One thing I've been wondering for a while, does the United Nations or any authority have plans to deal with the appearance of an alien fleet in the solar system? (Real aliens, not the UFO/conspiracy nonsense!) I know it's a very remote possibility, but surely some government has made plans just in case? (I did look around the UN site, but there is nothing.) I wonder what the procedure would be if alien spaceships were detected - would the public be told, or would the news be kept secret to avoid massive panic?
Depends who detects them, how they detect them, what the aliens do, etc.
In all likelyhood, if aliens do exist and actually manage to reach the solar system, and if the first group to spot them kept it quiet, they would soon be detected by another group, which might or might not make it public. If that group kept quiet, there would be a third, etc.
Furthermore, you assume that the UN or some government would be the first to spot the aliens, but more than likely it would be some group of astronomers or other, in which case they would likely make their findings known as soon as they were sure of what they were seeing. The government might not hear about it until it appeared as breaking news on CNN.
Even if a government was first to detect it, there would be great incentive not to keep it quiet, since giving other governments time to spot the aliens and claim discovery might lead to them getting PR that could otherwise have been yours.
If the aliens were trying to communicate they might be hard to ignore. Doubly so if they attack. In fact, if they're capable of getting here in the first place, their ships would be pretty hard to miss. If they get here in a way possible with known physics (ie, slower than light), they would have to be really clipping along (as in a good fraction of c) to get here in any reasonable amount of time. Unless they were just flying by, they'd have to shed most of that velocity in order to enter solar orbit, and all that kinetic energy would have to go somewhere. Their decelleration burn would be flat out impossible to miss.
If they get here by faster than light it's hard to say exactly how energetic their arrival would be, but assuming that FTL travel is possible it would have to conserve energy. So they'd have to make up for differences in speed, gravitational potential, etc. between the point they started out from and the point they wanted to get to (if they're jumping in from a star with a relative velocity of 100 km/s relative to the sun, they have to come up with the energy to change their velocity by 100 km/s somehow). Assuming that their FTL gizmo is less than 100% efficient, some energy is going to be wasted in the process of supplying the energy to equilize velocities. For a ship of any great size, the energy release would probably be fairly easy to see.
