B Ideas to protect the Earth from possible asteroid impacts

[Ratman]

Completely negligible

I can agree if the asteroid in question is pulverized and it happens far enough from the Earth – it would be less harmful than fallout from nuclear tests. But would you say the same if Bennu-sized object was converted to several thousand Chelyabinsk-sized "dirty bombs"? We know very little about the material properties of the asteroids. Rubble pile reacts in a different way than rigid rock or chunk of metal, and there are probably more varieties out there. Deflection, nuclear or conventional, is way safer than blowing up. In a particularly bad scenario (too big asteroid, too small yield and/or unfortunate mechanic properties), the gravity can re-form the threatening object, making us wish we changed its trajectory.

 

[LURCH]

I’m pretty sure that, in any scenario, a bunker buster nuke, also known as a GPW (Ground Penetrating Weapon), is the best course of action. A surface or near-surface detonation wastes most of the device’s energy into the vacuum, while relying on heat to vaporize a small amount of material on the surface. A sub-surface blast excavates a large amount of material and turns it into reaction mass. These devices already exist and could be converted to this purpose with relative ease, and they can be programmed to detonate at a predetermined depth below the surface. Anywhere between the surface and the center of mass will result in a much greater deflection than a stand-off detonation.

The fear of turning one object headed for Earth into thousands of objects headed for Earth only applies to those scenarios that are so popular in the sci-fi thrillers, where the people in charge always wait to detonate until less than a minute before impact (for dramatic effect, I suppose). We are all in agreement that the key is to detect the threat and take action early, while it is many years away. In that case, such a concern does not exist.

 

[unusually_wrong]

Surface or near-surface blast may be preferable, it has a more predictable effect - part of asteroid is ablated away, pushing the entire remaining asteroid, still in one piece. Especially if you have even just an approximate control over the location of the blast and thus, the direction of the push, this is a nice, controllable deflection rather than "let's go with a big BOOM and hope for the best" approach.

If it's a large asteroid, it could take hundreds of nukes (maybe even more) just to nudge it enough. I haven't seen anyone bring up the possibility of using lasers. I imagine that it would probably take a lot of time to do that though.

 

[Mlesnita Daniel]

I haven't seen anyone bring up the possibility of using lasers

If I remember correctly, laser technology is not that powerful yet. I don't think it can cause any sort of damage to an asteroid. I have to mention that last time when I checked it was like a year ago. Some stuff may have changed.

 

[alantheastronomer]

The City of Berkeley, for example, requires that nothing radioactive - even a single disintegration - enters the city limits.

So what do Berkeley hospitals do about the medical radioisotopes they need for medical treatments? Are they exempt?

 

[LURCH]

I believe all objects made of physical matter would have to be exempt. It’s possible that this was the intent; that the entire city should only be made of metaphysical constructs. I mean, this is Berkeley we’re talking about.

 

[Tiran]

Build an orbital soleta/solar sail for climate cooling from lunar aluminum. Change its shape to a parabolic reflector and use focused sunlight to vaporize the side of the asteroid, blowing it slowly off course.

With nukes, careful mapping and reevaluation after each strike would yield the best results by making sure each detonation acts most effectively. Vaporizing ice is probably more effective than blowing up rock. Detonations in craters more effective than on convex areas.

 

[Nik_2213]

First, you must detect the object with enough lead-time to plan.
http://spaceweather.com/ has interesting lists. Note some objects may only be detected upon impact or outbound..

Then, you must remember the 'Deep Impact' mission { NOT the movie } which whanged a 100 kg copper ingot into comet Tempel 1 (9P/Tempel) at a crossing speed of ~10 km/sec, made but a ~150 metre crater. It certainly did not affect the orbit to any measurable degree. Would ten such make a difference ? Twenty ? Fifty ??

I'm reluctant to venture into 'finger breaking' territory, but mitigating fallout from the 'whack it' approach would seem to need possibly-mythical shaped-charge nukes...

That still puts you into 'Hydra Killing' country. Even if your first, second or third thermo-nuke shatters the monster, you now have multiple city-killers inbound. Behind a protective cloud of debris that may thwart subsequent attacks...

With sufficient time to work, more elegant solutions such as an ion-engined orbiter playing 'gravity tug' may serve...

