I was watching firefly, and I couldn’t help wondering - they seem to be able to land on pretty much any planet without any alarms being raised. So, given currently deployed technology, how likely is it that anyone could land a space-ship on earth without being detected?
Stealth/cloaking devices or similar don’t count, and you’d need to have a reasonably slow speed to be able to land (no instantanious transporter devices or worm-holes).
Just to make my question clearer: I mean detected as in radar screens / observatories. If you land in the middle of manhattan, somebody’s going to spot you eventually 
You could probably land in the wilderness far away from big cities or military installations. Perhaps in a South American jungle or African desert.
You mean there’s a pretty small chance of being spotted by satellites looking for space-debris or ICBMs if you land out of the way? I was thinking of Antarctica myself.
If they use stealth technology capable of fooling our primitive little RADAR then they could land anywehere there is not any people.: Northern Canada, Alaska, the middle of the Sahara Desert, Antarctica, etc. There are really a whole lot of places they could land. You could probably even find some good ones in the lower United States.
The best way would be for the spaceship to resemble (on radar of course) something quite regular or normal. Probably some kind of meteorite. I guess to slow might mean more radar time… to fast might get too much attention. Size certainly would have to be smallish… hitching a ride on the Space Shuttle downside might make entry easy.
Yeah, but “stealth technology” isn’t just something you paint on. It’s a whole range of technologies incorporating shape changes, coatings, and so on. The degree of specialization required to make something “stealth” usually makes it unsuitable for reentry. Likewise, “heat shielding for reentry” isn’t something you can staple onto the outside of a design. You need the structure to absorb heat and the skin to ablate at the right rate. You also need some sort of control system (if manned) that will keep it from augering in at the end of its flight. ICBM reentry vehicles enter the atmosphere at Mach 20+ (but at those altitudes Mach numbers are sort of irrelevant) and impact at well over the speed of sound. Stealth aircraft can exceed the speed of sound but generally don’t do so.
Basically, if you want to go unnoticed, you’re going to have to stay away from any kind of specialized ballistic missile warning radars, which means landing in the southern hemisphere. I guess you could try to develop something that was able to reenter, but also able to avoid detection – but we don’t have any such thing yet, or it would be on every ICBM currently fielded (chaff and decoys don’t avoid detection, just identification).
You also want to avoid detection by geostationary satellites with infrared sensors. Because many of those sensors are specifically designed to distinguish man-made (sorry, “anthropogenic”) objects from other false positives, you need something that has the same spectral signature as a natural object.
I think your best bet is to land a retrofitting craft on a large asteroid, and catch a smaller asteroid in passing to off-load your exploration team. Core it out like an apple, and place a small Apollo-style capsule inside with the following subsystems:
Use your larger spacecraft to accelerate the meteor towards the “right spot” on earth, detaching once you get too close to earth to risk detection by NASA &co. – from here on out, the meteor-craft plummets on its own with minor course corrections. It enters the atmosphere just like a meteor, and once you get below a certain altitude, you deploy the parachute and fire the explosive charges. The rocks continue on their way and your craft pops free of the debris. You land unnoticed by all, unless your perfect landing zone happens to be someone’s Saharan village.