How Can We Effectively Destroy An Asteroid Headed for Earth?

[Astronuc]

Nuclear blasts in space do not produce shock waves like they do in air, water, etc, since there is no medium available to propogate a shock wave. So, a nuke would have to be detonated near enough to the surface of the asteroid to vaporize some of its mass in order to impart any momentum. Explosions in a vacuum generate very high energy projectiles. Explosions in a medium generate high energy shock waves, which is a far more effective way to impart kinetic energy to a mass. Detonating a nuke near enough to the surface of an asteroid to be effective risks fragmenting it.

Morbius certainly understands the physics of nuclear explosions in a vacuum.

The point of detonating a nuclear warhead is exactly to use the 'radiation/thermal' energy to vaporize some mass of the asteroid which will 'deflect' it. Most of the nuclear warhead will be vaporized, so high energy projectiles are not a significant problem. A nuclear blast will likely cause some fragmentation of the asteroid, but those fragments would likely be volatized.

The radiation and thermal energy from a nuclear detonation will reach the asteroid surface before any blast wave, so the blast wave will interact with vapor and liquid, rather than solid, so fragmentation should not be a problem. I would imagine that someone is doing simulations to predict the thermo-mechanical response of a nuclear blast on an asteroid.

 

[Morbius]

Perhaps you missed my point. Nuclear blasts in space do not produce shock waves like they do in air, water, etc, since there is no medium available to propogate a shock wave. So, a nuke would have to be detonated near enough to the surface of the asteroid to vaporize some of its mass in order to impart any momentum. Explosions in a vacuum generate very high energy projectiles. Explosions in a medium generate high energy shock waves, which is a far more effective way to impart kinetic energy to a mass. Detonating a nuke near enough to the surface of an asteroid to be effective risks fragmenting it.

Chronos,

How is the nuke going to fragment the asteroid? You're thinking like the nuke was
still in the atmosphere! When you blow up a nuke; you expect it to "fragment" or
blow apart the stuff that's near it - but that is due to the blast wave from the nuke.

However, as you correctly point out - there is no blast wave in the vacuum of space!

Astronuc is quite correct - the material that constituted the nuke will be vaporized -
[actually it's beyond being vaporized - it's a plasma] so you don't have projectiles.

No - the major effect of the detonation of a nuke in space on material nearby is the
radiation. Place the nuke at the proper distance from the asteroid that you wish to
deflect - and the radiation will vaporize some of the material on the surface of the
asteroid, and the resultant blow-off will yield a reaction force that will deflect the
asteroid.

The effect of the radiation heating of the surface of the asteroid DOES PRODUCE
a shock wave IN THE ASTEROID! There'e no conventional nuclear blast wave
because the detonation is in a vacuum. However, the radiation can cause shock
waves to be generated in the material the radiation hits.

What one does is to detonate the nuke at the correct distance from the asteroid, so
that the radiation effects generate the proper deflection force. This was well covered
in the papers at the Planetary Defense Workshop that I cited above.

Dr. Gregory Greenman
Physicist

 

[Morbius]

one only needs to find the asteroid before it gets anywhere close to Earth. within a time frame of about 5-10 years, do you not think that we can divert an asteroid with a satellite that can be trasported near it? It's more plausible than blowing it up with nuclear missles. or transporting a rocket there. A satellite like object near the asteroid can offset it's course by a very small amount, but if done at a large distance away from earth...it can significantly deflect the asteroid's path. use of missles are just plain ridiculous and may be considered if we detect this thing too late...but then again...it's too late.

POSITION_VECTOR,

If we are going to deflect an asteroid, which can be miles in diameter; we are going to
have to deflect it YEARS ahead of time. But even then, you have to apply some
very large forces to the asteroid. The gravitational force from some once Earth-bound
satellite isn't going to do it.

Besides, any deflection forces you get from the gravity of the satellite will have
come from the rockets used to put the satellite there.

Suppose I have a ball of magnetic material sitting on the table. I hold in my hand a
magnet, and I swipe the magnet close to the ball and deflect it. The ball rolls off.
Where did the energy to roll the ball come from? It came from ME. The magnetic
field merely conveyed the energy via the magnetic force. But the energy came
ultimately from me. Using the magnet actually made the process less efficient -
I could have done better by hitting the ball directly with my hand.