 

[Tiran]

With sufficient time to work, more elegant solutions such as an ion-engined orbiter playing 'gravity tug' may serve...

You have to get the ion drive there with enough fuel to actually accomplish something.

Using nukes like an Orion drive instead of for demolition might be faster and more efficient. You could always send out a pusher plate to protect the asteroid.

 

[LURCH]

Then, you must remember the 'Deep Impact' mission { NOT the movie } which whanged a 100 kg copper ingot into comet Tempel 1 (9P/Tempel) at a crossing speed of ~10 km/sec, made but a ~150 metre crater. It certainly did not affect the orbit to any measurable degree. Would ten such make a difference ? Twenty ? Fifty ??

I'm reluctant to venture into 'finger breaking' territory, but mitigating fallout from the 'whack it' approach would seem to need possibly-mythical shaped-charge nukes...

That still puts you into 'Hydra Killing' country. Even if your first, second or third thermo-nuke shatters the monster, you now have multiple city-killers inbound. Behind a protective cloud of debris that may thwart subsequent attacks...

I hear this statement a lot, and I strongly believe it to be false. The statement is usually phrased along the lines of; "A nuke will turn one object headed for Earth into many objects headed for Earth", but that is not cosistant with reality. One key to understanding this is to realize that the phrase, "many objects headed for Earth" claims two conditions; that the object has been broken into many pieces, and that those pieces are headed for Earth. I do not believe that these two conditions can both be realized.

Consider the scenario in which the original object is indeed shattered into a thousand pieces. If we examine the resulting debris field, we can see that it is expanding over time. For specificity, let us look at this from the perspective of one of the pieces of debris. If we observe the spatial relationship between that piece and its nearest neighbors at one second after the blast, and then again at two seconds, and again at thirty, we see that with each successive observation, the distance between any two of these objects has steadily increased. If they are all moving away from one another, getting farther apart over time, then they cannot all be headed for the same target. In fact, no two of them can arrive at the same destination (unless they are large enough to pull themselves back together gravitationally). This puts a very specific upper limit on the maximum number of fragments that might still be on a collision course with Earth. That maximum limit is one.

Even this maximum may not be possible. The only reason the object would break into many pieces would be because each of those pieces has undergone an acceleration. If each of them was originally on a collision course with Earth before being subjected to an acceleration, then it would seem that this course (the collision course) is the one path in all the universe along which none of them can now be progressing.

 

[Vanadium 50]

So what do Berkeley hospitals do about the medical radioisotopes they need for medical treatments? Are they exempt?

No idea. I try not to get sick inside Berkeley city limits. I'm afraid instead of real medicine they may want to align my chakras.

 

[TEFLing]

perhaps the most feasible way is "preventative medicine" -- detect collisions far far far in advance, and supply the future would be dangers with a slight nudge in the outer solar system... long long before they get up to speed and take full aim at Earth ?

Otherwise, sitting back on our heels, waiting for asteroids to show up on our solar doorstep at relative speeds of 11+ kps is probably impossible

 

[James Demers]

An object that takes us by surprise is not going to give us much of a response time, and "nuking" it and hoping for the best is the only option that seems likely to me.

For an object that we're already tracking, we're going to have a much wider window of opportunity. We already know how to land on these things, so one reasonable approach is to land a thruster on it - maybe an ion engine - and slowly nudge it into a harmless orbit. A tiny change in direction really adds up over millions of miles, so it won't take much of a nudge; the trick is to get to it early enough.

 

[Tiran]

We already know how to land on these things, so one reasonable approach is to land a thruster on it - maybe an ion engine - and slowly nudge it into a harmless orbit

The primary problem with this notion is that the ion engine AND all of its fuel have to get out to the incoming asteroid, change direction 180° to match its incoming velocity, land somewhere stable enough to thrust against and still have enough time/fuel to push it far enough off course to matter.

Ion engines are practical for spacecraft because spacecraft are relatively light compared to the fuel they carry. But an asteroid is hugely massive and none of that is ion engine fuel.

 

[bahamagreen]

What are you talking about?? Moving either the Earth or the Moon by even a microscopic amount is totally beyond our capabilities in the next few thousands of years. Moving an asteroid is way more practical.

