Let me start with #2. It means "Are we able to stop an asteroid?" It doesn't depend on a change of circumstances. Now #1 can either be the past tense of that -- i.e. "were we able to stop an asteroid?" -- or it's a more tentative or conditional expression of it, where we are hinting that we know er aren't able to stop it if circumstances stay the same, but it may become possible to do so if circumstances change -- we develop new technology, for example. Now in this particular example, the simple past interpretation doesn't really seem very likely (have you tried to stop any asteroids recently?), so the question is more likely to be inviting a reply where someone tells us about those new technologies. 1. "Could" has two meanings -- it's either the past tense of

COULD is the past tense of CAN. Put simply: "Could we stop an asteroid?" -- means "WERE we able to stop asteroid in the past?" "Can we stop an asteroid?" -- means "Are we able to stop an asteroid NOW?"