Sky & Telescope - November 2021 - 30

80 YEARS: WHERE WE'RE GOING by Camille M. Carlisle
LASERS IN SPACE NASA's Laser
Communications Relay Demonstration
will transmit data from
the International Space Station to
Earth using infrared lasers.
Beam the Data
Down, Scotty
Lasers will replace radio antennas as the
interplanetary communications system.
C
assini. New Horizons. The Hubble Space Telescope.
These spacecraft and so many others have given us
mesmerizing views of the cosmos, expanding our knowledge
and piquing our curiosity.
But it's no use sending instruments to space if we can't get
the data back. Without a communications link, there is no
exploration.
Throughout the Space Age, we've used radio wavelengths
to transmit data to and from our space emissaries. Radio has
major perks: It cuts through clouds above ground stations,
and its wavelengths are so long that you don't have to aim too
carefully in order to hit your antenna back on Earth.
But radio is also slow. Those same long wavelengths limit
how much information a signal can carry. Take, for instance,
the New Horizons spacecraft, which due to distance, bandwidth,
and network sharing needed more than a year to send
its 6.25 GB of Pluto flyby data back to antsy team scientists.
That's similar to the file size of a single HD movie.
NASA and other spacefarers are thus developing an alternative:
infrared laser communications.
30 NOVEMBER 2021 * SKY & TELESCOPE
Radio and laser communications operate on the same
principle: Encode information by modulating a wavelength's
properties, then transmit the signal to a receiver that can
decode the modified stream. It's the same idea behind your
Wi-Fi router, which uses microwaves to send information
from the internet to devices around your home.
But infrared wavelengths are roughly 10,000 times shorter
than the radio ones used to communicate with spacecraft,
so lasers pack information into tighter waves. They can thus
deliver more data in the same period of time than a radio signal
can. Switching to infrared lasers - which go by the misnomer
optical communications - would increase data transmission
rates by a factor of 10 or more. That would boost the
downlink rate from Mars - which is about 2 megabits per
second (Mbps) when the planet is at its closest - to some 25
Mbps, the minimum uplink requirement for U.S. broadband.
There are downsides, however. First, infrared lasers can't
pass through clouds, so ground stations need to be in clearsky
locations, such as Hawai'i and parts of California. Second,
the signal beam is narrow. Electromagnetic waves spread
out as they travel from their source; a radio beam transmitted
from Mars is larger than Earth's diameter by the time it
reaches our planet, making catching it easy. But a laser beam
shot from that same distance would only be the size of California
when it reached Earth, says Abhijit Biswas (Jet Propulsion
Laboratory). Successfully hitting the ground station is
like holding a meter-long soda straw so steady that the far
end doesn't dip by a micron. " If we miss, " he says, " there will
be no one to catch the bits. "
DAVE RYAN / NASA
Sky & Telescope - November 2021

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