Well, yes with sufficient lead time, but what if you're in a hurry?
Two questions arise:

- can you displace the earth?
- how quickly can you do this without "breaking it"?

power available (half revolution): 5x10^24W
half revolution period: 4.3x10^4s
earth mass: 6x10^24kg
displacement: Earth radius 6.4x10^6m
power=mass x acceleration x displacement / time so a=pt/md
I get a rough maximum acceleration of about 5mm/s^2

Actual result might be much less, but I don't know how little the Earth could tolerate (oceans, plates, etc...)

 

[rootone]

Something like a big ball of spongy stuff it must get through before hitting Earth.
This idea not copyright protected.

 

[alantheastronomer]

When asked this question, Neil DeGrasse Tyson offered two possible solutions, neither one practically viable: First, a "gravity tractor" and second, painting half the asteroid white so that the difference in solar reflectance would slowly push it off course...The problem with the second one is obvious - any deflection of the asteroid would be too minute to make enough of a difference by the time it arrives. While the problem with a gravity tractor is that you need a spacecraft with enough mass to pull the asteroid off course, but that mass is going to have to be launched from the surface of the Earth and that requires enough fuel to get the job done, but that would probably be prohibitive both in cost and supply. Then there's the dilemma of, if it's massive enough to be detected early it would be too massive to deflect, but if it's smaller it won't be detected until it's too late to effectively deflect it. There was a third solution proposed by the T.V. show "Salvation" which is to use a solar sail to deflect it, but solar radiation and even the solar wind wouldn't be able to provide enough thrust to alter the orbit of an asteroid. Wikipedia provides a few more possibilities: Attaching solid rocket boosters to gradually deflect it and alter it's orbit enough that it misses the Earth. Using mass drivers to expel material that would provide a thrust to alter the orbit. Jay Melosh, a planetary astrophysicist who is known for discovering lava tubes on the Moon, is the one who originally pointed out the shortcomings of using nuclear weapons to "destroy" the asteroid, came up with the most elegant solution, which Tiran also mentioned in post #37, and that is to use a parabolic solar reflector to ablate a portion of the asteroid to provide enough thrust to blow it off course. A refinement to this solution would be to use the reflector to pump an industrial size laser to more efficiently ablate the material...

 

[Janus]

The best approach to preventing a collision with an incoming asteroid relies heavily on its trajectory. Deflecting it to one side or another might not always be the best use of your resources. In some cases, speeding up the asteroid in its orbit might be best. A collision between asteroid and the Earth depends on both the Earth and asteroid arriving at the same point of Earth's orbit at the same time. Speeding up the asteroid will change its orbit; in the best case, both causing it to cross further along in Earth's orbit and crossing Earth orbit earlier, and thus crossing Earth orbit before the Earth has quite arrived there.
There is another reason why accelerating the asteroid could be the best answer. This involves the impact parameter. As the asteroid gets closer to the Earth,the Earth's own gravity will begin to act on it deflecting its trajectory. So even if the initial trajectory would have had the asteroid missing the Earth, Earth's gravity could draw it into a collision. The lower the relative velocity between asteroid and Earth, the more time Earth's gravity has to act on the asteroid and the larger the "target" the Earth makes. Accelerating the asteroid (under the right conditions) can increase the relative velocity between Earth and asteroid at orbital intersection, effectively making the Earth a smaller target to hit.

 

[alantheastronomer]

The best approach to preventing a collision...speeding up the asteroid in it's orbit might be best.

I agree! This would also make it easier to orient a reflector to be facing the sun while ablating it from behind. There's also an alternative option of deflecting it to a lower energy orbit so that the Earth passes first and the asteroid crosses Earth's orbit later...whichever method is used, we have to be careful that another collision is not realized at a later orbit.

 

[Tiran]

This involves the impact parameter. As the asteroid gets closer to the Earth,the Earth's own gravity will begin to act on it deflecting its trajectory. So even if the initial trajectory would have had the asteroid missing the Earth, Earth's gravity could draw it into a collision.

I see what you're saying, but how much of a velocity change is required vs. how much that is going to decrease the attraction of Earth's gravity on drawing the asteroid in?