The same is true with your satellite. Ultimately, the energy to deflect the asteroid
came from the rockets. By using the gravity of the satellite; you merely made the
process LESS EFFICIENT. It would be better to use the force of the rockets
directly. Deflecting an asteroid is going to take a LOT of force, and we don't have
any to spare; so we really won't be able to accommodate the inefficiencies of using
gravity.

As I stated before, the most efficient way to transport energy to some distant
place is via nuclear energy, hence it will be useful to have some nuclear weapons
on hand for the purpose. Perhaps that factored into President Clinton's decision
on August 11, 1995 to alter US policy to one that will retain nuclear weapons in
the US stockpile, for the indefinite future.

Dr. Gregory Greenman
Physicist

 

[Morbius]

Exactly. It's just a way of elliminating the problems of attaching a thruster to the asteroid if it's spinning, or if it's composition is a loose conglomorite rather than a solid rock. (Or if these properties are unknown)

LURCH,

Using gravity doesn't help you plant the rocket on the asteroid, even if it's spinning or
a loose consistency.

Gravity is a "central force". The force vector will be along the vector from the center
of mass of the asteroid to the center of mass of the rocket. Therefore, since the
force vector and radius vector are co-linear; the gravity can't induce a torque on the
rocket. Therefore, if the asteroid is spinning, the rocket WON'T spin up to match it.
You will still have the problem of a rocket attaching itself to a spinning object in any
case.

No - gravity is pretty useless here. Anything you can do with gravity, can be done
better by doing it directly.

If your asteroid is spinning, or even if it has a surface of loose consistency; if you
accellerate your rocket in space and slam it into the asteroid in a collision; the
asteroid is going to pick up the momentum. Where else would the momentum go?

Dr. Gregory Greenman
Physicist

 

[Morbius]

The radiation and thermal energy from a nuclear detonation will reach the asteroid surface before any blast wave, so the blast wave will interact with vapor and liquid, rather than solid, so fragmentation should not be a problem. I would imagine that someone is doing simulations to predict the thermo-mechanical response of a nuclear blast on an asteroid.

Astronuc,

EXACTLY CORRECT!

Yes - the interactions and responses of materials to the effects of nuclear explosions
indeed have been, and are; being simulated. Those are very complex calculations;
certainly not "back of the envelope" or "hand-waving" types. However, it's the type
of thing that been done for year for other reasons.

Dr. Gregory Greenman
Physicist

 

[LURCH]

LURCH,

Using gravity doesn't help you plant the rocket on the asteroid, even if it's spinning or
a loose consistency.

Gravity is a "central force". The force vector will be along the vector from the center
of mass of the asteroid to the center of mass of the rocket. Therefore, since the
force vector and radius vector are co-linear; the gravity can't induce a torque on the
rocket. Therefore, if the asteroid is spinning, the rocket WON'T spin up to match it.
You will still have the problem of a rocket attaching itself to a spinning object in any
case.

No - gravity is pretty useless here. Anything you can do with gravity, can be done
better by doing it directly.

If your asteroid is spinning, or even if it has a surface of loose consistency; if you
accellerate your rocket in space and slam it into the asteroid in a collision; the
asteroid is going to pick up the momentum. Where else would the momentum go?

Dr. Gregory Greenman
Physicist

You missunderstand the method being proposed. The idea is that the thruster be attached to the object by gravity alone; it never comes into physical, "material" contact with the object. This way, the rocket never has to sit on the surface. I believe the term used was "gravitational tethering".

It is precisely because gravity is a central force that the problems of spinning or loosely conglomerated targets are elliminated. If the object is spinning, the rocket, which is a short distance away, does not spin, and can point its nose (and its thrust) in one constant direction. Because the force and radius vectors are co-linear, no angular momentum is exchanged (well, not enough to mention, anyway). If the object is of loose composition, the tug of gravity will work on the whole group of objects as a single mass. Where a thruster on the surface or an impactor might just blow a hole right through the object, a thruster at a distance would pull on the center of mass for the group.

