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A New Plan for Keeping NASA's Oldest Explorers Going With careful
planning and dashes of creativity, engineers have been able to keep NASA's
Voyager 1 and 2 spacecraft flying for nearly 42 years - longer than any other
spacecraft in history. To ensure that these vintage robots continue to return
the best science data possible from the frontiers of space, mission engineers
are implementing a new plan to manage them. And that involves making difficult
choices, particularly about instruments and thrusters.
One key issue
is that both Voyagers, launched in 1977, have less and less power available
over time to run their science instruments and the heaters that keep them warm
in the coldness of deep space. Engineers have had to decide what parts get
power and what parts have to be turned off on both spacecraft. But those
decisions must be made sooner for Voyager 2 than Voyager 1 because Voyager 2
has one more science instrument collecting data - and drawing power - than its
sibling.
After
extensive discussions with the science team, mission managers recently turned
off a heater for the cosmic ray subsystem instrument (CRS) on Voyager 2 as part
of the new power management plan. The cosmic ray instrument played a crucial
role last November in determining that Voyager 2 had exited the
heliosphere, the protective bubble created by a constant outflow (or
wind) of ionized particles from the Sun. Ever since, the two Voyagers have been
sending back details of how our heliosphere interacts with the wind flowing in interstellar
space, the space between stars.
Not only are Voyager
mission findings providing humanity with observations of truly uncharted
territory, but they help us understand the very nature of energy and radiation
in space - key information for protecting NASA's missions and astronauts even
when closer to home.
Mission team
members can now preliminarily confirm that Voyager 2's cosmic ray instrument is
still returning data, despite dropping to a chilly minus 74 degrees Fahrenheit
(minus 59 degrees Celsius). This is lower than the temperatures at which CRS
was tested more than 42 years ago (down to minus 49 degrees Fahrenheit, or
minus 45 degrees Celsius). Another Voyager
instrument also continued to function for years after it dropped
below temperatures at which it was tested.
"It's
incredible that Voyagers' instruments have proved so hardy," said Voyager
Project Manager Suzanne Dodd, who is based at NASA's Jet Propulsion Laboratory
in Pasadena, California. "We're proud they've withstood the test of time.
The long lifetimes of the spacecraft mean we're dealing with scenarios we never
thought we'd encounter. We will continue to explore every option we have in
order to keep the Voyagers doing the best science possible."
Voyager 2 continues
to return data from five instruments as it travels through interstellar space. In
addition to the cosmic ray instrument, which detects fast-moving particles that
can originate from the Sun or from sources outside our solar system, the
spacecraft is operating two instruments dedicated to studying plasma (a gas in
which atoms have been ionized and electrons float freely) and a magnetometer
(which measures magnetic fields) for understanding the sparse clouds of
material in interstellar space.
Taking data
from a range of directions, the low-energy charged particle instrument is
particularly useful for studying the probe's transition away from our
heliosphere. Because CRS can look only in certain fixed directions, the Voyager
science team decided to turn off CRS's heater first.
Voyager 1, which
crossed into
interstellar space in August 2012, continues to collect data from
its cosmic ray instrument as well, plus from one plasma instrument, the
magnetometer and the low-energy charged particle instrument.
Why Turn Off Heaters?
Launched
separately in 1977, the two Voyagers are now over 11 billion miles (18 billion
kilometers) from the Sun and far from its warmth. Engineers have to carefully
control temperature on both spacecraft to keep them operating. For instance, if
fuel lines powering the thrusters that keep the spacecraft oriented were to
freeze, the Voyagers' antennae could stop pointing at Earth. That would prevent
engineers from sending commands to the spacecraft or receiving scientific data.
So the spacecraft were designed to heat themselves.
But running
heaters - and instruments - requires power, which is constantly diminishing on
both Voyagers.
Each of the
probes is powered by three radioisotope thermoelectric generators, or RTGs,
which produce heat via the natural decay of plutonium-238 radioisotopes and
convert that heat into electrical power. Because the heat energy of the
plutonium in the RTGs declines and their internal efficiency decreases over
time, each spacecraft is producing about 4 fewer watts of electrical power each
year. That means the generators produce about 40% less than what they did at
launch nearly 42 years ago, limiting the number of systems that can run on the
spacecraft.
The mission's
new power management plan explores multiple options for dealing with the
diminishing power supply on both spacecraft, including shutting off additional instrument
heaters over the next few years.
Revving Up Old Jet Packs
Another
challenge that engineers have faced is managing the degradation of some of the spacecraft
thrusters, which fire in tiny pulses, or puffs, to subtly rotate the spacecraft. This became an issue in 2017, when mission
controllers noticed that a set of thrusters on Voyager 1 needed to give off
more puffs to keep the spacecraft's antenna pointed at Earth. To make sure the
spacecraft could continue to maintain proper orientation, the team fired
up another set of thrusters on Voyager 1 that hadn't been used in 37
years.
Voyager 2's
current thrusters have started to degrade, too. Mission managers have decided
to make the same thruster switch on that probe this month. Voyager 2 last used
these thrusters (known as trajectory correction maneuver thrusters) during its
encounter with Neptune in 1989.
Many Miles to Go Before They Sleep
The engineers'
plan to manage power and aging parts should ensure that Voyager 1 and 2 can
continue to collect data from interstellar space for several years to come. Data from the Voyagers continue to provide scientists with
never-before-seen observations of our boundary with interstellar space, complementing
NASA's Interstellar Boundary Explorer (IBEX), a mission that is
remotely sensing that boundary. NASA is also preparing the Interstellar Mapping
and Acceleration Probe (IMAP), due to launch in 2024,to capitalize on the
Voyagers' observations.
"Both
Voyager probes are exploring regions never before visited, so every day is a
day of discovery," said Voyager Project Scientist Ed Stone, who is based
at Caltech. "Voyager is going to keep surprising us with new insights
about deep space."
The
Voyager spacecraft were built by JPL, which continues to operate both. JPL is a
division of Caltech in Pasadena. The Voyager missions are a part of the NASA
Heliophysics System Observatory, sponsored by the Heliophysics Division of the
Science Mission Directorate in Washington. For more information about the
Voyager spacecraft, visit:
https://www.nasa.gov/voyager
https://voyager.jpl.nasa.gov
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