Directed Energy Planetary Defense and Relativistic Probes | Hacker News Reader

Directed Energy Planetary Defense and Relativistic Probes

deepspace.ucsb.edu

53 pointschr133 comments

Happened to catch rockstar astrophysicist Neil deGrasse Tyson on Charlie Rose the other night and the subject of asteroid deflection came up. For small candidates, vaporization would be ideal. But for larger ones, any attempt at destroying them may have unwanted consequences. Debris fields, partial cleaving, etc. Sending a team of human miners with demolition expertise is not in the cards. So the current vogue in asteroid deflection would be to send something heavy to intercept the asteroid and deflect it a degree or two using only gravitational pull! A very clean and elegant solution that would no doubt make a great simulation or video game ;)

Mission Concepts and Operations for Asteroid Mitigation Involving Multiple Gravity Tractors

http://www.ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201...

This already exists in a video game. It's call kerbal space program. But instead of gravity you can attach thrusters to asteroids to deflect them. The mod was created by NASA.

What do you think the smilie was about...

A nice thing about this is that it doesn't need to destroy the large asteroid, it just vaporizes small part of it creating thrust and changing the trajectory. One of the videos mentioned bringing asteroids with useful resources closer as a possible usecase.

The point isn't the vaporize them, it's to nudge them ever so slightly.

Consider a comet- you fire one or more lasers to a specific spot on the comet, causing heat. The heat causes the water to expand, and shoot off from the comet. Does this destroy the comet? Certainly not much of it. But as that water vapor shoots away from it, Newtons Third Law comes into play and the comet, ever so slightly, is pushed in the opposite direction.

You may rightfully say that this would be a very minuscule push, so what good will it do? Well, if you play Kerbal Space Program you'll quickly learn that tiny nudges can have huge effects in the long-run, especially on big orbits. Add a few mm/s to the velocity of a comet two years ahead of time, and it can make the difference between that comet hitting a planet, or not. It can mean the comet is ever so slightly closer to Jupiter, causing it to be pulled by its gravity just a bit more.

But KSP does give a false impression. It is very much a "spheres in a vacuum" approach to orbital physics. It doesn't model many/most of the realworld forces.

When dealing with the truly minuscule forces such as would be created by lasers aimed at asteroids, one much accommodate all other similarly-sized forces before taking action. Rocks in space are subject to a force/pressure from the sun, a pressure comparable to that from a earthbound laser. Before trying to deflect, one would have to understand how the sun will be altering the course. That calculation is not easy and requires details (albedo/rotation) that are hard to come by for tiny rocks. You don't want to be countering the force from the sun only to push the rock into an intercept it would otherwise have missed.

Vaporization also need not mean destroying the rock. So long as the vaporized material achieves escape velocity (not hard on an asteroid) vaporizing the surface of an object creates a force, a thrust, which is greater than the pressure directly from the laser.

> Rocks in space are subject to a force/pressure from the sun, a pressure comparable to that from a earthbound laser.

Hum, is it? When near the Sun, yes, of course, but when away from it, we can probably throw way more energy on it than the Sun.

Yes, it does require details that we currently can't measure. The good new is that the laser doubles as an excellent instrument for measuring those details.

That very much depends on the laser. Most of the approaches I've seen discussed involve multiple 'attacks' on a rock spread out over a considerable period of time. You only fire when the rock is far enough above the horizon (say 25-30% of the time if you have 1 ground laser). And then you want to hit it when it is at some sort of apsis, preferably when it is far away with a relatively slow velocity. The sun has a 24/7 line of sight and is always firing.

Pressure from either sun light or laser is very small, and can be ignored for time intervals smaller than centuries. The method proposed in the article is only about thrust created by vaporization

The sun vaporizes. That's where comets get tails.

Does it? I thought that was heated gasses boiling off.

Boiling off == vaporized (ok pedantically, boiling off -> vaporized, but not the other way around).

The Sun only does that on icy rocks, and when they are near. Lasers can (in theory) do that on icy rocks that are much further away.

In space it often isn't so much boiling (liquid+heat=gas) as sublimation (solid+energy=gas). Sublimation can, weirdly, occur well below the freezing/boiling point. That one lone cosmic ray can heat a single molecule of water enough to cast it into space as a gas ... all in an environment where the average temp is well below freezing points. Temperatures in vacuum are strange.

