The damn thing is layered like a frozen onion.

When we see comets up in the sky they're really spectacular. But unless you get close to a comet, you can't really figure out what's going on.

We believe they hold the most primitive materials in the solar system and that they played a role in shaping some of the planets, but we really have more ideas about comets than facts.

For example, some boulders seem to have just disintegrated on the surface. We've also seen that some of the fine surface material moves downhill, filling low areas and creating flat surfaces in craters, even with Eros' low gravity. These are big puzzles and we need to get a better look.

To understand something sufficiently well to be able to program it for a computer does not mean to understand it to its ultimate depth. There can be no such ultimate understanding in practical affairs. Programming is rather a test of understanding. In this respect it is like writing; often when we think we understand something and attempt to write about it, our very act of composition reveals our lack of understanding even to ourselves. Our pen writes the word 'because' and suddenly stops. We thought we understood the 'why' of something, but discover that we don't. We begin a sentence with 'obviously,' and then see that what we meant to write is not obvious at all. Sometimes we connect two clauses with the word 'therefore,' only to then see that our chain of reasoning is defective. Programming is like that. It is, after all, writing, too. But in ordinary writing we sometimes obscure our lack of understanding, our failures in logic, by unwittingly appealing to the immense flexibility of a natural language and to its inherent ambiguity... An interpreter of programming-language-texts, a computer, is immune to the seductive influence of mere eloquence... A computer is a merciless critic.

To understand something sufficiently well to be able to program it for a computer does not mean to understand it to its ultimate depth. There can be no such ultimate understanding in practical affairs. Programming is rather a test of understanding. In this respect it is like writing; often when we think we understand something and attempt to write about it, our very act of composition reveals our lack of understanding even to ourselves. Our pen writes the word 'because' and suddenly stops. We thought we understood the 'why' of something, but discover that we don't. We begin a sentence with 'obviously,' and then see that what we meant to write is not obvious at all. Sometimes we connect two clauses with the word 'therefore,' only to then see that our chain of reasoning is defective. Programming is like that. It is, after all, writing, too. But in ordinary writing we sometimes obscure our lack of understanding, our failures in logic, by unwittingly appealing to the immense flexibility of a natural language and to its inherent ambiguity... An interpreter of of programming-language-texts, a computer, is immune to the seductive influence of mere eloquence... A computer is a merciless critic.