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Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

Posted January 25, 2010 8:10 AM

From Wired Top Stories:

If we're going to protect the Earth from an asteroid, we need to find the dangerous ones whizzing about in the emptiness of space. Unfortunately, the United States will not complete the survey of large near-Earth objects by 2020 as mandated, but not funded, by Congress in 2005. That's the conclusion of a new National Resource Council Report, Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies, released Friday.

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#1

Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/25/2010 8:22 AM

Somebody had better have Bruce Willis on speed-dial....

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#2

Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/25/2010 2:35 PM

The LSST and Pan-STARRS telescopes referenced in the article are ground-based, visible-light telescopes which cannot search for NEOs in the Deep-IR part of the electromagnetic spectrum where NEOs are brightest.

NEOs are best detected from space using instruments such as WISE, which can see them in the Deep-IR region and where Earth's warm atmosphere cannot blind the instrument to the faint thermal signatures.

I wouldn't fund that survey either if those two scopes are the best idea they can come up with.

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#3

Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/25/2010 2:48 PM

Absolutely, unquestionably, the most important factor in this whole debate is detection. Unless an object is first detected we cannot know if it's going to hit us. Of course this might be preferable to knowing that a dinosaur killer is on it's way but that our fearless leaders aren't going to do anything about it.

I have said over and over that the most likely scenario is that some amateur astronomer with a 60mm refractor is going to discover a new object that is going to turn out to be an asteroid the size of Ceres and less than 18 months from impact with Kansas. The question is, what are we going to do about it? And, unless our fearless (clue-less?) leaders do decide to allocate more funds to the effort, this is all too likely to be exactly what we will face.

So, to turn aside a dinosaur killer with the limited time we are likely to have is going to require the most powerful technologies we have at out disposal. And yes, by that I do mean nuclear power and nuclear explosives. To hope that we could deflect or destroy such an asteroid or comet with anything less is wishful thinking, and to allow aging hippies and tree-huggers to try to block such an effort would be suicide.

And by the way. The concept of a "gravity tractor" is absurd. To place a spacecraft near an asteroid with thrusters flaming (probably ion devices with their microscopic thrust) in hopes that gravitational attraction will cause the asteroid to follow is like imagining that a single ant could turn aside a charging elephant.

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#4
In reply to #3

Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/25/2010 4:24 PM

The author writes: "First, the asteroid could be hit with some kind of impactor, either conventional or nuclear. Second, a longer-term, more precise technique like a gravity tractor could be employed."

You need a subscription to read the "gravity-tractor" paper. The abstract doesn't describe the proposed technique in sufficient detail.

The author fails to mention what probably would be two of the most promising techniques for asteroid mitigation: thermonuclear ablation (for large asteroids) and plasmafication (for small ones).

Thermonuclear ablation refers to the technique whereby one or more thermonuclear devices (TNDs) are detonated at one side of a large (possibly rotating) NEO causing the surface material to vaporize momentarily, providing thrust in addition to the thrust due to radiation pressure from the explosion(s). Destroying the NEO is not the objective of this technique; deflection is.

The second technique could be used with smaller NEOs to completely vaporize them. Multiple small TNDs are positioned symmetrically about the NEO at some prescribed distance and configuration depending on the NEO's geometry. The TNDs are then detonated simultaneously - or nearly so - such that debris from the NEO is confined at the NEO and vaporized by the X-radiation from the explosions. If one doubts that radiation pressure is able to confine matter, consider that the radiation pressure near a W-80 TN warhead can exceed 1.4 billion bar (atmospheres) due to X-rays alone.

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#5
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Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/25/2010 4:59 PM

I agree that the abstract portion of the gravity tractor sounds implausible, at best. So implausible in fact that I doubt the abstract accurately describes the thrust of the paper. (Pun intended )

Your ablation and plasmafication techniques sounds quite plausible, europium. I seem to remember two earlier proposed long term orbit adjusting techniques of a controlled solar sail technique and solar panel with a mass accelerator technique. Both of these techniques suffer the drawback of requiring landing on the NEO, no simple feat. I wonder if this precludes either implementation.

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#6
In reply to #5

Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/25/2010 5:25 PM

Neither technique requires landing on the NEO. In both approaches the detonations take place at a distance from the object. Of course, the earlier the object(s) are detected, the more easily they are mitigated. Early detection is the key.

