The subject here is a scientific mystery that was well documented by European scientists in the 1920s and 1930s, well before any man made objects were launched into outer space, and then was largely forgotten. The phenomenon has since apparently disappeared, although anecdotal reports still pop up from time to time, and a low level of interest persists. We are discussing it here, because although no one knows for sure what caused the Long Delay Echoes, one plausible explanation is
Bracewell Probes. The Long Delay Echoes (LDEs) just possibly might be a clue to how to find such a probe.
Some Basic Background You may Wish to Skip
I'm sure you know that radio waves generally travel at the speed of light, and can be bounced off of various surfaces and also off the Earth's ionosphere if the radio wavelength is long enough. The speed of radio waves can be a bit slower if they are traveling through a medium such as a plasma or water, but generally the speed is not much less than the 300,000 kilometers per second we are used to. That means, if we send a radio signal out, and see it come back to us, then to calculate the distance to the reflector, we divide the time delay in seconds by 2 (since it went out and back), and then multiply by 300,000 kilometers per second to get the distance. Then, if we send out a radio signal and it comes back 2 seconds later, it must have traveled 300,000 km each way, or most of the way to the Moon. Amateur radio operators often enjoy bouncing their signals off the moon, which generally exhibits a very weak echo delayed about 2 and one half seconds. Strong echoes, or echoes delayed longer than 2.5 seconds, are not moon echoes.
Since the 1960s, there are many radio repeaters in Geosynchronous orbit, but the round trip delay is much shorter than the moon - less than half a second, and you generally won't see a satellite echo at all unless your signal parameters are just right - and probably illegal.
