Ask Dr. SETI ®

Ed, Alaska

The Doctor Responds:
We're always in search of interesting "magic frequencies," Ed. X-band is certainly a good part of the spectrum to monitor for intelligently generated extraterrestrial signals, and the equipment you describe could well be used for SETI, as well as for monitoring our various space probes. And, trying to second-guess ETI as to the frequencies on which they might be transmitting could well prove productive. But, do I expect ETI to be able to detect the downlinks from our space probes? Not really.

Spacecraft power budgets are extremely limited, so the downlinks use the lowest possible transmitter power, with high-gain antennas pointed back toward Earth to make up the link margin. I highly doubt that these signals can be detected over interstellar distances. Thus, listening on those frequencies for extraterrestrial replies to our spacecraft would probably be an exercise in futility.

On the other hand, those same spacecraft receive uplink telecommands from Earth, and the NASA Deep Space Network uses very powerful transmitters to send those commands. Some of those uplink signals are also in X-band, in the vicinity of 8.6 GHz, and it is conceivable that they are powerful enough to be detected by advanced extraterrestrial SETI scientists. So, on the offchance that ETI might reply on the same frequency it receives, it certainly couldn't hurt to monitor the DSN uplink frequencies, at least occasionally.