I ask because speeding up an incoming object is essentially the most difficult way to do it because you can't do it with electromagnetic or ballistic means, which means getting some sort of drive system out to the object. If the object is tumbling, you won't be able to dock a drive and will have to settle for something like a ersatz Orion. If it isn't tumbling, that means we'll have no data on the "dark side" where whatever device is going to have to act. That could be a disaster if the dark side has a composition incompatible with the drive mechanism. So speeding up really seems like the most difficult method that resists secondary methods, because you wouldn't want to speed it up and then only have slow-down back up techniques if the drive fails.

Which brings up another concern - redundancy. It would be best if whatever primary method was largely compatible with any secondary methods that could be built in parallel. They could act on the same vector, or complimentary vectors.

You can change the object's trajectory by pushing it in, out, up, down, right or left in the plane of the ecliptic, including combinations. What are problematic combinations are speeding it up while steering it down orbit, or slowing it down while steering it up orbit. In those cases you're cancelling out the action of either vector.

I don't know for sure, but my instinct is that the most efficient steering vector is going to be perpendicular to a line running from the center of the Earth through the potential impact zone.

 

[Nik_2213]

Unless a shattered object has enough time and delta-V to disperse far enough, Earth's gravity will draw in fragments.

And, even fragments that miss on the first pass may have their orbit altered to present a near-future threat...

whimsy:
Safest plan may be to collide it with the Moon...
/

 

[Tiran]

Safest plan may be to collide it with the Moon...

So you definitely end up with a ton of dangerous debris in Earth orbit?

 

[LURCH]

Unless a shattered object has enough time and delta-V to disperse far enough, Earth's gravity will draw in fragments.

Although this statement is true, it remains equally true for a shattered object or an object that remains whole. Enough delta v, delivered early enough, and the impact (or impacts) will be avoided. The only question, as far as I can see, is how to get the most delta v in the least amount of time.

 

[Janus]

Although this statement is true, it remains equally true for a shattered object or an object that remains whole. Enough delta v, delivered early enough, and the impact (or impacts) will be avoided. The only question, as far as I can see, is how to get the most delta v in the least amount of time.

It's not just a matter of magnitude of delta v, but also the best vector in which to apply it. The same delta v change can result in anything from a wide miss to there still being a collision. For instance, applying the delta v along a particular vector pushes the Earth orbit intersection point some 216,000 km further ahead of where it was, and two hrs later. Unfortunately, 216,000 km is how far the Earth travels in its orbit in two hrs. So you haven't avoided the collision, you've just changed the when and where.
For a newly discovered body, there is another complicating factor: the probable path. When you just discover an object you have a limited period of observation on which to base its trajectory, So what you have is a wide cone in which its actual path will fall. If the Earth falls within that cone, there is a chance that it could hit the Earth. At this point, the only way to insure a miss is to alter the course so that the Earth falls outside of the cone. If the Earth is close to the center line of that cone, then this could result in quite a large delta v. I if it is near the edge already it might not take much to nudge the object to ensure a miss.
A longer observation time means a better estimate of the trajectory, and the probable trajectory cone narrows. But an longer observation time can also mean a decrease in the Earth object distance and less time for the applied delta v to shift the trajectory. So it's a balance between waiting long enough to pin down the trajectory but not waiting so long that we can't alter the trajectory enough.

 

[Tiran]

But an longer observation time can also mean a decrease in the Earth object distance and less time for the applied delta v to shift the trajectory. So it's a balance between waiting long enough to pin down the trajectory but not waiting so long that we can't alter the trajectory enough.

I would think there is a delta vee solution that could be applied early enough that it wouldn't matter if the trajectory was 100% accurate enough or not because the early change would push the object well outside of possible impact.

In the process, the object's actual trajectory may pass through an Earth intercept (wrong direction), but would end up well outside an intercept on the other side.

But if you don't have the time or energy to make a big change in trajectory, then you have to be precise.

 

[Bortei]

If you could reduce the mass of the object, would that effect the trajectory? Could that be done using sonic or laser devices?

 

[jim mcnamara]

If you could reduce the mass of the object, would that effect the trajectory? Could that be done using sonic or laser devices?

I do not think it would be very effective.

