All you rocket scientists, an asteroid problem to work on.

[Aero]

A small asteroid, discovered yesterday, will pass close to Earth tonight.

http://www.space.com/scienceastronomy/0 ... -pass.html

A small asteroid will buzz the Earth late Friday EDT (early Saturday GMT), flying just inside the orbit of the moon. It should pass safely by our home planet, according to a crack team of NASA space rock trackers.

The space rock, named 2009 TM8, was just discovered Thursday by the Catalina Sky Survey in Arizona. It will get within 216,000 miles (348,000 km) of Earth when it zooms by at a speed of about 18,163 mph (29,232 kph).

"That's slightly closer than the orbit of our moon," NASA's Asteroid Watch team said Friday via Twitter.

The time of closest approach will be 0344 GMT Saturday, or 11:44 p.m. EDT tonight.

My problem to you is just this. If this asteroid actually was headed for a bulls eye strike on earth, what would it take to deflect it? Here are some questions to ponder:

1. If it actually struck Earth, what would its impact velocity really be? Certainly more than the quoted 18,000 mph, as Earth's escape velocity is like 25,000 mph. (I would guess impact velocity to be more like 40,000 mph.)

2. If it was a nickel iron asteroid, what would be its mass? I guess, assuming it is spherical, that it masses something over 2000 metric tons.

3. What would be its energy release on impact? I'm guessing it would be quite a bit more than the 4 kilotons quoted in the news article, more nearly 100 kilotons is my guess. What would be its radius of destruction?

Now for the fun part! With total freedom to use anything we have seriously discussed on this forum, what would it take to divert this hypothetical asteroid? That is, we have two days notice, so our system obviously has to be in place, docked somewhere, but not on ready alert though its crew should be stationed nearby. What will this system look like? I'm guessing that it is not a gravity tractor.

[chrismb]

err..... surely at that kind of speed, around the escape velocity, a close pass might simply mean it drops straight into an eccentric close-earth orbit and crash a few days later after a few wobbly laps around the earth?

"If it's typical density, it would create a 4 kiloton explosion in the Earth's atmosphere if it were to hit, which of course it won't," said Don Yeomans. - Those kind of 'certain' statements always get me worried!!!

[Professor Science]

2 days notice? I'd almost say it can't be done. But not doing the math, i don't know for sure.

[Aero]

"If it's typical density, it would create a 4 kiloton explosion in the Earth's atmosphere if it were to hit, which of course it won't," said Don Yeomans.

I agree, they worry me too - seriously downplaying a potential disaster just keeps space watch unfunded. Of course over playing it could be just as negative. The honest truth is probably best and the honest truth as I see it is that if it is typical density and it did hit Earth it would deliver more energy than the Hiroshima bomb.

As for dropping into orbit, its not likely. Of course the Earth's gravity will change the orbit, but the asteroid is moving much to fast to go into orbit. The escape velocity of 25,000 mph is as measured from the surface of the earth. Consider that low Earth orbital velocity is about 17,000 mph, while the moon's orbital velocity is about what? One revolution in about 28 days. I guess that would be

Moon velocity = 2 * 3.14159 * 240000/ (28 * 24) = about 2244 mph

Gravity obeys the inverse square law, 2244 * (240000/216000) ^ 2 gives 2770 mph for orbital velocity at 216000 miles orbital radius. I don't think the asteroid will drop into orbit but it might come back around the sun.

[Mindblast]

Earth escape velocity of ~40000kmh is for surface level (a bit lower for low earth orbit). If its closest distance will be 348000 km it won't be captured. For example the moons orbital velocity is somewhere around 1000 m/s (3600 kmh).

Edit: Ah right you answered your own question already..

[Aero]

2 days notice? I'd almost say it can't be done. But not doing the math, i don't know for sure.

That's not playing the game. And anyway, all you need is a cargo ship with 30 ft. loading doors and 2500 ton cargo capacity. Then just go match orbits, de-spin it and snatch it. What's the big deal? Well, you're right, a two day time limit is a big deal and so is that cargo ship. I'm hoping for a more economical, near term solution with maybe ME thrusters, FRC thrusters, or even inertial drives. And I think de-spinning the asteroid is a different problem, interesting in its own right but a separate problem.

[D Tibbets]

With only several days notice there is not much you can do. If the asteroid was not to small and not to large you might desperatly try to launch a nuclear missle and try to hit it near the peak of the balliatic flight of the missle. If the asteroid is stony, or if it is a comet you might break it up some so that the upper atmosphere would finish the job and thereby spare the surface. But, I dought that nuclear missles are accurate enough. Fallout would presumably not be to bad as the dust would hopefully end up in the upper atmosphere and be dilluted before drifting to the surface.

