Built to Last: JPL's Amazingly Long-Lived Missions
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When a rocket carries a spacecraft up through the atmosphere sending it outside of Earth’s orbit, it might seem untouchable. It can talk to us and us to it. But whatever set of hardware is in its innards and strapped to its outsides when it leaves Earth’s bonds is what sees it through its mission.
While that’s self-evidently true, mission scientists and managers at NASA’s Jet Propulsion Laboratory (JPL), managed by Caltech in Pasadena, have performed some long-distance tinkering on many active projects, helping to extend those probes and satellites’ lifetimes many times beyond their original scheduled mission. The pictures and data that continue to pour back years and decades later are a result of smart planning coupled with ample helpings of plain old good luck.
Scientists can’t rely on luck, but they welcome it when choices made to ensure success in the primary mission turn out — not really accidentally—to have other benefits, especially many years after launch. As Mike Werner, a longtime scientist on the Spitzer satellite, says, “nature has cooperated, but we exploited.”
Here, we look at four of JPL’s veteran spacecraft, how they’re faring, and what’s kept them in good trim.
A Cold Telescope Sees Unexpected Heat Signatures
When the Spitzer Space Telescope was launched in 2003, its scientists knew that it was something special because it contained an extremely sensitive infrared detector. Earth’s atmosphere filters light that reaches our planet. But infrared radiation, which we feel as heat, is so predominant on Earth that it drowns out the light’s faint invisible glow from far-distant places. Because these long-wave emanations reveal a lot about the universe’s formation and current structure, astronomers had blind spots in their understanding of many phenomena.
Werner, Spitzer’s project scientist, says the telescope’s detector is 1,000 times more sensitive than if it were on the ground. “What we knew with Spitzer was that we had a huge gain in capability over anything we had in the past,” he says. Werner began work on the mission in 1984, when planning was already underway. The design changed substantially as it became clear from emerging science that it had to be farther from Earth than originally intended to avoid infrared noise (Werner shares his experiences in a new book he co-authored, “More Things in the Heavens”).
When launched in 2003, Spitzer relied on several techniques to keep its detectors extremely cool. Its orbit trails Earth, avoiding most of its heat and light. Shields and reflective paint and metal also keep heat away. And its science instruments were placed in a compartment cooled by liquid helium that slowed things down to a few degrees above absolute zero.
Equipped with a telescope that focused infrared frequencies onto three instruments, Spitzer was designed to pick up burned-out stars, stellar “nurseries” in which new suns form, certain kinds of organic molecules, the centers of galaxies and more.
Spitzer’s liquid helium supply would eventually dissipate, which happened as projected, about five and a half years into the mission. But due to the orbit and design, part of one of the three instruments could remain active and produce good measurements even at the “hot” temperature of about 30 degrees Celsius above absolute zero.
With this limited detection capability, Spitzer continued to deliver vastly more than expected, as infrared light has proven to be more useful at detecting phenomena than known at its launch, such as planets orbiting other stars, or exoplanets. Werner says during the mission’s early stages, “exoplanets had not yet been discovered.” Now NASA considers over 4,000 confirmed.
That includes the seven planets discovered in the TRAPPIST-1 system. Scientists using a pair of telescopes in Chile and Morocco published a paper on three of them in 2016 and worked with Spitzer and ground-based telescopes to confirm them and uncover four more in 2017. “In order to get the data on the TRAPPIST-1 system, we had to observe for 20 days continuously,” says Werner, or for around 500 hours.
Most probes make deep observations into limited areas, as they’re constrained by tightly focused science instrument designs. But “some combination of good luck and good engineering” led to broad insights with Spitzer, Werner says. In 2018, on its 15th year in space, JPL published a list of Spitzer's 15 top discoveries, which span an unusually broad array of astronomical areas.
Spitzer’s life is now measured in months, however. Some onboard systems have had to switch to backups, Werner says, and continue to work correctly. But as a solar-powered craft that trails the Earth ever farther, the satellite has a smaller and smaller window when it can turn from the Sun and transmit to Earth using charged batteries. The mission’s official cutoff date is January 30, 2020.