 

[theCandyman]

For gravity, F = G mM/r^2. [G = 6.7 * 10^-11 N m^2/ kg^2]

Now use this equation and calculate how much force using just gravity will affect the asteroid. Now only that, but if the asteroid is of a threating size, it might be impossible to make a rocket massive enough to affect it.

 

[DaveC426913]

For gravity, F = G mM/r^2. [G = 6.7 * 10^-11 N m^2/ kg^2]

Now use this equation and calculate how much force using just gravity will affect the asteroid. Now only that, but if the asteroid is of a threating size, it might be impossible to make a rocket massive enough to affect it.

The rocket is not supposed to be massive; the rocket acts by applying thrust.

And how much it affects the asteroid is determined only by how much time it has to do so. Even a small rocket will have an effect; it's only question of 'is it enough to do the trick?'

 

[DaveC426913]

You missunderstand the method being proposed. The idea is that the thruster be attached to the object by gravity alone; it never comes into physical, "material" contact with the object. This way, the rocket never has to sit on the surface. I believe the term used was "gravitational tethering".

So, if I understand correctly, the rocket's acceleration is constrained by the escape velocity of the asteroid. No matter how powerful the rocket has the potential to thrust, it must stay below escape velocity or it will break free.

 

[Morbius]

So, if I understand correctly, the rocket's acceleration is constrained by the escape velocity of the asteroid. No matter how powerful the rocket has the potential to thrust, it must stay below escape velocity or it will break free.

Dave,

You are correct. The amount of force the rocket can exert on the asteroid is limited
by the mutual gravitational attraction between the rocket and asteroid, and the
requirement that the rocket stay below escape velocity.

This SEVERELY restrincts the amount of force that can be applied. Even if we
deflect the asteroid YEARS in advance; it is going to take some VERY LARGE forces
to deflect asteroids of a size that would be a threat to the Earth.

Dr. Gregory Greenman
Physicist

 

[LURCH]

Yes, in this method detection would have to be even earlier than the other techniques mentioned. And, as we all were reminded last week,

https://www.physicsforums.com/showthread.php?t=130554

We don't have very adequate early detection.

This SEVERELY restrincts the amount of force that can be applied. Even if we
deflect the asteroid YEARS in advance; it is going to take some VERY LARGE forces
to deflect asteroids of a size that would be a threat to the Earth.

Also very true. But on the plus side, this method would put a direct correlation between the amount of energy available (in practicale terms) and the amount needed. Larger objects would require more thrust, but they would also have a stronger gravitational tether, and be able to handle more thrust. Nevertheless, the amount of thrust needed compared to the amount that could be applied per unite of time is such a huge ratio, we'd have to start with much more advanced warning than we're likely to get.

And we'd have to know the strength of the gravitational field around the object before we could begin thrusting, so we don't just escape. This is notoriusly difficult to calculate for an object traveling through open space. I suppose we could find that out by firing a preliminary projectile near the object (kind-of a shot across its bow), and measure how the path of that projectile is altered by the close encounter. But the margin of error would be pretty big. I think we'd have to send an engine with veriable thrust, so we could dial it back if it started to escape, or crank it up if it started to get closer to the object. Not impossible, but pretty complicated.

And of course, as has been stated, this method reduces the amount of effect we could have on the object. Current scenarios are dependant upon detection years in advance, so that the small effect we could have on the cousre of the object would be sufficient when multiplied by time to impact. Gravitational tethering makes the effect we could have so small that the time to impact would have to be many decades; [probably about a century. This raises two big questions,
1)Can we accurately predict a collision event 100 yrs in advance?
I'm doubtfull, we have a cattelog of a few objects that might hit in the next twenty years, and we can't even speek with much certainty about them. They "probably won't".
2)Projecting the kind of tech we are likely to have 100 yrs from now, why bother?
If we have a hundred years of warning, then we can be pretty sure that we'll have a much more reliable method in place in plenty of time.

I still say the best solution is to blast 'em!

 

[Morbius]

I think we'd have to send an engine with veriable thrust, so we could dial it back if it started to escape, or crank it up if it started to get closer to the object. Not impossible, but pretty complicated.

LURCH,

The main engines on the space shuttle are variable thrust. During the launch of a
space shuttle, the engines throttle back to reduce aerodynamic stress on the vehicle,
and then as the air gets thinnner with altitude, the engines throttle up again.

