Bad Universe on the Discovery Channel

[volcano]

Why would it be necessary to stop rotation? It seems to me that the effectiveness of this technique would only depend on the whether the laser had sufficient power to vaporize a bit of whatever it was pointed at. If it does, you get that action/reaction thing going, and the asteroid moves one way while the gas blows off the other. It shouldn't be necessary to keep the gas blowing off the same spot, should it? Having it blow off in a ring around the asteroid should still work, since the direction it's blowing off at any given instant is the opposite of the way you want the asteroid to go.

I guess it depends on how instantaneously the laser causes vaporization. If it needs to be pointed at the same spot for a while to cause material to blow off, then the rotation poses a challenge. Likewise, it depends on how quickly the vaporization stops-- if the previously-heated spot continues to blow off material for a while after rotating away from the laser, this could be a problem. Perhaps the solution is a pulsed laser, timed with the rotation of the asteroid (so that the laser always hits roughly the same spot of the asteroid as it rotates).

Dropping a rocket onto the surface of the asteroid and having it land "butt-end-up" seems a bit easier, even if you'd still have to pulse the rocket to deal with the asteroid's rotation (you only want the rocket to fire when it's oriented in the direction you want the asteroid to move).

As an aside, back in my first semester of college physics I had a professor demonstrate this principle (Newton's first law, I suppose) by sitting in a Radio Flyer wagon and propelling himself across the room by discharging a fire extinguisher.

 

[dasmiller]

I guess it depends on how instantaneously the laser causes vaporization. If it needs to be pointed at the same spot for a while to cause material to blow off, then the rotation poses a challenge. Likewise, it depends on how quickly the vaporization stops-- if the previously-heated spot continues to blow off material for a while after rotating away from the laser, this could be a problem. Perhaps the solution is a pulsed laser, timed with the rotation of the asteroid (so that the laser always hits roughly the same spot of the asteroid as it rotates).

All true.

Bear in mind that the "spot" on an asteroid that's illuminated by the laser is likely to be the entire face of the asteroid.

I indulged in a little math. At a range of 20 million km (which is pretty close for this sort of thing), with a 5 meter optic and 430 nm wavelength (I think that's a rich blue), the "spot" is about 2 km across. If your asteroid is less than 2 km in diameter, the spot will cover an entire face. And much of the laser's energy will simply miss the asteroid.

If it's in an orbit similar to Earth's, and we wanted to hit it while it was on the far side of the orbit, it would be 280 million km away, and the laser's spot would be almost 30 km across (assuming the laser was in Earth's vicinity).

Bigger optics and a shorter wavelength would make the spot smaller, but would make the pointing problem more difficult. For the case stated, you'd need to maintain pointing to <0.04 arc seconds. Trust me, for space applications, that's . . . challenging.

 

[volcano]

I indulged in a little math. At a range of 20 million km (which is pretty close for this sort of thing), with a 5 meter optic and 430 nm wavelength (I think that's a rich blue), the "spot" is about 2 km across. If your asteroid is less than 2 km in diameter, the spot will cover an entire face. And much of the laser's energy will simply miss the asteroid.

Yeah, it seems that a laser is probably the most technically challenging way of moving an asteroid (at least with current technology).

I use a 193 nm laser to blast holes in rocks in the lab, but the beam only travels about a meter and only blasts a hole about 100 um across. Not quite potent enough for space applications (but fun to use, nonetheless).

 

[bokonon]

At a range of 20 million km (which is pretty close for this sort of thing), with a 5 meter optic and 430 nm wavelength (I think that's a rich blue), the "spot" is about 2 km across.

Why would you attempt such a thing from 20 million km away?

I haven't studied it in any detail, but my general impression is that the laser nudging method would require quite a bit of time for an effective course correction, which would imply that the laser would be traveling alongside the asteroid in question. Is that not the case?

 

[dasmiller]

Why would you attempt such a thing from 20 million km away?

I haven't studied it in any detail, but my general impression is that the laser nudging method would require quite a bit of time for an effective course correction, which would imply that the laser would be traveling alongside the asteroid in question. Is that not the case?

Well, that gets into the part about the laser being in Earth's vicinity. It's a design trade; if you want to co-orbit the laser, then the laser will be quite a bit smaller (for a given booster capability, you can put a lot more mass into LEO than into some solar orbit) and likely much later (it will probably take many months to get to the matching orbit. Then there are all the autonomy issues, and maintenance . . .

Of course, if you co-orbit the laser, then you don't have the pointing issues and you won't lose energy due to spillover.

<shrug> I'd assumed a LEO laser platform, but you'd have to come up with some hypothetical cases and assumptions and simply run the trade.

 

[Dr.Sid]

There were also X-ray lasers pumped by nukes (one shot only) in the Star Wars program. Anyway for one big target, nuke alone, fired far enough it leaves the asteroid intact, but close enough for thermal effects, seem to me as best and achievable solution. We can launch such think, navigate it, and for sure fire it.