In 2021, the James Webb Space Telescope is slated to launch with far more sensitive infrared detection, ready to pick up where Spitzer left off.
From Martian Monitoring to Star Tracking
Mars is the only planet we know of populated entirely by robots. Landers and rovers have reached its surface across several decades, and two remain operational: The Curiosity rover and the InSight lander, both handled by JPL.
But if Curiosity turned its eyes to the sky, it would see crowded airspace. Several satellites that actively scan the planet remain in orbit, including craft from NASA, the European Space Agency, Russia’s Roscosmos (in partnership with the European Space Agency) and India. JPL manages two of those: Mars Odyssey, launched in 2001, and the Mars Reconnaissance Orbiter (MRO), launched in 2005.
Odyssey holds the record by far for remaining functional on or around Mars at 18 years, doubling the previous record. It was designed to measure attributes of Mars’ composition, such as the kind and distribution of minerals, and the presence of water just beneath the surface. It also creates thermal maps of the surface and maps the planet’s terrain.
MRO has a somewhat different mission. Its focus is water on the ground and in the air mixed with dust. Both Odyssey and MRO relay data from ground robotic craft, such as Curiosity, and previously the twin Spirit and Opportunity rovers, back to Earth.
However, MRO’s most significant component is the highest-resolution camera ever launched beyond Earth, called HiRISE, paired with a telescope that lets it take close-ups of the surface. It creates uncompressed images nearly 20 gigabytes in size and is paired with the highest-speed transmission system ever sent to deep space, at up to six megabits per second — slow for cable broadband, remarkably fast for Mars. So far, MRO has sent back 45 terabytes of data, vastly more than all other deep-space missions combined.
Both satellites have faced challenges in operation. Odyssey lost its radiation detector, designed to provide insights into safety for a future crewed mission to Mars, in 2003. A large solar bombardment that year appeared to knock out of one of the detector’s chips, rendering it unusable, although it can use other instruments for some radiation monitoring, according to Laura Kerber, the deputy project scientist on Odyssey.
Odyssey also had trouble keeping itself in the right orientation in 2012. It normally relies on three reaction wheels, which spin to rotate the satellite in opposing axes to align with Earth and Mars. One of the wheels failed, but redundancy planning meant a backup was in place and could be spun up and put into service. If another wheel stops working, the mission team can use the remaining two, plus thrusters, but would run out of fuel within a year, says Kerber.
MRO had to work through an interlocked set of problems with its batteries and HiRISE camera. As components age, they show wear in different ways. Odyssey and MRO are both solar-powered and contain batteries that are charged to keep them running during the eclipse — when Mars is between the satellites and the sun — that can occur in every two-hour orbit of Mars. For the MRO, that can be half an hour in each cycle; for Odyssey, it might be as long as 17 minutes, but Kerber says the current orbit keeps Odyssey eclipse-free for most of the year.
The HiRISE camera developed noise in its image sensors, but that could be reduced by adding a warm-up phase for those circuits before capturing images. When initially added, the warm-up happened during the eclipse. As concerns about battery life added up, however, it had to be moved to daylight hours. “You’re trying to turn off spacecraft heaters during eclipses, so you’re not drawing into the battery,” says Dan Johnston, MRO’s project manager.
Starting late 2017, the camera developed blurriness as Mars was moving to its farthest point from Earth — probably a result of not being sufficiently warm despite the pre-heating. Meanwhile, in February 2018, MRO went into emergency standby mode when the batteries dropped below their expected range during its dark period.
Johnston says a team of battery experts developed a plan that successfully reconditioned the batteries — something familiar to those on Earth who have used older generations of rechargeables. The blurring problem was solved by keeping a heater in the telescope part of the camera system running continuously while photos were taken. Ironically, it was designed to turn off during active image exposure to reduce noise.
