38,000-mile-per-hour flight farther into deep space,
an acoustic phenomenon known as the Doppler effect
slightly stretches its signal’s wavelength—the same way
the tone in a siren’s wail warps as an ambulance speeds
by. The change tells the ground crew how far Voyager
has flown between its daily check-in and the 20-ish
hours it takes the call to reach us. It also helps them
continue charting the interstellar pioneer’s course. If
they know where the thing’s going, they know where to
turn those giant antennas to listen for it again.
Now that each probe has completed its primary
mission, the new goal is “how can we stretch it out
and stretch it out—how long can we make it go?” says
Suzanne Dodd, project manager for Voyager and
head of JPL’s Interplanetary Network Directorate.
Delivering commands to these extra-solar
explorers— working to slow our waning opportunity
for deep-space insight—is mainly about managing
the probes’ power. Every redundant system on board
has, at this point, been turned off. That means both
craft are generating very little heat in the extreme in-
terstellar cold, so the hydrazine propellant in the fuel
lines might freeze. Mission
Control cycles through sys-
tems, seeing what might be
worth keeping alive for the
sole purpose of warming
the lines. It’s a patch job,
on some of the oldest com-
puters still going.the data, carried by the signal. The signal is the
bus. Flight engineers and the staffers tending
Mission Control care about the data, the same
way almost everyone cares about the kids. But
radio scientists find the vehicle itself more inter-
esting because it’s filled with noise.
If you study the bus carefully, you can figure
out what it went through as it wound toward its
destination. Marks, imperfections—the ugly,
misshapen bits—tell you about the journey: not
just the road traveled, but the other vehicles,
the weather, the traffic along the route. Folks
like Oudrhiri scrutinize these myriad faults on a
scale, oh, about the size of the universe.
Many of the earliest radio-science experiments
were unintentional. Back in 1971, when the Mariner
9 probe passed Mars, its signal traveled through
the Red Planet’s atmosphere, which collided with
and changed the wave. “People in telecom saw
it as an interference, but others saw that if you
studied the interference, you could determine
the density, the pressure, even the temperature
of the atmosphere on Mars,” Oudrhiri continues.
“That was the start of radio science.”
Since then, looking carefully at space-borne
noise has deepened our understanding of the so-
lar system. Disturbances in the Cassini probe’s
transmissions, for instance, helped reveal that
Saturn’s colorful rings formed much later than
the planet itself did—10 million to 100 million
years ago, versus 4.5 billion. NASA’s
GRAIL lunar mission in 2012 in-
volved two craft pinging radio waves
back and forth to learn about the in-
terior of the moon; inspecting how
gravity fields interfered with the
transmissions helped prove that
most of the orbiter’s crust isn’t as
dense as we previously thought.
Oudrhiri loves radio science for
its simplicity. A signal is a wave with
amplitude (the highs and lows),
phase (the pattern of those peaks and
troughs), and frequency (the number
of dips and spikes in a given span).
Distortion in these features is easy
to spot. If you know approximately
how the ripples should appear, you
know when they’ve changed. It’s like
a smoke signal blowing away before
you can make out the pattern, which
tips you off to an unfelt breeze.
A Voyager probe’s cosmic hum al-
ways contains one vital piece of radio
science. As either craft continues its
POPSCI.COM -- WINTER 2019 PG -- 70Mission Critical
The so-called
Dark Room at JPL
keeps tabs on more
than three dozen
craft. Staff tend
their monitors
around the clock,
receiving data
from deep space.