Alexander Zaitsev’s latest contribution to the debate over sending messages to the stars is a short paper that looks at how visible our planet might be thanks to transmissions from planetary radars like Arecibo, Goldstone or the Evpatoria site from which directed transmissions have already been sent. METI (Messaging to Extra-Terrestrial Intelligence) is broadly dedicated to transmitting messages to stars likely to have habitable planets, but so far the number of transmissions is relatively sparse. The debate over METI discusses the wisdom of continuing them without broader discussion.

But tucked within that debate is the specific question of our civilization’s visibility. For in addition to the messages that have already been sent, beginning with the Arecibo message in 1974 and continuing in the far more targeted transmissions from Evpatoria between 1999 and 2003, we are using our planetary radars to perform crucial astronomical studies. The work these dishes do in refining our knowledge of potentially dangerous asteroids is too significant to stop, but Zaitsev (Institute of Radio Engineering and Electronics, Russia) argues that their radar signature is far more obvious than the METI messages.

The argument goes like this: Roughly 1400 sets of radar transmissions have been produced from the three sites, with a distribution covering a broad swath of sky compared to the small number (16) of METI transmissions, which covered an area 2000 times smaller. Zaitsev then factors in the total duration of the radar transmissions, which exceeds the METI broadcasts by a factor of 500. He concludes that the radar work on asteroids and other objects is one million times more likely to be detectable than the signals sent as communications to other stars.

If this is the case, then making our civilization less visible involves shutting down activities necessary for planetary protection, an obviously dangerous move. But we still have a problem of degrees. A directed signal sent to a star with potentially habitable planets brightens our planetary signature significantly to receivers near that star, a place pre-selected for its astrobiological interest. The fleeting pulses of planetary radar transmissions covering broad areas of sky should likewise be detectable, but I find it hard to agree that this kind of transmission is at the same order of visibility, precisely because it is far more widely and randomly dispersed. [Addendum: See Dr. Zaitsev’s comment below re my misuse of the term ‘pulses’ in this context. These are not pulsed systems.]

The more we learn about how our activities might be detected elsewhere, the better, and I think that raising the issue of ongoing radar transmissions is completely valid. Similarly, we have much to learn about how the radio and television transmissions of the past century might or might not be receivable at various distances. The METI debate takes place in a context of apparent degrees of visibility, and questions whether making a specific attempt to raise that visibility to carefully chosen targets is wise. Settling that debate should involve not just astronomers and physicists but a broad spectrum of informed opinion. It is a debate that Dr. Zaitsev is fully engaged in, but one that most media outlets (with striking exceptions like SEED Magazine) have chosen to ignore.

The paper is Zaitsev, “Detection Probability of Terrestrial Radio Signals by a Hostile Super-civilization,” available online.