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#7
In reply to #4

Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/25/2010 5:39 PM

Very well, allow me to say why I think the idea of a gravitational tractor is absurd.

First of all, we are going to need a very large spacecraft, on the order of millions of tons, to provide the necessary gravitational attraction. Do we have anything like this big in space? Are we likely to anytime soon?

Second is the time involved. Because of the very low thrust involved on the very large objects in question, we are going to need years on station. Are we likely to have this much time?

Third is that this is only applicable to rather small objects, up to maybe a hundred meters. These kinds of asteroids could rather more easily be swatted aside by nuclear detonations.

The point being that by the time we have these kinds of capabilities in place, we will already have the ability to deal with even much larger asteroids, and may even have moved several into local orbits. So, what's the point?

As for the other methods you mention, thermonuclear ablation seems to me to be the most promising technique. This is very similar to the nuclear pulse propulsion that was explored in the late 50s and called Orion. Furthermore, Orion type ships would be the only craft we could build right now that would have any hope of getting a large, manned mission off the ground and to such an asteroid as I have mentioned in my previous post with a large enough payload in time to do anything about it. And do keep in mind that for the foreseeable future at least, any such mission would have to depart from Earth surface. And I do say manned, because it seems likely we are going to need human minds on station to deal with the situation. Remote control and long delays could all to easily mean disaster.

As to thermonuclear vaporization, it sounds viable for smaller objects, but I wonder how big an object we could hope to successfully vaporize?

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#8
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Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/25/2010 10:39 PM

The 1908 Tunguska Event was an explosion in the 10-15 megaton range. This is comparable to the energy released by the Castle Bravo nuclear test. Castle Bravo was the largest above-ground nuclear device ever detonated by the United States.*

It is not known exactly what caused the Tunguska explosion. The general consensus is that the explosion was an air burst of either an asteroid or the rock/ice core of a dead comet roughly 30 meters in diameter. The air burst flattened 2000 km2 of forest and set trees ablaze as far away as 60 km.

If that same object exploded over a major city, it would kill at least one million people and quite possibly kill many more than that. It would be a major catastrophe.

A 30m diameter asteroid/comet core could easily be vaporized by the second method, or deflected by the first. The choice would depend on how close it is to Earth. Deflection of NEOs is applicable only when they are sufficiently far away to make this approach practical. Smaller NEOs are harder to see because of their small size, making their probable late discovery more likely to require the second approach.

* At 52 MT, Tsar Bomba was the largest weapon ever detonated by the Soviet Union - or by anyone else. See also here and here.

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#9
In reply to #8

Re: Bigger, Better Telescopes Needed to Find Near-Earth Asteroids

01/26/2010 12:04 AM

A very good point, and a Good Answer from me. The problem is that an incoming rock or snowball that small is damned hard to spot. Of course even just shattering something that small would go a long way towards mitigating it's effects.

What is really needed is a vastly expanded human presence in space, with something like the old NORAD radar net to keep an close eye on space out to a couple of light minutes, and maybe a somewhat less close eye all the way out to the Oort cloud and covering the entire electromagnetic spectrum.

As you say though, a rock or snowball that size could be easily dealt with if spotted with even a few days to impact. It's the really big ones, the dinosaur killers and bigger, that worry me. Something the size of Ceres could easily fracture the planet's crust, and that would be really, REALLY BAD.

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#10
In reply to #9

The Right Kind of Telescopes Needed to Find Near-Earth Asteroids

01/27/2010 12:25 AM

Fragmentation of smaller objects is certainly an option and could be used in conjunction with a variant of the second approach for mid-sized NEOs. In this scenario the NEO is not completely vaporized but is reduced to a more manageable size where it could be fragmented by subsequent TN detonations.

Opinion: The title of this thread is irksome in that it mirrors the same myoptic approach to NEO detection that I've seen again and again over the last 30 years. For some reason these folks stay stuck on the same approach and certainly one of the worst: detecting these objects in visible light. Bigger, Better Telescopes Needed indeed!

Not Bigger and Better telescopes; rather, The Right Kind of Telescopes and More of Them. Earth-Orbit and Deep-Space Wide-Field Deep-IR Telescopes. Ones like WISE.

NEOs appear very dark in the visible part of the electromagnetic spectrum. They have low albedos making them hard to detect against the backdrop of space. But not at DIR wavelengths (≥10μm).