Dr. Gregory Greenman
Physicist

 

[LURCH]

Ah yes, of course. I was still thinking of ion propulsion. If this method were to be viable at all, I think that the low specific impulse and long duration of Ion Drive would be a necessity.

VASIMR's (ion drives with veriable specific impulse) exist, but last I heard there were only prototypes in testing. Have any been flown? I thought I remembered eharing that the ESU was going to use that propulsion system for its next Lunar orbiter, but I don't know if that's still in the future.

Aslo, here's an interesting article on this very topic;

http://flux.aps.org/meetings/YR04/DPP04/baps/abs/S1905.html

NASA's Prometheus program seeks to develop new generations of spacecraft nuclear-power and ion propulsion systems for applications to future planetary missions...

...Another potential Prometheus mission of high science interest would be an extended tour of primitive bodies in the solar system, including asteroids, Jupiter family comets, Centaurs, and Kuiper Belt Objects (KBO). The final landed phase of this mission might include an active keplerian experiment for detectable (via downlink radio doppler shift) acceleration of a small kilometer-size Centaur or KBO object, likely the satellite of a larger object observable from Earth. This would have obvious application to testing of mitigation techniques for Earth impact hazards.

 

[Astronuc]

VASIMR's (ion drives with veriable specific impulse) exist, but last I heard there were only prototypes in testing. Have any been flown?

VASIMR have yet to be perfected - they are still problematic and have little thrust. None have been flown.

 

[Morbius]

Ah yes, of course. I was still thinking of ion propulsion.

LURCH,

"It beats me. But isn't she a beauty? Interesting design.
I've never seen anything like her. And ion propulsion at that.
They could teach us a thing or two."

--LCDR Montongomery Scott, from the Star Trek episode "Spock's Brain"

Dr. Gregory Greenman
Physicist

 

[Orion1]

I performed a calculation using the Teller equation and information from this thread for subsurface nuclear explosion blast cavity radius and produced the result:

Teller equation:
$$r(h) = C \frac{Y^{\frac{1}{3}}}{(\rho h)^\frac{1}{4}}}$$

$$Y = 1 \cdot 10^6 \; \text{kt}$$ - proposed yield (for complete destruction)
$$\rho_{Fe} = 7.87 \; \text{g} \cdot \text{cc}^{-1}$$ - Ferrous density

$$r(h) = 80 \cdot \frac{(1 \cdot 10^6)^{\frac{1}{3}}}{(7.87 h)^{\frac{1}{4}}} = \frac{4.776 \; \text{km}}{h^{\frac{1}{4}}}$$

$$r(h) = \frac{4.776 \; \text{km}}{h^{\frac{1}{4}}}$$

h = penetrator depth, although the constant C is for Granite, the result should be close.

 

[SAZAR]

They say if we needed to destroy and asteroid headed for Earth the fastest easiest way is witha nuclear bomb. If we used it it would destroy it but it would create thousands of pieces falling to Earth which would be much worse.

In a special it was said that it would take a bomb on the order of 1,000 megatons to completely destroy an asteroid. It would not only be the biggest bomb ever built but it would need to be put on the biggest rocket ever built. My soloution could be simple. INstead of building one big bomb build about 10 smaller ones. launch 10 smaller nuclear warheads and attach them all to the asteroid. THen at the precise moment they all go off.

SInce there would be 10 each one would be around 100 megatons. And since it not one concentrated into one place they would have a better chance of destroying all the asteroid. I think 100 megatons is well within our bombmaking capabilitites. If we make them light enough we could put 2 on each rocket only requiring 5 launches. This sounds easy and simple. Why hasn't anybody ever thought of this?

Movie: "Meteor"

http://www.imdb.com/title/tt0079550/
(watch that movie)

PS: pay attention to the year it was made.

 

[SAZAR]

About the theme itself...
...No doubt; some sort of bomb is in order: nuclear or even anti-matter...
...But, it would be interesting to see some more ingenious (but effective!) solutions...

 

[the_force]

About the theme itself...
...No doubt; some sort of bomb is in order: nuclear or even anti-matter...
...But, it would be interesting to see some more ingenious (but effective!) solutions...