Again, if the asteroid is large enough to instill a fear that it might reach the ground or lower atmosphere before breaking up (Boom!), but not too big, a Standard missle III or other antisatallite missle might intercept it outside the atmosphere and damage/ shock it enough that it might break up higher in the atmosphere.
I dought that the trajectory of the asteroid could be changed much on such a short time scale.
A two day warning would allow time to at least try to evacuate the impact area/ vunerable costline (if tsunomi threat), provided the rock is not to big. There is a ranking system for potential impactors, based on probability of hitting and potential for devistation. It is called the Torino scale

http://neo.jpl.nasa.gov/torino_scale.html

A site that calculates the effects of an asteroid strike based on makeup, size, speed and angle to the ground is at:

http://www.lpl.arizona.edu/impacteffects/

Dan Tibbets

[Aero]

Well, according to the Arizona web site, even the high density nickel - iron asteroid wouldn't be noticeable at the surface because it would destroy itself in an air burst. I thought that solid metal asteroids didn't burst and disintegrate but the web site indicates that they do. Double the asteroid diameter then there will be damage. That is a 25 year event.

No, I don't believe that web site. I calculated a 25 meter diameter asteroid strike with the same parameters, and the web site concluded that there would be an air burst at 500 meters, on the order of 1.86 x 100 Megatons, and it would break some glass.

Maybe I'm not using the site correctly, but a 186 Megaton air burst at 1500 feet will cause some damage, more than just breaking glass.

[krenshala]

The uneven air pressure combined with ablation due to its speed will chew up even a metal meteor. This is why most meteorite fragments are just that, fragments of the larger original. A decent number of the pieces may make it to the ground but most will be (much) smaller than a bowling ball. Or, at least, thats what my previous reading on the subject has shown/indicated.

Of course, this is only information on meteorites from the last 100 years or so, and most of the data from before WW2 is pretty sketchy.

Personally, I think a high-powered laser (powered by a Polywell, perhaps? ) would be able to heat one side of the asteroid fast enough to cause vaporization of some of the rock causing it to act like an attitude jet. Properly placing the beam impact point can either deflect the asteroid or perhaps even cause it to become a new moon for Earth. 8.12 km/s is fast, and they don't mention if it is an opposing orbit or not (i'm assuming not considering the listed velocity), but giving a day or so there should be enough time to nudge it aside. At the least to the point where it spends more time in the upper atmosphere, which will cause it to break up at a higher altitude, than it would otherwise.

[blaisepascal]

A small asteroid, discovered yesterday, will pass close to Earth tonight.

My problem to you is just this. If this asteroid actually was headed for a bulls eye strike on earth, what would it take to deflect it? Here are some questions to ponder:

1. If it actually struck Earth, what would its impact velocity really be? Certainly more than the quoted 18,000 mph, as Earth's escape velocity is like 25,000 mph. (I would guess impact velocity to be more like 40,000 mph.)

The velocities don't add, but the energies do. The specific kinetic energy of the asteroid is (8100 m/s)^2, while the specific kinetic energy it would get from falling to the Earth is at most (11200 m/s)^2. Add that together, and you get a specific kinetic energy of 190000000 m^2/s^2, for a impact speed of the square root of that, or about 14km/s (31,000 mph).

2. If it was a nickel iron asteroid, what would be its mass? I guess, assuming it is spherical, that it masses something over 2000 metric tons.

Nickel-iron asteroids are about 7-8tonnes/m^3, so a 7m diameter spherical asteroid would be... about 1250-1400tonnes. Call it 1300 tonnes.

But a spherical rock 7m in diameter would be the exception, not the rule. Most asteroids are irregularly shaped, and therefore smaller than they would be for the same diameter than if they were spherical. Two common asteroid shapes are "cigar-like" and "dumbbell". If it were dumbbell shaped, we could approximate it as two spheres each 3.5m in diameter, and that would cut it's mass by a factor of 4, down to about 350tonne.

3. What would be its energy release on impact? I'm guessing it would be quite a bit more than the 4 kilotons quoted in the news article, more nearly 100 kilotons is my guess. What would be its radius of destruction?

For 1300 tonnes at 14km/s, I get 60 kton tnt.
For 1300 tonnes at 8.1km/s (their quoted speed), I get 20kton tnt.

That's for nickel-iron. The vast majority of meteorites are not nickel-iron, and have a density of about half that, and thus about half the energy (so about 10-30kton tnt). Add in the irregular shape, and you lose another factor of 4, so we're talking 2.5-7.5tkon tnt.

But that's the total energy dumped into the Earth. It doesn't account for energy dissipated in the atmosphere burning up the asteroid, etc. It wouldn't surprise me in the least if a "typical" asteroid of that size had a low-altitude airburst in the kton range, at most. Remember that Little Boy (Hiroshima) was about 10-15kton tnt.

Now for the fun part! With total freedom to use anything we have seriously discussed on this forum, what would it take to divert this hypothetical asteroid? That is, we have two days notice, so our system obviously has to be in place, docked somewhere, but not on ready alert though its crew should be stationed nearby. What will this system look like? I'm guessing that it is not a gravity tractor.