Bigger and Better ground-based telescopes are not the answer. Worse (far worse, actually), these projects would siphon funds away from the very projects that would stand the best chance of detecting NEOs/NEAs.

As of January 23, 2010, 6692 Near-Earth Objects have been discovered. 1086 of these NEOs are asteroids with a diameter approximately 1 kilometer or larger. Also, 146 of these NEOs have been classified as Potentially Hazardous Asteroids (PHAs).

Most of these objects were discovered at visible wavelengths. You can just imagine what horrors they'll find when folks open their DIR eyes and take a look around.

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#11
In reply to #10

Re: The Right Kind of Telescopes Needed to Find Near-Earth Asteroids

01/27/2010 10:02 AM

Excellent point! Deep infra red imaging should show many more NEO objects since the light absorbed by their low albedo gets eventually converted to infrared radiation. Hopefully these extra terrestrial scopes will have a fine enough resolution to detect objects smaller than whatever it was in Tunguska.

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#12
In reply to #11

Re: The Right Kind of Telescopes Needed to Find Near-Earth Asteroids

01/27/2010 2:43 PM

Officially I've left CR4, but this subject is close to my heart and one I feel needs to be discussed.

On Jan 12 WISE (Wide-field Infrared Survey Explorer) ...

... discovered its first Near-Earth Object, never-before-seen 2010 AB78, barely two weeks after the telescope became operational. The 1 km asteroid was at a distance of 158 million km (98 million miles) in an orbit that poses no immediate threat to Earth.

2010 AB78 is the red dot in the center of this false-color image and in the one below.

WISE can "see" in four IR wavelengths: 3.4, 4.6, 12 and 22 microns (μm). The red in these images corresponds to a wavelength of 12 μm, with the green and blue corresponding to 4.6 μm- and 3.4 μm-wavelength infrared light, respectively.

WISE is expected to discover hundreds - if not thousands - of the estimated 100,000 or so undetected NEOs out there. More than half of these NEOs are estimated to be larger than the Tunguska object. And speaking of which ...

... many people seem to imagine that the Tunguska air burst was fairly stationary, not unlike a high-altitude TND detonation. Something like this, a 62 kiloton stationary asteroid exploding with an energy of 5 megatons at 5 km above the surface (the Tunguska explosion was around twice to three times as powerful). The movie window is 15 km wide and about 8 km high. Bright colors of the fireball indicate temperature, ranging from steam (dull red) to rock vapor (white). Gray background indicates air density and shows a spherical blast wave that reflects from the ground. The fireball rises buoyantly and cools as it recedes, limiting the thermal effects on the surface. This simulation shows what happens when momentum is ignored to simplify the problem as scientists have done in the past. The air burst is approximated by a "point source" explosion similar to a nuclear detonation.

Nothing could be further from the truth. This simulation shows what happens when momentum is not ignored, an approach that is allowed with modern supercomputers and codes. The same asteroid is now moving through the atmosphere at a typical impact velocity (20 km/s). For illustration purposes, extra energy is deposited into the asteroid when it reaches 5 km, for a total of 5 megatons. Momentum carries the hot fireball down to the surface, which enhances heat and wind effects on the ground.

(Imagine New York or London under this thing.)

Close-up of the previous simulation. The box dimensions are 4 km wide and 3 km high. The colors indicate the energy associated with vorticity, the swirling, tornado-like eddies generated by the downward motion. High velocity winds can be sustained at ground level by vortex flow.

3D simulation of a 15 megaton explosion that is initiated 18 km above the surface, for an asteroid entering at an angle of 35 degrees above the horizontal. Box dimensions are 40 km wide, 20 km high. Colors indicate speed. The hot fireball descends to the surface and slides downrange at high velocities, subjecting the landscape to blast-furnace conditions. This did not happen at Tunguska.

The Tunguska meteorite was chicken feed compared to the rock that killed off three-quarters of Earth's species some 65 million years ago. That one was over 330 times larger and thirty-seven million times more massive than the Tunguska object. We can't even conceive of a nuclear bomb that big, with that kind of power. Thirty seconds after impact, every single forest and grassland from the Yucatan Peninsula northward to what would someday be called the US-Canadian border was a blazing inferno, as were the lands from the impact point south, to northern Argentina. An Extinction-Level event.

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