First optical telescope dedicated to SETI begins operation

Almost 50 years ago, Philip Morrison and Giuseppe Cocconi famously suggested that the most likely electromagnetic wavelength at which an advanced alien civilization would try to signal its existence is the ubiquitous 21-cm microwave emission line of neutral atomic hydrogen. And until recently, the search for extraterrestrial intelligence (SETI) has been conducted almost exclusively by radio astronomers sifting through 21-cm radiation from space in search of anything that might be a message.

Half a century ago, the 21-cm line was the only known interstellar microwave emission line, and lasers had not yet been developed. Since then, radio astronomers have found many shorter-wavelength microwave lines that would serve better because they suffer less interstellar dispersion. Furthermore, today’s laser technology makes it possible for petawatt (10¹⁵ W) lasers to emit highly collimated nanosecond optical pulses that briefly outshine the Sun by a factor of 10 000. Because no known astrophysical source could put out a bright nanosecond optical pulse, some SETI searchers have concluded that looking for signals from technologically advanced aliens is more promising with optical telescopes than with radio telescopes.

The optical search for extraterrestrial intelligence—called OSETI—now has its first dedicated telescope. Completed in April and already in operation, the Planetary Society’s Optical SETI Telescope at the Oak Ridge Observatory in Massachusetts is directed by Harvard University physicist Paul Horowitz. Its 1.8-m primary mirror is, despite its modest size, the largest of any US optical telescope east of the Mississippi.

Horowitz and his OSETI collaborators had been observing parasitically on the observatory’s 1.5-m Wyeth Telescope for six years. In that time they searched some 5000 sunlike stars within 1000 light-years of Earth for bright nanosecond pulses (see A. Howard et al., Astrophys. J., 613, 1270, 2004 http://dx.doi.org/10.1086/423300 ). “With the new instrument, we exceeded that total in the first few hours,” says Horowitz. That’s primarily because the new telescope, optimized for OSETI, has a much larger field of view, monitored by an array of about a thousand ultrahigh-speed photodetectors. The group expects to cover the entire celestial sphere (except for far southern precincts inaccessible at the latitude of Massachusetts) within a year or two. The telescope monitors any one star for about a minute as Earth’s rotation takes it across the field of view.

The project’s cost, less than $400 000, was borne mostly by the Planetary Society. “It’s so inexpensive,” says Horowitz, “because all we need for OSETI is an efficient photon bucket.” The telescope’s two-arcminute angular resolution is no better than that of the human eye. But it suffices for the purpose. And one is not looking for spectroscopic precision or for the kind of phase-encoded messaging one seeks with radio-telescope heterodyning.

How could a petawatt laser call attention to itself over interstellar distances by outshining the Sun? Current optical technology can collimate the output of a petawatt laser at the top of the atmosphere into a beam whose angular spread Δθ is only 10⁻⁷ radians. All of the pulse’s power is therefore concentrated on (Δθ)²/4π, or 1/10¹⁵, of the celestial sphere. Distant observers who happen to be within the spread of that beam would see a nanosecond pulse much brighter than our parent star. But observers outside the beam would see no pulse. Presumably, thinks Horowitz, an alien civilization seeking contact would direct such a pulsed beam at one promising star system after another. The beam could be made wide enough to cover a diameter of many Earth orbits.

“If we find nanosecond pulses, we can’t lose,” says Horowitz. “If it’s not from an alien civilization, at least we will have discovered an astrophysical phenomenon that no one anticipated. Not a bad consolation prize.”