2007 Planetary Defense Conference

Belter · 2018-10-11 10:28:43

The penetration wouldn't need to be very deep, but deep enough that the radiant energy is more entirely captured. By detonating at the side, you're losing just over 50% of the energy. Just a few meters into it and you'd be much closer to 100%. And the chances of any large debris getting aligned to a collision course with Earth is incredibly small, unless this happens within lunar orbit. And if it happens within lunar orbit, we'd be wanting 90% of that blast energy, not 45% of it.

GW Johnson · 2018-10-11 13:39:17

Once you penetrate at all, you create a shock wave in a medium, that medium being the solid material of the asteroid. At nuke levels of energy release, that's a sure-fire recipe for disrupting a body of fairly substantial cohesion. Which most of these things are apparently not.

Note also what I said about solid particles over a quarter inch in size surviving entry to strike the surface. A few inches across is a crater a few meters across. Do this as a last ditch defense, and thousands or even millions of such debris particles strike the Earth as a shotgun blast.

That's what the physics says. Which is why you are far better off achieving deflection instead of disruption. Some of these things are very non-cohesive bodies, which makes deflection instead of disruption difficult indeed.

The other thing I pointed out was that, as yet, we don't know how to tell who is cohesive and who is not, or how much cohesion there really is.

It's way too soon to say we have any sort of asteroid defense capability. Much of the science, and most of the technology and hardware, is still missing.

You have to do this job "right". Doing it wrong usually makes the problem worse.

GW

Last edited by GW Johnson (2018-10-11 13:39:49)

Belter · 2018-10-11 14:19:15

I agree, there are definitely variables. But most lack of cohesion is going to more likely found in comet rather than in actual asteroids Most of the NEA are most likely pretty solid rocks. The likelihood that large rocks will stay together over millions or billions of years huddled together is pretty slim. Not sure we've found such a body that didn't have a lot of frozen materials among it. If the asteroid gets so close that we need to make a last ditch effort, we are screwed or less screwed, so I'll go with less screwed. But....if we can see this thing coming and intercept it millions of miles away, the shrapnel from the explosion that strong is going to be sent virtually 100% out of the collision course. This is one of the very first things that pissed me off about Gravity. That the explosion didn't scatter the debris in all directions, but just happened to put most all of it in an unlucky collision course. In any case, I don't see any risk in hitting it hard at a distance and it is pretty much nothing but mandatory to hit it with everything if it's inside lunar orbit. We could try to give it some nudges first and see how it goes.

I think the real solution longer term, since we won't have a short term one, is an array of ships, perhaps a milion or several million miles out, that would have nuclear weapons ready for launch and that would have enough time to get into position and launch missile strikes to change the paths. However, these things would almost certainly fall apart long before they would ever be needed. But for the much more common smaller asteroid, that might intersect lunar orbit, we might want to give it a nudge just because we have the ability and want the practice. And hope we don't make it worse by doing so.

Actually, a better idea and one more likely is that within 100 years, we're going to be annihilating those asteroids with self-replicating mining and printing robots that will attack any asteroid that gets close and turn it into spacecraft and space stations and using up any raw materials they have, even if it is just to build more robots that will fly off and eat another asteroid.

Last edited by Belter (2018-10-11 14:20:19)

elderflower · 2018-10-12 05:45:54

I don't like the fleet of ships with Nuclear arms on board. It might sound like a bad spy movie, but what a temptation for a bunch of criminals or a psychopathic head of a large state!
There would be a blast wave in space. I don't know how strong it would be but it would propagate through the medium of vaporised bomb structure.

Terraformer · 2018-10-12 10:58:55

Why use nukes, when you can use a giant mirror to concentrate sunlight?

Belter · 2018-10-12 15:20:37

elderflower wrote:

I don't like the fleet of ships with Nuclear arms on board. It might sound like a bad spy movie, but what a temptation for a bunch of criminals or a psychopathic head of a large state!
There would be a blast wave in space. I don't know how strong it would be but it would propagate through the medium of vaporised bomb structure.

But it would make for a great novel or movie. BFS-C ships could probably pull this off when it is finished, though not sure how big the missiles would have to be and how many could be carried.

Belter · 2018-10-12 15:21:42

Terraformer wrote:

Why use nukes, when you can use a giant mirror to concentrate sunlight?

Because nuclear missiles are easier and less expensive to make.

GW Johnson · 2018-10-12 16:43:54

Nukes would only be last resort. The community of scientists and engineers that go to those asteroid defense conferences prefer impactors and gravity tractors.

