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detect some of these extremely small leaks.

The problem is the leaks aren't big. They're small. Even a small leak is not acceptable, so they're just having a bear of a time trying to find the leaks. They actually found one and thought they had solved the problem. They got back on the launch pad, loaded the hydrogen in and discovered that they had, not one, but two, and they had only found one of them. So, they had to get off the launch pad again. Hopefully, they'll find the leaks in Columbia, but they haven't found them yet. And it's not from lack of trying.

Question: Are the Shuttle's main engines, which are basically liquid oxygen and liquid hydrogen engines, safer than the solid rockets, because you can shut off the shuttle engines and you can't shut off the solids?

That argument has had a lot of attention, as you might imagine. There are things to be said on both sides.

First, let me say that minutes after the Challenger accident, if you had taken a poll of what had gone wrong among the astronauts, and probably everyone at the Johnson Space Center, 95 percent would have said the main engines. They wouldn't have said the solid rockets, because the main engines are state-of-the-art. They are very delicate pieces of machinery. They are real high-tech and difficult to get to work right.

It's true that you can shut them off, and it's true that you can't shut off the solids, but the main engines have lots of rotating machinery, hydrogen pumps, oxygen pumps, etc. Lots of things that can go wrong, including things that sound as simple as just cracked blades that would be catastrophic. So the Shuttle's main engines are one of the things that received the most attention before the shuttle flights, because they were considered to be one of the most technically challenging parts of the Shuttle.

The solid rockets, on the other hand, are easy to understand. You light it and it burns. There are no moving parts. It just burns and when it's done burning, it stops burning. People had actually assumed that because of that, solids were safer. The solid material you can, to a large extent, transport. It's fairly safe. It's not as volatile once it's been cured and it's in the solid form. You have to drop it from high places to get it to explode, as people have demonstrated recently. It was considered to be pretty stable stuff and safer. Well, we've proved that that's not right.

But I don't think it's an easy case to make, even though it's true that you can't turn off the solid rockets, and you're stuck with them as long as they're burning. That essentially means that if anything goes wrong with the solid rockets, there's literally nothing you can do. We don't know how to build an escape system that would get astronauts safely away from burning solid rockets. It's not at all clear that you'd be better off if you jumped to the solid rockets and went to a liquid propulsion system.