Sound waves (sonic) do not propagate in a vacuum. If by "reduce mass", you mean break into smaller pieces, what you are doing in effect (analogy warning) is turning a very large cannon ball into a shotgun blast of many smaller cannonballs. If the smaller cannonballs (or anyway most of them) were less than the size where they are destroyed by entry into the atmosphere, then you would mitigate some of the damage. Air bursts from large meteors exploding in the atmosphere cause lots of damage, too. So it may not be a 100% win.

The "cannon balls" would have close to the same orbital properties with very small changes to the original path, so you would have to do this fracture attempt very far away to get enough deflection so most cannon balls miss Earth entirely.

Russian superbolide:
https://en.wikipedia.org/wiki/Chelyabinsk_meteor

I don't know about lasers - the energy required would be VERY large.

 

[LURCH]

If you could reduce the mass of the object, would that effect the trajectory? Could that be done using sonic or laser devices?

Unfortunately no, changing the mass would not change the trajectory by any useful amount. That is what Galileo famously showed. Bearing in mind that orbital trajectories are determined by gravity, we can look at his experiments dropping heavy and light objects, and observe that they fall at the same rate. So the progres of an orbiting object remains practically unaltered as the mass changes.

However, the manner in which the mass changes makes all the difference in the world. Any mass lost by the object must go somewhere, and in some direction. This is the so-called “reaction mass” that people often talk about when discussing these kinds of problems (or space travel in general). When some mass leaves the object in one direction (x), the remaining mass of the object gets a push in the opposite direction (-x).

I think this would also be a good place to mention something about gravity tractors. The question is often brought up about the mass of the tractor itself. In fact, I believe it may have been mentioned earlier in this same conversation that the tractor would need enough mass to exert a gravitational pull on the object. Actually, the gravity tractor idea relies entirely on the object’s gravitational pull. This is the bond that holds the object and the tractor together, so that a push that moves one moves both. I still believe this plan to have many problems, but the mass of he tractor is not among them.

 

[Tiran]

I think this would also be a good place to mention something about gravity tractors. The question is often brought up about the mass of the tractor itself. In fact, I believe it may have been mentioned earlier in this same conversation that the tractor would need enough mass to exert a gravitational pull on the object. Actually, the gravity tractor idea relies entirely on the object’s gravitational pull. This is the bond that holds the object and the tractor together, so that a push that moves one moves both.

I really don't see how you can have gravitational attraction without mass. The tractor balances gravity with thrust, keeping the tractor in a state of balance between the attraction and thrust away from the object. If the tractor has very little mass, than the amount of thrust it can apply and not break "orbit" will be miniscule. The tractor needs enough mass to get its thrust into a useful range to effect enough change over time.

A rocket hovering at a fixed height on its exhaust above the North Pole is also a gravity tractor. But the rocket is so tiny that it isn't going to move the Earth toward the North on any reasonable timeline. But a small moon with an engine could.

 

[mfb]

Space is big. We know more than 10,000 near Earth asteroids, there are many more, and they all happily orbit the Sun without hitting Earth. If you split an object on an intersection course into many fragments most of these fragments will happily orbit the Sun without hitting Earth, too. If 0.1% or even 1% of the mass happens to end up on Earth in a few smaller impacts you still reduced the damage a lot. A 10 km object is an extinction risk, a 1 km object destroys or severely damages a country, a 100 m object can destroy a town, a 10 m object won't do damage. Each step is 0.1% of the mass of the previous one. Getting hit by a few of them is much better than getting hit by one of the higher category.

While a deflection is more controlled "blowing it up" can be an emergency measure if there is no time for other methods.

If we have multiple orbits (and multiple years) as warning time then acceleration or deceleration is typically the approach that needs the least velocity change. Changing the orbital period makes the position difference accumulate over multiple orbits, changing the orbit shape but not the period does not. To change the period you want to change the speed of the object as much as possible (ideally in perihelion, but that might be impractical to reach).

power available (half revolution): 5x10^24W
half revolution period: 4.3x10^4s
earth mass: 6x10^24kg
displacement: Earth radius 6.4x10^6m
power=mass x acceleration x displacement / time so a=pt/md
I get a rough maximum acceleration of about 5mm/s^2

That calculation makes no sense. No, you can't change the position of Earth by any measurable amount, and randomly multiplying some numbers doesn't help.