Ask Dr. SETI ®

A Student (via email)

The Doctor Responds:
There's no easy answer to that question, because the Arecibo radio telescope is only one half (the receive part) of a communications link. The transmit side of the link is the particular signal being observed, and its strength will determine range. If I asked you at what distance you could see a light, wouldn't you ask whether I meant a flashlight or a searchlight?

For extremely powerful emitters (such as quasars) it should be possible for Arecibo to receive over many billions of light years. Weaker emitters (say, interstellar hydrogen clouds) are detectable over hundreds to thousands of light years.

Frank Drake once calculated that as a telecommunications facility, Arecibo could communicate with its theoretical twin anywhere in the Milky Way galaxy. That 100,000 light year range seems overly optimistic to me. When I ran the numbers, I came up with a more modest result of perhaps 10,000 LY (still a substantial distance).

A few years ago, the Arecibo radio observatory was able to recover the ten-Watt microwave beacon aboard the Pioneer 10 spacecraft, out beyond the orbit of Pluto. Today, however, that particular signal from beyond the edge of our solar system is no longer detectable at Arecibo, or anywhere else on Earth.

Used as a planetary radar, Arecibo has mapped the Moon and Venus, but not the outer planets. So take your pick as to the nature of the source, and that will help you to determine Arecibo's range.