For the "larger ones" that fitzwatermellow was mentioning, the problem is that your mental model of a big rock starts breaking down. You'd be closer to think "big gravel pile sort of iced together, sort of held together by local gravity because there's nothing else pulling them apart". You shoot a little pulse at the gravel pile and break a piece off and it may simply fly out into space, taking almost all the momentum with it and not affecting the bigger body at all. You apply a big force and you might just send rocks through the pile, with, again, the rocks going through taking most of the momentum you applied with them. (Same for landing a booster on it... there may be nowhere you can boost where the booster doesn't just drill right through and come out the other side.) Hit it harder and it may simply spread out.

The advantage of the gravity towing approach is that gravity affects the entire pile equally; it doesn't matter how tenuously the whole thing is held together. This approach could literally deflect a mountain-sized ball of pure sandbox-grade sand, completely and correctly. That's not something a lot of other approaches can say. It's only downside is that it's miserably slow, so you have to call the problem years in advance.

That is a really intuitive explanation. Thanks.

The problem with gravity pull is you first need to mach orbits which takes a long time or a lot of delta V. From an energy and complexity perspective your best bet is to impact with a large low mass object like foam or lots of tiny objects like a cloud of sand. The advantage being just about any orbit that gets you there works and high relative velocity's are good. The problem is it's easy to miss.

A solar sail is a solid candidate as you get a large low mass object and 'free' delta V at the same time.

PS: Due to risk of malfunction you likely want to send several ships anyway, while the miss chance is an issue, the cheaper direct approach lets you take more chances.

If we break asteroid into smaller pieces wouldn't that be enough? Small pieces won't reach surface of earth and won't cause any damage.

> You shoot a little pulse at the gravel pile and break a piece off and it may simply fly out into space, taking almost all the momentum with it and not affecting the bigger body at all.

this is true for shooting with projectile, but not for vaporizing with laser. With laser you don't apply any external momentum to the asteroid, vapor small debris fly off the asteroid with some momentum because of expanding under heat, and remaining part gets same momentum in the other direction.

can this be weaponised to direct asteroids to a particular place on earth? just asking for a friend.

My guess is no. Reason: steering towards is a smaller target than steering away. If you can only point a laser from over here, it's hard to steer an asteroid any way except "thataway" (normal to the most visible surface).

unless the asteroid was closer to the sun than the earth, then you could push it so it hit earth 6 months from when you pushed it.

The available thrust directions are limited given that the laser is presumably in orbit or perhaps at a Lagrange point.

Of course once you have a multi-mega (or giga) watt laser in orbit you can pretty much dictate terms to anyone else who wants to use space. After all one person's 'space junk' is another persons 'operating surveillance satellite' :-)

I'm a much bigger fan of the idea of using drone space craft with an ion engine which has essentially a gooey blob/harpoon on the front of it, that you attach to/spear the desired object, and then turn on your ion engine to provide enough delta-v to put it where it won't hurt anyone.

Probably, but it'd require nation-state level of resources to build and get the system into orbit, plus months/years of lasering.

It'd become pretty quickly apparent what you're doing and your satellite would fairly rapidly become an expanding cloud of debris.

What is our next move when it shoots back?

LOL!

Welp, it's officially the future once we have planetary defense laser bases.

> DE-STAR or Directed Energy System for Targeting of Asteroids and exploRation

Ugh! "exploRation"?

Naming it DEath-STAR is probably not the best way to build public support.

Have you met geeks?

> sufficient warning

Well, yes, quite. Wouldn't we be better off making sure we can reliably find them first?

> Wouldn't we be better off making sure we can reliably find them first?

People are already are working on that. For example, there's the NEOWISE project[0], which used the Wide-field Infrared Survey Explorer to look for potentially dangerous astroids. And there is a ongoing effort for a new space mission, NEOCAM[1], specifically designed to do this same thing and find more objects.

[0] http://neo.jpl.nasa.gov/programs/neowise.html

[1] https://en.wikipedia.org/wiki/Near-Earth_Object_Camera

They propose additions use cases for the array in their papers. These include a LIDAR based detection system. I'm on my phone right now so I can't link the appropriate one. Let me know if you are interested and I can find it later.

With powerful systems you never know whether they are going to save you or evaporate you. Funny shit like this doesn't get funding unless there are military options to it I'd wager.

has anyone checked out the relativistic probes claim? what potential speeds are we talking about? and, i presume, there are no brakes? :D

Trying to force asteroids away are we?

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