RENEE MONTAGNE, host:
Britain says the 21 people arrested in an alleged plot to blow up planes include the main suspects behind the planning.
Britain has cancelled all flights today between London's Heathrow Airport and points in Britain, Europe, and Libya.
In this country, Homeland Security Secretary Michael Chertoff confirmed at a news conference this morning that this plot involved liquid explosives, apparently to be carried aboard aircraft, possibly a separate component to be put together aboard the plane.
Joining us now to discuss liquid explosives is NPR science correspondent Jon Hamilton.
JON HAMILTON reporting:
MONTAGNE: And Jon, why would a terrorist want to use liquid explosives? I mean, I can figure out that it's easier to get on the plane at this moment, but that and what else?
HAMILTON: Well, they're relatively easy to make or to buy. You can make them out of household items like acetone, which is really just nail polish remover and hair bleach, which is hydrogen peroxide. And they are very hard to detect, as we talked about before. And they can be very powerful explosives. It doesn't take a lot to make a very big bang.
MONTAGNE: And how do you set these things off, and, I guess, intentionally rather than accidentally?
HAMILTON: Well, I was going to say, the biggest problem is how not to set them off. The explosive that people are probably most familiar with is nitroglycerine, and Alfred Nobel, when he was working on discovering this, nearly blew himself up. And that's been the entire history of liquid explosives, is that once they are put together, they can be set off by almost anything.
And so, once they're on a plane, for instance, you could set them off with a little bit of electricity as has been tried. A couple of nine volt batteries would do it. Friction, shock, heat, and in some cases, even light. So setting these things off is not a problem.
MONTAGNE: How about bringing down a big passenger airliner? How much liquid explosive would one need?
HAMILTON: Well, certainly, it depends on which liquid explosive you're talking about. Nitroglycerine, for instance, is very powerful and if you think about the amount of nitroglycerine that you could put in, say, a modest-size contact lens solution container, that would be enough blow a very big hole in an airplane. And of course, you don't have to blow up the whole airplane to bring an airplane down.
For instance, if you set off a relatively small explosion, but it happens to be near the fuel tanks, you can get a much bigger explosion. And so, it really depends on where it is and how much goes off, but as far as finding enough of this stuff to - or getting enough of this stuff on an airplane to bring down an airliner, it's not a whole lot.
MONTAGNE: Now, we haven't heard this morning any announcement of what type of liquid explosive exactly these suspects were suspected of using, but what would you guess or what would you know?
HAMILTON: Well, we have two things to go on. One is history, and in the past, al-Qaida has favored nitroglycerine. There's a plot that took place in the Philippines that sounds a little similar to this one that involved the use of nitroglycerine.
However, some of the remarks that Chertoff made suggest that maybe it was something a little more complicated. He suggested that perhaps there was a plan to mix elements together on the plane. That would solve the problem of the explosive being so volatile. The problem with these is once they're mixed, they tend to go off.
So if you - they would be easier to detect if it weren't mixed until you got on the plane, and that opens up a whole range of liquid explosives and we just don't know which one at this point.
MONTAGNE: Thanks, Jon.
HAMILTON: You're welcome.
MONTAGNE: NPR's science correspondent, Jon Hamilton.
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