Light echoes

Occasionally some astronomical event — a gamma-ray burst, a supernova, a magnetar flare, or whatever — will go off, lighting up the sky more or less spectacularly and then fading. Normally that's the end of that event, and we're left to wait for the next one. (In particular, this means that if you study this sort of thing, you need to be prepared for one of these things to happen at any time, so that you suddenly need to write target-of-opportunity proposals, analyze data, and release preliminary results in a tearing hurry. This typically happens while you're supposed to be on vacation.) Once in a while, though, we see a peculiar phenomenon called "light echoes".

A "light echo" arises a little like an ordinary (sound) echo: some event happens producing a bright flash (loud noise) and in addition to the light (sound) making its way to you directly, some of the light (sound) goes in a different direction, bounces off something, and makes its way from the other object to you, arriving a little later. From the place I usually stand to watch the summer fireworks competition, you hear the big skyrockets go off, then a second or two later you hear the echo from a nearby building. Of course, on human time and distance scales, the light from the fireworks reaches us instantaneously, so it's obvious that both the original sound and its echo are delayed. In an astronomical setting, we only receive light, and it takes very much longer. But it's still possible to receive a delayed echo, and studying these echoes can be very informative.

(Photo, courtesy of ESA, to the right is X-ray dust echoes around the magnetar 1E 1547.0-5408, one of the objects people in the group here at McGill study. This interesting dust-echo work is from another group, though. The echoes are from a massive X-ray outburst, which we think was caused when the extremely strong internal magnetic field stresses cracked and twisted a piece of its crust; this twisted the external magnetic field, and the twisted magnetic field produced and accelerated massive numbers of electrons and positrons, which blasted out a torrent of X-rays. At least we think that's how it happened; we saw the torrent of X-rays.)

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From the arxiv: The Potato Radius: a Lower Minimum Size for Dwarf Planets, by Lineweaver and Norman.

This article is kind of neat, not least for the charming way it uses "potato" (and its adjectival variant "potatoid") as a technical term. The point of it is to try to work out how big an object has to be for it to be round.

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From the arxiv: Momentum Transfer by Laser Ablation of Irregularly Shaped Space Debris, by Liedahl et al.

I don't have a whole lot to say about this one, but how could I ignore a paper about Giant Space Lasers? They're talking about using them to clean some of the junk out of low Earth orbit. While it would take a great deal of power to completely vaporize space junk, all you really need to do is give it enough of a shove (~100-200 m/s) that it starts to dip into the atmosphere (~200 km altitude), where it will slow down and burn up. Just how much of a shove you can get by zapping it with a laser so that some evaporates is not easy to predict, hence the paper.

The particular kind of Giant Space Laser they're talking about is left to some degree unspecified, but it's clear that you want short pulses, so that you get explosive vaporization (gas flow velocity of ~1000 m/s) rather than gentle heating, and they're talking about 10 J pulses (producing 0.1-1 m/s change in velocity for a 1 g target). So it's an awful lot of short powerful pulses. They also mention, in the usual understated scientific way, the possibility of "structural modification" of the debris — that is, the possibility that the bolt or paint flake or whatever will be blown to pieces or bent out of shape by the laser (in addition to the ~10% of the mass that will be outright vaporized). They suggest laboratory experimentation, to which I say, can I help zap random pieces of junk with a high-powered laser and see what happens? Please?

(Photo to the right is of a NASA laser ranging experiment, not actually zapping space debris. Unfortunately.)

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Space Race

In honour of Yuri's Night, I'd like to point to this rather nice BBC documentary:

It casts the space race as a personal competition between Sergei Korolyov and Werner von Braun, and is mostly recreations and dramatizations (including the peculiar decision to have the Russians speak English with a Russian accent, except for commands like "Ignition" and "Launch", which the actors could presumably learn and are subtitled). But it's generally well made, and doesn't shy away from either von Braun's Nazi past or the military applications that drove the space race itself: a rocket that could put a man into orbit could also deliver a warhead to a major city, and vice versa. From a public relations point of view, announcing a new milestone in the space race generally went over better than announcing one's ability to more effectively kill millions of people.

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