Unfortunately, those are effective only with long warning times, measured in years. That applies to near Earth asteroids, of which only those further out than Earth from the sun are currently observable. The sunward near Earth asteroids still elude detection, until only hours before the hit, and not all of those. Chelyabinsk was not detected at all.

There are the "comets" in the strongly-elongate elliptic orbits. Only a few are known. Most are surprises, detected only days before the closest pass. That will always be true. That is threat for which nukes are the only option, despite the severe side effects risks (disruption). Those are also the hardest to reach, in terms of launch vehicle capability.

Sounds like the means to send both equipment and men across the inner solar system, on a time scale of weeks, is a high priority, doesn't it?

Ever notice that almost nobody is working on that problem?

Sleep tight, knowing that.

GW

SpaceNut · 2018-10-12 20:34:22

I think that I have talked about making an optic telescope sonar like system but it will take many of them facing outward circling the earth mars band at a very slow rate to allow for it to work.

elderflower · 2018-10-13 04:10:09

Just to remind folks and perhaps inform this debate....
Not long after the Chernobyl meteorite, another impactor came in, undetected until the last minute, over the desert in Sudan. If that had hit a city there would have been considerable damage.
It was only the low entry angle and lack of cohesion that stopped the Chernobyl meteor from being a disaster. So we got away with it twice in fairly quick succession, although we had no detection and no defence.

Belter · 2018-10-13 08:31:13

Largish meteors are still only hitting us about once every 25,000 years. The really huge ones only happen every 1 million and the biggest ones only every tens of millions. The Earth has largely cleared its orbit like good planet. We could build a planetary defense system that only gets used every few thousand years.

A better idea might be to drop some satellites into a slightly elliptical solar orbits that have the ability to "tag" these with mini landers that can help us map their orbits, do soil samples, maybe hop around and look for minerals.

Last edited by Belter (2018-10-13 08:48:43)

GW Johnson · 2018-10-13 10:44:12

What I've been trying to communicate here (unsuccessfully, it seems) is that this issue is complex. There is no one "pat" answer. I know that is uncomfortable, but you have to deal with it.

Yes, the big ones are both rare and mostly identified by the search conducted so far. Those are the 1 km+ size extinction event objects and larger. That search has taken years to accomplish. The search down to 140+ m extinction-event objects is underway, and has not gotten all that far yet. Of the 4 planned telescopes for this purpose, only one (PANSTARSS) has been put into operation.

But we get multiple 1-10 m objects every year! Those are city-buster objects! Most of those are surprises, especially the ones coming from sunward of Earth's orbit like Chelyabinsk did. We do NOT have the detection capability for these. If we did have detection capability, there would be enough warning time to do impactor or gravity tractor deflection. Orbital periods measured in years.

Things on elongate comet-like orbits will always be short-warning surprises, but fairly rare. Human lifetime scales, it would seem (about the same rate as comet discovery, crudely speaking), for the smaller dimmer 10-100 m (city buster to regional catastrophe objects).

Those require nukes on big launchers within hours or days, because that's how long these things are near the sun, moving at very high speeds. Hopefully, the almost-inevitable disruption takes place far enough away for the spreading debris to mostly miss the Earth, thus reducing the damage to some city-busters or less. Although that debris will come around again, unless the orbit is hyperbolic.

The detection we need should be a belt of satellites well inward of Earth's orbit, looking outward, some visible, some IR, plus a radar capability. Something about the orbit of Venus has been proposed. Nobody is doing this. It would not be all that expensive to do. That could help us find the bulk of the 1 m or larger near-Earth city-buster asteroids. But that still leaves the cometlike objects as an undetectable threat, except as a last minute warning.

When I attended the 2009 conference, there was no Falcon-Heavy, and SLS looked to be more fiction than fact. The rockets to carry the impactors, any gravity tractor spacecraft, or the re-equipped and re-fuzed nukes just did not exist. Now, at least one exists (Falcon-Heavy), and the other (SLS) may soon exist, although not many of those will ever be available. You’re looking at a need to fling a few tons to very high C3 energies indeed, so don’t be fooled into thinking an Atlas-V 552 or anything similar could do this job.

Impactors are just slugs, but they need to be launched from a carrier spacecraft with precision guidance. So, who is working on one?

The revised fuzing for the nukes does not exist yet. They need the same sort of precision-guided carrier spacecraft as the impactors, although probably not exactly the same one. So, who is working on that?