 

[Skeptic Ginger]

The asteroids depicted in that link are the ones we can SEE from Earth, through that damned pesky atmosphere (or from Earth Orbit), which makes them sizable objects, not "specks of dust"

He was describing a scaled version.

 

[Notrump]

I was disappointed. For years Plait has been criticizing those who profit by spreading fear through sensationalism. He outdid even them as he was grinning ear to ear over the prospect of creating explosions. It was another example of the dumbing down we’ve seen in recent years by once respectable cable networks such as Discovery and History.

 

[LandR]

Is this going to be shown on Discovery in the UK ?

 

[DeiRenDopa]

Comment #27 from ChazInMT seems relevant here.

27. ChazInMT Says:
August 27th, 2010 at 11:57 am
I try and educate people regarding size relationships as an astronomer/presenter all the time. The Sci Fi movies, 100% of all science related TV programs, and indeed, this image presented above are grossly out of scale. I have begged in the past that Phil set things straight regarding this and present our solar system to scale on one of his TV ventures. I have never seen the scale of the solar system properly or adequately represented in the main stream media.
Some “Facts of Note” to try and get your head around:
If the Sun were scaled down to a bowling ball in size, the Earth would be a very small 2mm diameter bead orbiting 26 Yards from the bowling ball, Mars is a 1mm bead 40 yards away, and Jupiter is a 22mm diameter marble orbiting 135 Yards from the bowling ball sun. Neptune is a pea sized object 777 Yards out, close to half a mile. On this scale, there would be 10 amazingly small specks of dust representing asteroids per square yard between Jupiter and Mars. Assuming as Phil has that they are about a foot deep, this would be about 1 Asteroid in every cubic foot of space for the 500,000 Asteroids covering an area the size of 10 football fields. If there are 2 million Asteroids, this would still just be 4 specks of dust in a cubic foot of space.
From what I understand, when NASA sends a spacecraft through this “Crowded” asteroid belt beyond Mars, they completely disregard any chance of anything hitting the spacecraft or vice versa, because the probability is so remote.
BTW, let me bounce this off you, if the Earth were a basketball, the crust we live on, and 99% of the atmosphere would each be about 3 sheets of paper thick on average covering the basketball. Earth and Sky are 6 sheets of paper thick. A chicken egg shell is 6 times thicker to scale than our earths crust, food for thought..

To summarize:

4 tiny specks of dust per cubic foot to represent average asteroid density.

Mars 40 yards from the Sun, Jupiter 135 yards, thus 95 yards between them.

Assuming for the sake of argument that the telephoto lens at 1 LY could carve out a 1 cubic foot field of view, we'd be looking at compressing 95*3=285 feet of asteroid belt cross-section into a 1 square foot image. At four specks of dust per foot, that's 1140 specks in our field of view.

Yeah, that's probably not going to look like a floating rock pile unless you take some liberties with the size of the specks. I think you could justify showing half a dozen big rocks cavorting, but more than that would seem unlikely to "line up" properly for the tiny field of view you'd need to make them look menacing.

The scaling is ~1.6 x 10^-8 (2mm = 12,700 km).

Assume a camera with a megapixel per square foot (of image).

1 pixel is thus ~0.3 mm, which would correspond to an object ~2,000 km in diameter.

Assume the average asteroid is 10 km in diameter; each of the 1140 dust specks would thus take up, on average, 0.05 (5%) of a pixel.

So, there'd be ~1000 dust specks, each far smaller than 1 pixel, in an ~81 gigapixel image.

Definitely not your average floating rock pile, and no 'rocks' at all!

 

[bokonon]

The scaling is ~1.6 x 10^-8 (2mm = 12,700 km).

Assume a camera with a megapixel per square foot (of image).

1 pixel is thus ~0.3 mm, which would correspond to an object ~2,000 km in diameter.

Assume the average asteroid is 10 km in diameter; each of the 1140 dust specks would thus take up, on average, 0.05 (5%) of a pixel.

So, there'd be ~1000 dust specks, each far smaller than 1 pixel, in an ~81 gigapixel image.

Definitely not your average floating rock pile, and no 'rocks' at all!

I'm not following your calculations here. Our image was one square foot, which you initially pegged at 1 megapixel (1000x1000 pixels). Where is the 81 gigapixel image originating?

 

[arthwollipot]

Is this going to be shown on Discovery in the UK ?

No-one knows...

 

[DeiRenDopa]

I'm not following your calculations here. Our image was one square foot, which you initially pegged at 1 megapixel (1000x1000 pixels). Where is the 81 gigapixel image originating?

4 dust specks per square foot; 1140 in the 285 x 285 square feet thingie ...

 

[bokonon]

4 dust specks per square foot; 1140 in the 285 x 285 square feet thingie ...

So you're multiplying 285 by 285? Why?

285 feet is (to scale) the linear distance in feet between the orbit of Mars and the orbit of Jupiter. Our "square foot image" represents a projection across that 285-foot section of radius, compressing the 285-cubit feet along a single line of sight into a one square-foot image. It's as though you had a 1-square-foot window at the orbit of one planet, and walked away from or toward the sun, letting the 4 dust specks in each cubic foot adhere to the window, until you reached the orbit of the other planet.

