B Ideas to protect the Earth from possible asteroid impacts

[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.