Nobody I know of is working on a real gravity tractor spacecraft design, although NASA does seem to be working on some up-sized ion thrusters, of a type that a tractor craft might need.

And we still know next to nothing about predicting cohesion properties or spallation properties of these objects. We won't know for many years yet. And probably nothing "for sure" until men actually visit many of these things.

That's the state of the art, and it ain't good. Not yet, anyway.

GW

Belter · 2018-10-13 11:52:47

We probably have thousands of years to solve it still. We survived the last 100,000 years with no NASA. Even if a meteor hit NY City, humans would be fine. We are far more likely to die in a nuclear war of our own making than by asteroid.

Terraformer · 2018-10-13 15:16:12

The thing to remember about impactors is Rick's Law - An object impacting at 3 km/sec delivers kinetic energy equal to its mass in TNT.

To get a feel for how destructive an impactor could be, divide it's impact velocity by three and square it, telling you how many Ricks of damage it can do. Then multiply that by it's mass in tonnes.

A retrograde object falling from the outer solar system would hit with a velocity of up to 72 km/s. That's 576 Ricks. A 10m diameter ball of ice would hit 288 kilotons of explosive power. That should cause plenty of damage.

Fortunately, those are rare. Far more likely would be impacts in the 15-20 km/s range. To get comparable damage with a 25 Rick impactor, you'd need 10,000 tonnes. But a similar 10m diameter made out of silicates would still be more powerful than Little Boy. How often do they happen?

SpaceNut · 2018-10-13 16:30:05

The mass of the object is as well important to the impact but the shape as well plus angle of entry may slow the object down as it slices through the atmospher as does a lifting body with a circular shape not so much.

GW Johnson · 2018-10-15 09:27:04

I refer y'all to Mark Boslough's published work on simulation of asteroid impacts. The kinetic energy of the thing at entry is comparable to the energy yield of a nuclear weapon, yes, but that is not the whole picture. The asteroid (or comet) has a high momentum that the bomb does not. Boslough works at Sandia Labs.

Most of these are unconsolidated objects sometimes called carbonaceous chondrites (although I think those categories will need revision once we have visited a bunch of these things). Such tend to explode during entry before they can impact. But, depending upon entry angle and speed, that explosion can be quite low, as in Tunguska, instead of high, as with Chelyabinsk.

The fireball has a big momentum along the trajectory, completely unlike that of a bomb, and can strike the ground. In that event, you get widespread incineration, even if there is no impact crater. How often that happens is not very clear, as craters are preserved in the geologic record. Incineration events, not so very much.

These things don't have to be very large to bust cities like a nuclear weapon. At or just under a meter dimension will do it. Smaller objects like that are far more numerous, and happen to us more often. Mis-using the data without statistics, similar objects hit in Tunguska a century ago and recently in Chelyabinsk.

So the problem time seems closer to a century or so, not thousands of years. (You can probably get far better data and statistics from the Minor Planet Center that keeps the records on these things, or from Lindley Johnson at NASA.) It would therefore seem prudent to be working on these issues with some seriousness. What better reason could there be for a space program, than planetary protection?

I won't argue that we are more likely to incinerate ourselves with nuclear war, but I cannot see ignoring the asteroid threat, either! We've avoided the nuclear war for something over half a century now. That time is getting to be comparable to the time interval between Tunguska and Chelyabinsk. I just let those particular data speak for themselves.

GW

Last edited by GW Johnson (2018-10-15 09:32:49)

SpaceNut · 2018-10-15 20:45:25

The secondary effects of a ground penetrating piece of rock depending on the mass that it keeps plowing its way there will cause a ton of soils to be displaced into the air causing more damage to the surrounding area that is being shot at by hot materials from the impact.
Slice and dicing up the rock while farther out in space would be an option if we had the man power and equipment enroute to mining it before it could do any damage.

GW Johnson · 2018-10-16 14:01:08

Any surface impact will raise a tremendous hot debris cloud. The bigger the object, the bigger the cloud, the higher it goes, and the further the effects spread.

The "incineration events" I mentioned in my post just above, also include massive blast effects. Just like Tunguska. If the fireball touches the surface, you add massive incineration to the effects of massive blast. You pretty well get the blast no matter what, at one level or another. Depends upon how high up it explodes, if it explodes at all. No explosion up in the air, then it hits the surface.