At least, that's what I had intended to describe; I may have been unclear.

 

[Wowbagger]

For years Plait has been criticizing those who profit by spreading fear through sensationalism. He outdid even them as he was grinning ear to ear over the prospect of creating explosions.

I think the explosions he used in the show were appropriately scaled experiments, not exaggerated or sensationalized depictions.

The threat from space is a REAL one to be considered. Not something he just made up, and he was honest about the probabilities of it happening, what is being tracked in the sky, and strategies for dealing with it.

I don't think he was spreading fear through sensationalism. I think he was spreading pertinent information for managing a realistic threat, and summarizing it via model explosions.

 

[DeiRenDopa]

So you're multiplying 285 by 285? Why?

285 feet is (to scale) the linear distance in feet between the orbit of Mars and the orbit of Jupiter. Our "square foot image" represents a projection across that 285-foot section of radius, compressing the 285-cubit feet along a single line of sight into a one square-foot image. It's as though you had a 1-square-foot window at the orbit of one planet, and walked away from or toward the sun, letting the 4 dust specks in each cubic foot adhere to the window, until you reached the orbit of the other planet.

At least, that's what I had intended to describe; I may have been unclear.

That may be so.

Here's the original:

"Assuming for the sake of argument that the telephoto lens at 1 LY could carve out a 1 cubic foot field of view, we'd be looking at compressing 95*3=285 feet of asteroid belt cross-section into a 1 square foot image. At four specks of dust per foot, that's 1140 specks in our field of view."

It seems there is some confusion over units, if nothing else:

* "carve out a 1 cubic foot field of view" - a field of view should have dimensions of L x L, not L x L x L, surely?

* "compressing 95*3=285 feet of asteroid belt cross-section" - a cross-section should have dimensions of L x L, not L, surely?

* "four specks of dust per foot" - if it's a cross-section, it should have dimensions of L x L, not L; if it's a density, it should have dimensions of L x L x L, not L, surely?

 

[bokonon]

That may be so.

Here's the original:

"Assuming for the sake of argument that the telephoto lens at 1 LY could carve out a 1 cubic foot field of view, we'd be looking at compressing 95*3=285 feet of asteroid belt cross-section into a 1 square foot image. At four specks of dust per foot, that's 1140 specks in our field of view."

It seems there is some confusion over units, if nothing else:

* "carve out a 1 cubic foot field of view" - a field of view should have dimensions of L x L, not L x L x L, surely?

You're correct; it should read "carve out a one square foot field of view." By the time I'd noticed that, it was too late to edit it.

* "compressing 95*3=285 feet of asteroid belt cross-section" - a cross-section should have dimensions of L x L, not L, surely?

Not really. It's a cross section of the disk containing the asteroids, and we're slicing across the entire disk, 285 linear feet. We're slicing across it with a view window which is 1 foot square, but the cross section, in my mind, is 285 feet.

* "four specks of dust per foot" - if it's a cross-section, it should have dimensions of L x L, not L; if it's a density, it should have dimensions of L x L x L, not L, surely?

Four specks of dust are contained in a cubic foot of asteroid belt, which is projected onto our one square foot view window. Our view window picks up an additional 4 specks of dust every time it "moves" through a cubic foot of asteroid belt. The square foot view window moves through a cubic foot of asteroid belt by moving one linear foot across the asteroid belt. "Four specks of dust per foot" is thus per linear foot.

 

[Chucky]

Thanks to bluskool and Skeptic Gingerfor the heads up about the torrent. I'll try that.

 

[alfaniner]

Considering Phil's recent TAM talk, maybe it should have been titled "DBAD Universe"?

 

[DeiRenDopa]

You're correct; it should read "carve out a one square foot field of view." By the time I'd noticed that, it was too late to edit it.

OK.

Not really. It's a cross section of the disk containing the asteroids, and we're slicing across the entire disk, 285 linear feet. We're slicing across it with a view window which is 1 foot square, but the cross section, in my mind, is 285 feet.

So the geometry is that you're looking towards the Sun, in the plane of the orbits of the (true) planets?

If so, then the Main Belt asteroids do not form a disk, certainly not one whose scale height is approx the same as that of the planets!

While the distribution of the inclinations of Main Belt asteroids is centered (more or less) on the ecliptic, it's a wide distribution, with several streams at well-defined inclinations (> 0^o).

Four specks of dust are contained in a cubic foot of asteroid belt, which is projected onto our one square foot view window. Our view window picks up an additional 4 specks of dust every time it "moves" through a cubic foot of asteroid belt. The square foot view window moves through a cubic foot of asteroid belt by moving one linear foot across the asteroid belt. "Four specks of dust per foot" is thus per linear foot.

Got it, thanks.

Not too bad for an initial OOM (order of magnitude) estimate, but the distribution of Main Belt asteroids, by semi-major axis, is far from uniform. Not only are there pronounced gaps, but the belt starts quite some distance from Mars' (from the Sun), and peters out well before Jupiter's.