What is an Anti-matter bomb? :)

Do we even have a rocket that can take a 1000MT nuke? Maybe use a couple of the Soviets Tsar designs. lol

Only way - Mutate out minds and get smarter. Might need to wait until the next human mutation takes place :)

 

[Astronuc]

What is an Anti-matter bomb? :)

Do we even have a rocket that can take a 1000MT nuke? Maybe use a couple of the Soviets Tsar designs.

An antimatter bomb, as the name implies, is a bomb in which the active material is antimatter - most likely cryogenic anti-hydrogen, since making anti-atoms of Li or heavier atoms (elements) is exceedingly difficult. Even making pico-grams of anti-hydrogen takes a long time and it's not all at once. Storage is another exceedingly difficult problem.

Assuming one could amass a gram scale quantity of anti-matter - how would it work? Likely a reaction would be initiated between matter and antimatter, which would cause a burst of mesons and gamma rays, which would disperse the remaining solid anti-matter, which would interact with whatever matter it encountered, so likely the area would be a large volume of plasma. The energy release rate would be limited by how quickly the matter and anti-matter interact.

A 1000MT (1 GT) nuke would be very large, and that has not been the focus of mainstream reasearch. The main goal of late has been smaller more compact nukes for specific targets rather than large areas like cities.

 

[the_force]

Hey

Yea, we are making more bombs like SRAM-T, insted of the megaton nukes that were created in the cold war. I read a journal that stated the USA still has Nuclear missiles locked onto Russia and vice versa.

Does anyone think we will see a nuclear exchange in our life time?

 

[Morbius]

Yea, we are making more bombs like SRAM-T, insted of the megaton nukes that were created in the cold war. I read a journal that stated the USA still has Nuclear missiles locked onto Russia and vice versa.

the_force,

All that stuff about the USA and Russia having missiles "locked onto"
each other is a bunch of BALONEY!

The missiles have auto-pilots; like an airplane. Right now, all those
auto-pilots are OFF - the missiles are OFF.

When you want to fire the missile; you "spin it up" and boot-up the
auto-pilot. Then you enter the coordinates of the target in the
auto-pilot's computer.

Saying that we have missiles "locked on" Russia is like saying that your
powered-down and off computer is "locked on" your favorite website.

Dr. Gregory Greenman
Physicist

 

[Morbius]

Yea, we are making more bombs like SRAM-T, insted of the megaton nukes that were created in the cold war.

the_force,

Actually the USA hasn't produced a new nuclear weapon in YEARS!

The last nuclear weapon design that entered the stockpile was the
"nuclear bunker buster" the B-61 Mod 11 which entered service in
1997, and was developed at the behest of President Clinton:

http://www.fas.org/faspir/2001/v54n1/weapons.htm

The B-61 Mod 11 was a modification of the B-61 family of gravity bomb,
rather than a "clean sheet of paper" design.

The last totally new designs were the W-87 Peacekeeper ICBM warhead
designed by Livermore, and the W-88 Trident D-5 SLBM warhead designed
by Los Alamos. Both of these were commissioned in the '70s under the
Carter Administration [ the Peacekeeper was originally called "MX"].

The SRAM warheads, W89 for SRAM II, and W91 for SRAM-T never made
it beyond the feasibility study stage. The SRAM program was canceled
over a decade ago.

Dr. Gregory Greenman
Physicist

 

[the_force]

Hey

Thanks Morbius for clearing up the BALONEY :)

Does Russia still make nuclear weapons?

 

[Morbius]

Thanks Morbius for clearing up the BALONEY :)

Does Russia still make nuclear weapons?

the_force,

A while back in 2004, Russian President Vladamir Putin announced that Russia
was working on new nuclear weapons:

http://www.cbc.ca/world/story/2004/11/17/russia-missiles-041117.html

Dr. Gregory Greenman
Physicist

 

[the_force]

Hey

Thanks for that article, good read!

Geez, I think Putin wants the cold war back in light of recent events

 

[quetzalcoatl9]

what if the device to deflect the asteroid contained disk of polymer "propellant" that would be vaporized by the thermonuclear detonation in the direction of the asteroid, a la Orion? Would this be a more efficient transfer of energy to the asteroid than direct radiation interaction with the asteroid material?