Both satellites rely on inertial measurement devices that measure changes in speed and rotation, as well as provide an accurate position relative to the rest of the universe. However, these devices have moving parts that will fail over time. Both tested switching to a star tracker, a fixed-position camera which takes pictures of the sky and compares them against known star positions. Odyssey’s Kerber says the thought was, “Why don’t we take advantage of this convenient functional redundancy?” MRO has swapped over to mostly using the star tracker (which has a backup in both satellites, too), while Odyssey plans a transition soon.
Odyssey and MRO likely have many years ahead of them. Both were launched with sufficient fuel to manage orbital adjustments. Odyssey now sits in an orbit that relies less on its batteries in eclipse, while MRO’s battery reconditioning reduced concerns there.
Odyssey and MRO could twinkle down on Mars’ robot population for many years to come. Odyssey has no official mission end date, while MRO has a formal extension to the mid-2020s.
Now, Now, Voyager
The Voyager 1 and 2 probes that were launched in 1977 and now travel in interstellar space were preceded by other deep-space missions that had shorter lives. Even those that were wildly successful, like Pioneer 10 and 11, have been far outpaced by the Voyagers.
Those earlier craft help provide insight into design planning that helped give both Voyagers their longevity. Pioneer 10 revealed that more radiation hardening was needed on the Voyagers, resulting in a fast redesign of some components.
Voyager 1 and 2 were conceived in 1965, when astronomers recognized that an alignment of the solar system’s four gas giants that occurs once every 175 years was coming up. The two crafts’ flight plans allowed both to pass by Jupiter and Saturn with the added potential for Voyager 2 to also visit Uranus and Neptune.
The probes sent back relatively huge amounts of data and photography. Among many discoveries, they identified dozens of new moons across the four planets, found active volcanoes on Jupiter’s moon Io, detected a magnetic bubble around Uranus and Neptune, and revealed new rings and new details about existing ones on all four planets. They also revealed the workings of the barrier between the sun’s magnetosphere — the reach of its magnetic influence — and the medium outside it in which interstellar gases exist.
Some of the tens of thousands of pictures made by each Voyager were almost missed. Each craft has an instrument platform on a boom several feet from the main body of the probe that allowed movement in two directions. Voyager 1’s gear ground to a halt in 1978, and JPL determined a piece of plastic was left in place before launch. By sending commands to move the gears, the plastic was ground out, and motion was restored.
Voyager 2 hit a snag later in the mission as it flew on the far side of Saturn. Following that swing, the platform was frozen in one direction. The control team made efforts to dislodge it, but ultimately rotated the craft 90 degrees and relied on the other direction’s motor and gearing to shoot pictures of Uranus and Neptune.
That was critical because with less light at Uranus and far less at Neptune, JPL had to adjust camera exposures to be longer and have the craft compensate with motion to avoid smearing the images.
The greater distance also meant that techniques used to send data from Jupiter and Saturn would no longer allow fast enough transmissions to capture many images. The mission team shifted the programming of one the craft’s backup computers to perform image compression, reducing file size without losing detail.
They also flipped on an experimental system that substantially decreased the amount of redundant data sent back to correct any signals distorted between the far planets and receivers on Earth. That was coupled with improvements to the Deep Space Network that allowed fainter transmissions to be received better around the planet. Without all those efforts, we would only have learned a fraction of what we did about Uranus and Neptune than about Jupiter and Saturn.
The Voyagers are powered by the heat generated from the decay of a radioactive compound, so they produce less energy as each day passes. They will keep operating through the mid-2020s with ever fewer scientific instruments. But they may be remembered best by the last picture Voyager 1 took when it turned around to look back at Earth and snapped “the family portrait” of Neptune, Uranus, Saturn, Jupiter, Earth and Venus. Earth, so distant, was just a pale blue dot.
Top Image: Newborn stars peek out from beneath their natal blanket of dust in this dynamic image of the Rho Ophiuchi dark cloud from NASA's Spitzer Space Telescope. | Flickr/NASA Marshall Space Flight Center/Creative Commons (CC BY-NC 2.0)