaginary tube in space that we're 99.9999% sure the comet will stay inside. After just a few observations, we'll almost always know that Earth is outside that tube. Space is large; very, very few comets will pose any threat even after just a couple of observations.

Given that we see the things a decade or two in advance, there is no "other side of the Sun," so I don't know where that idea is coming from. A year is enough time to make a series of very precise observations and even determine parallax. Anything that still has a better than one in one million chance of hitting Earth at that point probably ought to be treated as a real threat.

Even with this "fire on warning" criterion, I seriously doubt we'd need to do it even once in a thousand years. Note than in all of recorded history, the closest approach seems to have been about 5 million km.

http://ssd.jpl.nasa.gov/?great_comets

Of course, something smaller and darker than Halley's Comet might need something better than PANSTARRS, but smaller also means easier to move.

--Greg

[alan]

The better question is how soon something like PANSTARRS or LSST could detect such a thing....

PANSTARRS says they'll be able to spot things down to almost magnitude 30....

--Greg

I found this on the PANSTARRS web page

A single observation with the broadband filter will reach a 5σ depth of 24 magnitude. This are type of observation will be used to search for solar system objects. By adding observations taken over several years, Pan-STARRS should be able to reach a maximum depth of magnitude 29.4.

I assume the magnitude 24 limit applies to moving objects and the magnitude 29.4 limit is produced by adding individual images taken over several years and thus only applies to stationary objects.

At a limiting magnitude of 24 a dormant Halley sized object (10 km) would be detectable out to roughly 10 AU giving us only 2.3 years warning.

One of the proposals linked on the PANSTARRS site estimates an all sky survey for comets at a limiting magnitude of 24 (30seconds/image) would require 0.1 years of telescope time per year. If PANSTARRS was devoted only to a year round survey for distant objects the limiting magnitude could be increased to 26.5 using 300 seconds/image. Then a Halley sized object could be detected out to about 18 AU increasing the warning time to 5.5 years.

[Greg Hullender]

Good catch, Alan. I seem to have skimmed that too fast.

The LSST might do a better job:

http://lsst.org/lsst/faq/science-faq#q2

Since it would scan half the sky in under 4 days to magnitude 24.5, one could at least imagine extending the exposures to hit magnitude 29.5 and still cover the southern sky once annually.

But it definitely seems that if you wanted to spot things as small as 1 km (so 1/10 the radius of Halley) as far away as the Kuiper Belt, then you'd really want some facility located in space so you could take the ultra-long exposures needed to see them.

--Greg

[nprev]

Little bit of comet trivia: All observed comets so far on hyperbolic trajectories seem to have been in the process of ejection from the Solar System via interaction with Jupiter. An inbound comet on a hyperbola would presumably be extrasolar in origin, and therefore a target of extreme scientific interest.

Re the LSST: A comprehensive KBO survey, and perhaps even an inner Oort Cloud survey, seem well within its capabilities. If the latter was accomplished it might provide enough interesting data to justify JWST time to do a really thorough job.

[Greg Hullender]

. . . it might provide enough interesting data to justify JWST time to do a really thorough job.

Hmmm, if PANSTARRS could scan the sky once a year, all other things being equal, JWST would take 7,000 years. The JWST is meant to look deep into space, not broadly. The field of view on the JWST is 2.2 arc minutes.

http://www.stsci.edu/jwst/overview/design/

PANSTARRS has a 3-degree field of view. They've got a nice site that illustrates the differences.

http://pan-starrs.ifa.hawaii.edu/public/de...res/wf-wang.htm

It doesn't show JWST, but it does show Hubble, which has a 3 arc-minute field of view, so just remember JWST will be 2.2 and hence smaller.

--Greg

[SpaceListener]

New space-based infrared systems, combined with shared ground-based assets, could reduce the overall time to reach the 90 percent goal by at least three years. Space systems have additional benefits as well as costs and risks compared to ground-based alternatives.

Extracted article from Near-Earth Object Survey and Definition Analysis of Alternatives, Report to Congress March 2007
Hence, the idea to have a space surveillance would be the best candidate.

[nprev]

Hmmm, if PANSTARRS could scan the sky once a year, all other things being equal, JWST would take 7,000 years.

Well, THAT ain't gonna work, now, is it? Okay, so a dedicated spaceborne IR outer Solar System mapping instrument does look more appealing.

[dvandorn]

Here's another nightmare scenario -- a large comet rounds the sun, its projected path bringing it close to Earth but showing a miss prior to aphelion. Comet breaks up into several fragments as it rounds the sun in a fairly tight aphelion, some of them larger than 1km, and one of the fragments manages to alter course just enough to go into an Earth-impact trajectory.

Fortunately, we have at least some assets monitoring the Sun from other viewing angles than our own, we might be aware that the comet had broken up -- but we'd still have a relatively short period of time to react.

-the other Doug

[Greg Hullender]

Here's another nightmare scenario

That'd have to be an awfully close miss. Based on other comets we've seen break up, the pieces drift apart very slowly. Anything that had a chance of doing this would (under the rules I suggested earlier) be treated as a threat as soon as it was detected.

Note that Shoemaker-Levey 9 broke into pieces in 1992. Two years later, when it hit Jupiter, the pieces were still so close together that essentially all of them hit the planet. A comet passing the Sun might take a month or two from perihelion to Earth's orbit, but that's it. So that original near miss probably needed to be on the order of tens of thousands of km -- if that.

http://en.wikipedia.org/wiki/Shoemaker-Levy

--Greg

[SteveM]

The International Academy of Astronautics held its first conference on protecting the Earth from Asteroids in Granada (Spain) on 27 - 30 April 2009. Some may find the program and abstracts interesting.

Steve M