 

[Morbius]

what if the device to deflect the asteroid contained disk of polymer "propellant" that would be vaporized by the thermonuclear detonation in the direction of the asteroid, a la Orion? Would this be a more efficient transfer of energy to the asteroid than direct radiation interaction with the asteroid material?

quetzalcoatl9,

If I get your question; you want a disk of material that sits between the bomb and the
asteroid; which the bomb will vaporize, and the expanding vaporized propellant would
then impact the asteroid and transfer momentum to it.

My question is why put anything between the asteroid and the bomb. If the propellant is
vaporized by the bomb; the hot vaporized material will expand in both directions - toward
the bomb, and away from the bomb. Only the half that is heading toward the bomb will
contribute to deflecting the asteroid.

In essence, only HALF the aborbed energy is then available for deflecting the asteroid.

Why not let all the radiation from the bomb "impact" on the asteroid?

What are you trying to accomplish with this disk of propellant? It isn't needed.

Dr. Gregory Greenman
Physicist

 

[quetzalcoatl9]

Why not let all the radiation from the bomb "impact" on the asteroid?

good point - i guess my thinking was that the polymer disk was proposed for use by the Orion craft to better transfer the energy. However, I also forgot that an Orion objective was for the spacecraft to not be destroyed (i.e. the pusher plate stay intact and get irradiated+ablated as little as possible), a constraint that need not hold with the asteroid :)

 

[Alphabit]

I read from somewhere that there was a way to divert the path of an asteroid by applying gravity to it... I assume they would do it through applying a constant force over a certain period of time.

 

[Morbius]

I read from somewhere that there was a way to divert the path of an asteroid by applying gravity to it... I assume they would do it through applying a constant force over a certain period of time.

Alphabit,

Yes - that's one proposal. However, gravity is a very weak force unless you have huge
amounts of mass. The idea was to put a spacecraft near the asteroid and let the gravity
of the spacecraft deflect the asteroid.

Naturally the gravity of anything of a mass small enough for us to boost into space is going
to be EXTEMELY small. So this scheme works ONLY if you have a few DECADES of
lead time.

For example, suppose we were to discover that one of the asteroids that had an Earth
crossing orbit was going to impact the Earth a few decades from now. This asteroid is
going to orbit the Sun many, many times before impacting Earth. Such an object would
succumb to a gentle push for many years to deflect it. Then you might be able to use gravity
to deflect the asteroid.

However, suppose we discover an asteroid or comet that is coming at us from the region
of space out beyond Pluto. This asteroid isn't going to loop around the Sun for many
decades giving us time to gently deflect it. Suppose this asteroid is on a collision course
with Earth on THIS orbit. This asteroid needs to be given a BIG PUSH and NOW in order
to make it miss Earth.

A gravity type deflection scheme would be ABSOLUTELY USELESS against such
an asteroid. That would be where we would need some way of applying much, much
larger forces.

Dr. Gregory Greenman
Physicist

 

[joema]

...this scheme works ONLY if you have a few DECADES of
lead time...However, suppose we discover an asteroid or comet that is coming at us from the region of space out beyond Pluto...Suppose this asteroid is on a collision course with Earth on THIS orbit...A gravity type deflection scheme would be ABSOLUTELY USELESS against such an asteroid...

A little more on this: the gravity tractor works by making the asteroid miss the "gravitational keyhole", not by deflecting it to miss the Earth's angular diameter. These regions are very small -- I think 400 meters wide for Apophis. Yet to achieve this takes about 1 year, and is generally at least 2 orbital periods out.

Apophis has an orbital period of about 1 year, so maybe the minimum lead time would be about 2 years, not including prep time for launch plus travel time. So figure 3 years as the absolute minimum, and that assumes having mostly off-the shelf capability.

The proposed gravity tractor is not a small, simple vehicle. It's 18 metric tons (40,000 lbs), uses a nuclear reactor and ion engines. The technology is taken from the now-cancelled NASA Prometheus project: http://en.wikipedia.org/wiki/Project_Prometheus.