Tunguska and Chelyabinsk were both estimated as objects on the 10-100 m range, spaced a tad over a century apart, and neither cohesive enough to strike the surface. Had they been cohesive surface impactors, that size class is planetary-scale catastrophe. As non-cohesive air bursts, they fall closer to city busters.

1 m objects are more common and so their events happen on shorter time scales. Smaller still is more often yet. Only 1-10 m is a major city buster to a regional catastrophe event, should the objects be cohesive and impact the surface. Not so bad as air bursts high up.

My point: this is quite the complicated set of phenomena, that doesn't just correlate on simple object size. Cohesiveness is just as big a driver of the effects as simple size. And we know next-to-nothing about cohesiveness. Which also affects mining techniques, by the way.

GW

kbd512 · 2018-10-16 14:13:51

GW,

To sum it all up, it sounds like you're saying that the problem is really complicated and we're in deep trouble if one of these things is on a collision course because we're not putting nearly enough money and manpower into mitigate the threat?

It doesn't seem like we even have reasonably effective methods for dealing with one of these high-speed space rocks. We spent hundreds of billions of dollars to acquire reliable and deliverable nukes, but some of these rocks make our most powerful nukes look like firecrackers.

Something doesn't seem quite right here. Maybe this should be higher priority, for everyone's sake.

SpaceNut · 2018-10-16 17:29:03

I can hear the cries now with, why so much money on space when we need so much of it here to fix roads to feed the poor ect....

Its hard to hit a target if you are not moving or are unable to detect it....

Terraformer · 2018-10-17 03:59:38

Humans are really bad at preparing for low risk, high impact events. Not spending money on asteroid defence has a regular modest upside (saving the money each year), combined with an absolutely massive but improbably downside (city destruction, megadeath events etc).

GW Johnson · 2018-10-17 19:34:52

What Kbd512 said in post 44 is exactly right.

What Spacenut sad in post 45 about hitting a target not detected is exactly right.

What Terraformer said in post 46 about high-impact but improbable events is exactly right. If the impact is just unacceptable, then evaluating risk as the usual product of impact x probability is entirely inappropriate.

GW

elderflower · 2018-10-18 04:27:35

If the solution for an inbound object is an intercept mission and we have at least a few hours to do this, we will need suitable vehicles and payloads to be available in the right place on earth or in orbit, or maybe on the Moon. Wherever they are they will need to be fuelled and ready to go, so solid boosters and storable liquid propellants will be required. Cryogenic propellants will not do as they take too long to load or require continuous servicing. This is why liquid fuelled rockets were abandoned for deterrent service years ago..

GW Johnson · 2018-10-18 10:48:37

Exactly correct, Elderflower!

ICBM's have something like 5-6 km/s deliverable delta-vee capability, to lob their warheads on a suborbital trajectory about halfway around the globe. If such a vehicle were parked in low Earth orbit ready to fire, it might achieve something around 13-14 km/s velocity leaving the Earth. That starts to approach what might be needed for a fast intercept mission.

A guess on my part says we need the instant-use of the solid, and we need something near 15-17 km/s departure velocity from Earth. That's only a 3 or 4 stage solid from low Earth orbit. It is one huge heavy lift launch vehicle from the surface. Payload: 3 to 5 bombs totalling under a ton, plus about a ton or so carrier spacecraft.

We need the right kind of vehicle guidance to make the intercept, the right sequencing for warhead release, and the right fuzing for those warheads. This could be done fairly quickly and cheaply. It need not take hundreds of billions of $. The biggest cost is likely launching the stages to orbit, and assembling the vehicles there.

Nuclear weapons in orbit mounted to multi-stage missiles? Such a vehicle is entirely unnecessary to attack surface targets. All you need is a small de-orbit burn at the right time to drop a warhead on a target. The intercept stages are all way too large to serve that role. Same applies to high-orbit basing, or even basing on the moon. But no matter where or how you base them, that nukes-in-space objection will be raised.

GW

Update 10-19-18 -- minor edits above plus this: how about basing in a solar orbit outside cis-lunar space?

Last edited by GW Johnson (2018-10-19 10:01:57)

elderflower · 2018-10-24 05:05:15

Solar orbits, apart from Lagrange ones, are not available to be tested and serviced at any time that they might need. Also you won't be able to deliver an intercept without a large penalty in delta v for an object coming from above or below the ecliptic -unless you had a huge constellation in many solar orbital planes. You could easily guarantee an intercept from Earth orbit for short notice targets as the only ones you need to deal with are by definition on an earth intercepting orbit themselves.

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