NASA Aircraft Videos Hayabusa Re-Entry

astroengine writes "Flying above the Australian Outback, NASA's converted DC-8 jet videoed the violent re-entry of the Japanese Hayabusa spacecraft. Flying in front of the disintegrating probe, the mission's sample return capsule can be seen speeding though the atmosphere. According to reports, the capsule landed safely and will be collected by helicopter in the morning." "Bad Astronomer" Phil Plait posts about the successful return as well.

Re:Good point by the Bad Astronomer

[AK Marc]

Well, one reason it doesn't make sense is that because it's wrong. When it hits the ground, a lot of the energy is "lost" in forms other than heating the atmosphere. It heats the rock, breaks the rock, imparts kinetic energy to the rock, and such.

What is true is that the energy in a single large meteor is sufficient to make the temperature of the air unbreathable over a large area. However, as you've noted, having the energy to do so and actually doing so are two separate ideas. I've not seen anything that went beyond "it could" into the mechanisms by which it would. Or, to say it another way, which would be more disruptive to the planet, a single piece of lead of 10,000 tons slamming into the earth, or 10,000 tons of feathers entering the atmosphere at the same point and with the same energy as the lead? Sure, they have the same energy, but you tell me which will cause more damage and why. No one knows, but he asserted the feathers would be more damaging, and you said something to the effect of "I don't believe you." I think I'd lean towards your side, but again, I don't think there's a genuine analysis of why and how that difference would happen.

Re:Good point by the Bad Astronomer

[AJWM]

What is going to couple the energy transfer from asteroid to atmosphere more effectively, one big rock or a bunch of little ones with the same net energy? The little ones, of course.

Now, the big rock will make a much bigger splash when it hits, creating quite the fireball -- which is a good thing, since heat radiates proportionally to the fourth power of the temperature difference, so a hot fireball will lose its energy to space a lot faster than a lot of smaller, cooler (but still hot!) ones. Yeah, the immediate vicinity is toast, but a lot of the energy is just, in effect, melting the ashes rather than heating up someplace else.

Or to put it another way (no, not a car analogy): would you rather have your whole body exposed to a temperature of say 400 degrees or one finger exposed to say 10,000? Hurts like hell either way, but the former kills you, the latter doesn't.