Even if you knew 3+ years in advance, I don't think there's a currently operational launch vehicle capable of lifting 18 metric tons to Earth escape velocity, plus enough surplus propellant to rendezvous with an asteroid moving at 30 km/sec.

Taking Apophis as an example, if it were initially discovered at the farthest distance from earth, and determined to be on a collision course, you'd have about 6 months. If an asteroid came from the sun's direction (where telescopes can't see) you might have 2 months. In such cases (as already mentioned) a gravity tractor would be useless.

 

[Morbius]

The proposed gravity tractor is not a small, simple vehicle. It's 18 metric tons (40,000 lbs), uses a nuclear reactor and ion engines.

joema,

In the celestial mechanics of natural stellar satellites; 18 metric tons is a PITTANCE!

Dr. Gregory Greenman
Physicist

 

[joema]

joema,

In the celestial mechanics of natural stellar satellites; 18 metric tons is a PITTANCE!...

Yes, agreed. My point was it might require (by the standards of previous deep space probes) a very large and powerful vehicle, using nuclear-electric propulsion. This in turn requires a very large launch vehicle, larger than any currently available.

However there are two high-level scenarios:

(a) Deflect asteroid to miss small gravitation keyhole in space, which if achieved will cause asteroid to miss Earth on a subsequent encounter. This can be done with a very small probe, using solar-electric propulsion and existing launch vehicles. It could be similar the Deep Space 1 probe, already tested: http://en.wikipedia.org/wiki/Deep_Space_1

(b) Deflect asteroid to miss Earth (it's on a direct earth-impact trajectory). Small vehicles like Deep Space 1 could only handle a small asteroid (150-200 m dia), and only if detected pretty far out. The Schweikart paper discussed these: http://arxiv.org/pdf/physics/0608157

If the object is larger or detected later, a much heavier nuclear-electric gravity tractor would be required, probably around 20 metric tons. This was described here: http://space.newscientist.com/article/dn8291--gravity-tractor-to-deflect-earthbound-asteroids.html

The gravity tractor concept is very interesting and elegant, but as you explained it only works in some circumstances. You need years (likely many years) of warning time.

There are various approach trajectories where the object wouldn't be seen in time with current detection technology. In those cases a nuclear deflection, probably using a non-fragmenting stand-off detonation, would seem the only option. The lead time to launch could be very short -- possibly months, conceivably weeks, since existing ICBMs could be used.

 

[Morbius]

The Schweikart paper discussed these: http://arxiv.org/pdf/physics/0608157

A physicist colleague of mine who works in the field of asteroid deflection was telling
me recently about a debate he had with Schweikart. He was not AT ALL impressed
with Schweikart's understanding of the problems and much of the relevant physics.

I would certainly quibble with Scweikart's contention that only the gravity tractor
deflection scheme can be "fully controlled". I would like to see more of the plans
for the "nuclear - electric" propulsion systems. What I have seen of these have
been pretty lacking in design details.

In those cases a nuclear deflection, probably using a non-fragmenting stand-off detonation, would seem the only option. The lead time to launch could be very short -- possibly months, conceivably weeks, since existing ICBMs could be used.

Many of the schemes, like gravity tractors, etc - work only in one end of the threat
spectrum - the asteroids in low eccentricity orbits that will require many orbits before
they impact Earth - hence a long enough lead time for a relatively weak application of
force to deflect the asteroid.

The gravity schemes don't work when you have asteroids or comets in highly eccentric
orbits that are "way out there"; but are heading for impact on their current orbit. These
are the ones that will offer very short lead times from the time we detect the object to
the time it has to be deflected. Those are the ones that are going to need a big push
since we don't have years for the ultimate effect of the push to accrue.

The stand-off nuclear weapon detonation would seem to be our only hope for these. This
technique will also work for the asteroids that would succumb to the gravity tractor. In
other words, the stand-off nuclear weapon detonation covers a much greater fraction of
the threat spectrum. It's even good for "rubble piles" - an object composed of many
discrete objects held together by mutual gravity. You can't push on a rubble pile with
spacecraft . Only a "body force" like a gravity tractor, or the ablation due to a stand-off
nuke will work well on a "rubble pile".

Dr. Gregory Greenman
Physicist