[Vertigo]

Some interesting responses there and Agent to the Stars is one of the books that got me thinking about this.

First I don't think they would know much of anything about us before they arrived. Stuff I have read reckons that even in our heyday of undirected radio signals those signals would only have been detectable up to about a light year away. More recently we have transmitted much more efficiently and I doubt our presence could be detected even that close. The only transmissions likely to have reached across interstallar space would be things like military radar but what are the chance of such a thing happening to point in just the right direction to catch another inhabited planet? And even if it did, it would move off them again in seconds (microseconds?).

So I think it unlikely my aliens had foreknowledge of our presence.

Now could they arrive undetected? If they think the solar system is a possible haven of life and if maybe prior experience has taught them that first contact could be fraught, they might try to arrive "quietly". Apart from anything else if the life (us) could see them well ahead of time then they might make militaty preparations and everything could go pear shaped. How could they do this? If they have time (and interstellar travel without FTL kind of presumes that) then I think it might be possible, though I don't know enough about astrogation to be sure.

The way I figure it, to be efficient a ships drive would be very focused sending out whatever drive exhaust it uses in a very tight beam. So if you plotted your course so that you are "missing" the system whilst decelerating, you should be able to take up a remote comet like orbit around the system without ever actually pointing your drive directly at it. From there it should be easy to use much the same tactic to move in closer. You will of course be detected eventually but not before you are much closer.

However here's another idea. Just before you start decelerating you fire a flyby probe at the system. No drive would be necessary as it has all your existing velocity. Send another one later maybe after you've killed some speed. The probe(s) would simply fly by and transmit as much information as they can gather as they pass. Being small and with no drive they are very unllikely to be detected.

Do you stop at the Moon, or better yet Mars and wait for the natives to calm down?

I like that idea Metryq and I really can't thing of anything much better. Stay well back, present as little threat as possible and talk fast

Rather than a voice message they could send something in code, so complex that only the experts could decode it and then take it to the authorities.

Again I quite like that idea, maybe in combination with Metryq's above. Except it still doesn't solve the problem, it only delays it. At some point the masses have to be told before it leaks out in an uncontrolled manner.

It's only fair though. We'd do it to any planet we ever managed to discover.

Maybe that's the point. If we ever go out there these are the decisions we might have to take.