Sky & Telescope - November 2021 - 31

NASA successfully demonstrated laser communications
from lunar orbit in 2013, achieving a downlink speed of
622 Mbps. Other experiments have followed. The European
Space Agency now uses optical links to connect some of its
low-orbit satellites with two geostationary relays (which then
send the data down to Earth via radio), and the Starlink project
has begun using inter-satellite lasers, too.
NASA's Laser Communications Relay Demonstration
(LCRD) aims to be the first relay system based entirely on
optical. From geosynchronous orbit, it will transmit data sent
from the International Space Station at a rate of 1.2 Gbps,
about double that of the 2013 lunar experiment. LCRD's June
2021 launch was delayed due to rocket concerns, however,
and no revised launch date had been announced when this
article went to press.
But explorers want to push beyond near-Earth space. Slated
to launch in 2022, the Deep Space Optical Communications
(DSOC) experiment will piggyback on NASA's Psyche asteroid
mission to test how well laser links work beyond Mars. Using
a setup similar to an off-axis telescope, DSOC will point a
micron-wavelength laser at a 22-cm mirror and shoot it back
to Earth, transmitting canned data that team members will
check for accuracy. (They'll lock onto Earth thanks to a beacon
beamed at a slightly different wavelength.)
DSOC's ground station will be the 5.1-meter Hale Telescope
at Palomar Observatory in California - " which is on
the skinny side for what we'd like for these distances, " admits
Biswas, who serves as DSOC project technologist. Ideally, he
says, laser communications will utilize 8- to 10-meter telescopes
in the future.
The DSOC team's goal is to downlink at 100 Mbps from
a quarter of an astronomical unit away and about 2 Mbps
from 2 a.u., which would be well into the asteroid belt. (For
comparison, Psyche's main telecommunications system will
probably achieve a rate one-tenth as fast.) Beyond 2 a.u., the
spacecraft will be in the daytime sky and thus unobservable.
It may be 20 years before laser communications are routine
on space missions, but routine they must become. The
TO DEEP SPACE Team members test the Deep Space Optical Communications
experiment in a JPL clean room. The 22-cm primary mirror
(center, pointed at camera) will transmit and receive infrared lasers to and
from ground stations on Earth. The aluminum base plate will bolt to the
Psyche spacecraft.
aging Hubble Space Telescope collects an average of 18 GB
of science data each week. The Transiting Exoplanet Survey
Satellite (TESS), launched in 2018, downloads 94 GB of
compressed data on a 2-week cycle, and the new James Webb
Space Telescope (see page 20) is expected to send about 30 GB
over 4 hours, twice a day. But future flagship missions will
likely collect terabytes of data each day, says astrophysics chief
technologist Mario Perez (NASA). Radio networks cannot
handle the data deluge.
Lasers can. In the future, laser communications may
enable astronauts to send high-def videos from the Moon.
We'll receive streaming imagery from Mars. A fleet of laserenabled,
TESS-like satellites could take exoplanet observations
to a new level. And maybe we'll position our receivers in
space, avoiding the cloud and daylight problems entirely.
¢ Science Editor CAMILLE M. CARLISLE learned more about
Wi-Fi and downlink speeds during the pandemic than she ever
thought she'd need to know.
Radio communications beam
Laser communications beam
MARS
EARTH
Radio transmission data rate
Laser transmission data rate
2 Mbps
25 Mbps
DOWNLINK Laser beams arrive at Earth much narrower than radio beams do, making precise pointing crucial. But lasers also transmit data 10
times faster than radio does.
sk yand tele scope .o r g * NOVEMBER 2021 31
COMMUNICATIONS INFOGRAPHIC: LEAH TISCIONE / S&T,
SOURCE: NASA; LAB: NASA / JPL-CALTECH

http://skyandtelescope.org

Sky & Telescope - November 2021

Table of Contents for the Digital Edition of Sky & Telescope - November 2021