Sky & Telescope - August 2022 - 38

NIROSETI: © LAURIE HATCH; DYSON SPHERE CONCEPT: KEVIN GILL / CC BY
2.0; BREAKTHROUGH STARSHOT: M. WEISS / CENTER FOR ASTROPHYSICS
The Search Continues
flashes from other civilizations go
back decades, those largely relied on
focusing light onto a photomultiplier
tube - the equivalent of a single-pixel
camera. Last year, Gillum instead
began installing a low-cost camerabased
system named LaserSETI for
what he calls the first " all-sky-allthe-time "
search. He designed the
instruments to seek short pulses of
coherent light like those that James
Benford envisioned. With it, he could
capture the light from a propulsion
beam as it sweeps across the sky, as
both the beam and Earth move.
To detect such technosignatures, Gillum points two pairs
of wide-angle cameras at the same area of sky from widely
separated spots. Each camera has a 75° field of view - nearly
twice the height of Ursa Major - and is set at right angles to
its partner. The initial observations have had one pair looking
west from California and the other looking east from
Hawai'i. The system, which uses transmission gratings to
obtain spectra, looks for sources displaying only a narrow
range of wavelengths, as you'd expect from a laser. A burst
of laser-like light would leave a telltale signal in the data collected
by the camera pair.
" Breakthrough Starshot is our ideal scenario, " Gillum says,
adding that a civilization using laser-powered satellites could
be visible from far away. " Those lasers are extremely bright,
NIROSETI The near-infrared/optical
instrument mounted on the Nickel 1-meter
telescope at Lick Observatory didn't find
nanosecond-long pulses expected from faroff
lasers, but its field of view was small.
and [detection] does not require anyone
trying to communicate. "
With two pairs of cameras monitoring
the same region of sky simultaneously,
combining data from the
two can filter out errant detections.
Observing from two locations also
provides the source's parallax; Gillum
says astronomers should be able to
measure distances up to 25 times the
distance of the Moon, so the system will not mistake a terrestrial
spacecraft for E.T. In the long term, Gillum hopes to
monitor the sky from about 70 sites.
Expanding the View
Another new thrust in search efforts is the doubling of the
spectral range under study. SETI has long relied on silicon
CCD detectors, which can record infrared light out to about
1,000 nanometers. Longer infrared wavelengths pass more
successfully through interstellar dust than shorter wavelengths
do; however, detecting them requires camera chips
made from more exotic semiconductors, which have historically
been beyond limited SETI budgets. More recently,
though, mass production of indium-gallium-arsenide
MEGASTRUCTURE The late futurist Freeman Dyson imagined that
alien civilizations might need to utilize more energy from their host stars
than we do. Such energy usage could ultimately come in the form of
a " Dyson sphere, " shown here in an artist's concept. Such a structure
would block some or all of the visible light from the central star, but it
would heat up and re-radiate that energy at infrared wavelengths.
38 AUGUST 2022 * SKY & TELESCOPE
STARSHOT Lasers on Earth shining on light sails could power small
spacecraft on interstellar journeys, as imagined here for the project
Breakthrough Starshot. Alien civilizations might have thought of similar
technology, from which we could detect laser " leaks. " It's even possible
that extraterrestrials might power such laser beacons as a means to
communicate intentionally.
Sky & Telescope - August 2022

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