Probably not a gravity tractor, but let's see what we can do... I'm going to relax the conditions a bit, and assume we can get to this asteroid with 2 days to spare. At that point, it would be 1.4GM away. Let's say we wish to divert it so that it misses the center of the earth by 20Mm (about 3 Earth radiuses, or two Earth radiuses above the surface). The delta-V required would be 115m/s.

The rocket equation says the mass ratio (initial/final) of a rocket needs to be equal to e^(delta-V/exhaust-V). For the Shuttle engines, the exhaust-V is about 4400m/s, so the mass ratio is 1.03. To move a 1300tonne rock, you'd only need 39 tonnes of fuel. An SSME could produce that much delta-V in under 2 minutes, and add only 3.2tonnes. Not bad, a Saturn V could lift that much payload (but not really with enough to spare for matching speed with the rock).

With a VASIMR, and an exhaust-V of 100km/s, the mass ratio is even lower, so only 1.5 tonnes of fuel would be needed. If we spread that over 2 days, that's less than 1000N of thrust, which could be handled by 200 VF-200 engines, each at 300kg, which would unfortunately weigh in at 60tonne. The chemical rocket is the better choice.

[D Tibbets]

Well, according to the Arizona web site, even the high density nickel - iron asteroid wouldn't be noticeable at the surface because it would destroy itself in an air burst. I thought that solid metal asteroids didn't burst and disintegrate but the web site indicates that they do. Double the asteroid diameter then there will be damage. That is a 25 year event.

No, I don't believe that web site. I calculated a 25 meter diameter asteroid strike with the same parameters, and the web site concluded that there would be an air burst at 500 meters, on the order of 1.86 x 100 Megatons, and it would break some glass.

Maybe I'm not using the site correctly, but a 186 Megaton air burst at 1500 feet will cause some damage, more than just breaking glass.

When I plug in the numbers for a 25 meter iron meteorite entering the atmosphere at 10 meters /s (~ 32,000 KM/hr) and a 90 degree angle, it would breaK up at ~ 5000 meters with ~ 600 ktons of energy (is that the total energy- the air burst and fragnent impacts with the ground?). The fragments would create a crater ~ 900 meters wide (or perhaps a lot of small craters covering that area (?). The effects you would feel depends on the distance from ground zero that you pluged in.

Dan Tibbets

[hanelyp]

Given energy delivery in the 10s of kT, where would the projected impact point be? You may not want to divert it. If it was headed towards a heavily populated area that you cared about diverting by a few 100km might be a viable plan.

[Aero]

Yes, the site seems to give inconsistent answers, one run to the next. But then, what is "Consistent" in this case?

Did you notice the fast and loose conversion from feet to meters in the original quoted article? Thirty feet converted to 7 meters. How accurate is that? Units converter gives 30 feet = 9.144 meter which will change the mass of the asteroid substantially for all shapes. But on another tack, Earth strikes by asteroids of this size are said to be a weekly event. That being the case the numbers showing no damage are probably correct, or nearly so. Otherwise we would have noticed.

And that being the case, there is no reason to divert the asteroid so the whole problem goes away. In order for the asteroid to be a problem it needs to be 25 - 30 meters in diameter, that is, a 200 year event. The shuttle main engines could still nudge its orbit, except one is not allowed to intercept the asteroid before it is discovered. The problem as originally stated gave 48 hours from discovery to impact, that was what made it a challenge.

[chrismb]

So, did it hit anywhere on your side of the planet?

Phew, another one gone. I get the feeling like we're the Jack in a game of intergalactic boules!

Remeber, it's not *if* but *when*, an astronomical reality that many seem to be happily ignoring in their comfy complacency over life.

[Tom Ligon]

I just missed an opportunity to attend an asteroid deflection conference, and did attend one a year ago.

The guess that an impact with Earth would be around 40,000 mph is probably pretty good on average. Escape velocity plus whatever orbital velocity component the particular bolus has relative to us as a rough gage.

Our conference spent a lot of time looking at Aphophis, the one that will make an 18,000 mile pass around 2029. It is a couple of hundred meters long. For that one, one possiblity was to use a "gravity tug", a small space probe equipped with about 8 of the same ion motors DS-1 uses. Rather than orbiting the asteroid, you park off to one side using the thrusters to station keep there. The gravitational attraction the spacecraft exerts is also felt by the asteroid, so you can gently move it.

How much? Over a decade or so, you could pull it sufficiently to change its point of passage about 30 km. Not much. For Aphophis, it might be enough to avoid a "keyhole" that would send it into Earth on the next orbit.

Otherwise, various nuclear options and violent impacts are envisioned, but they still need years of prep to be ready, and months or years to launch a mission. They also need testing, and nuclear tests in space are a hard sell.

I keep hoping I'll be able to offer meaningful thrust for the effort.

The project to identify all the dangerous objects is underway and is making excellent progress. That's why we've been spotting all the near passers. They were always out there, we just never looked.

As one Air Force guy mentioned last year, if you know the danger is coming and you can't do anything about it, you just die tensed up.