There was a general murmur of assent, so Weichart went on:

“Now let’s suppose that the cloud is moving sideways, as well as towards us and let’s suppose that the motion sideways is about as fast as the motion towards us. Then the cloud will move about like this. Now if you consider the motion from A to B you’ll see that there are two effects — the cloud will seem bigger at B than it was at A, exactly as in the previous case, but now the centre will have moved. And it will move through the angle AEB which must be something of the order of thirty degrees.”

“I don’t think the centre has moved through an angle of more than a quarter of a degree,” remarked Marlowe.

“Then the sideways motion can’t be more than about one per cent of the motion towards us. It looks as though the cloud is heading towards the solar system like a bullet at a target.”

“You mean, Dave, that there’s no chance of the cloud missing the solar system, of it being a near-miss, let us say?”

“On the facts as they’ve been given to us that cloud is going to score a bull’s eye, plumb in the middle of the target. Remember that it’s already two and a half degrees in diameter. The transverse velocity would have to be as much as ten per cent or so of the radial velocity if it were to miss us. And that would imply a far greater angular motion of the centre than Dr Marlowe says has taken place. The other question I’d like to ask is, why wasn’t the cloud detected sooner? I don’t want to be rude about it, but it seems very surprising that it wasn’t picked up quite a while ago, say ten years ago.”

“That of course was the first thing that sprang to my mind,” answered Marlowe.