An elaborate cooling system is designed to protect the space probe through sizzling flybys
Over the past six decades, 12 people have walked on the moon, spacecraft have visited every planet from Mercury to Neptune, and four rovers have racked up more than 60 kilometers traveling on the surface of Mars. And yet, despite the billions of dollars spent on the world’s civilian space programs, never has a probe journeyed very close to the sun. The nearest approach, by the Helios B probe in 1976, came no closer than 43 million km.
Why is that? There’s been no lack of interest in the sun—quite the opposite. Of all extraterrestrial bodies, the sun has the largest influence on us: It controls the radiation doses that astronauts experience and also affects the electronics in the myriad satellites on which we increasingly rely. Solar storms can even disrupt electric power grids, as famously happened in 1989, when one such storm blacked out the entire province of Quebec and caused ripple effects on electric grids in the United States.
So there’s no shortage of practical reasons to study the sun. And it still holds many deep scientific mysteries. Unlike other bodies in the solar system, the atmosphere high above it is more than two orders of magnitude hotter than it is at the surface. What causes that phenomenon, known as coronal heating? And what mechanisms create the solar wind, accelerating parts of the sun’s atmosphere to velocities ranging from 300 to 700 kilometers per second? These questions, among others, have baffled scientists for decades.
Indeed, the case for studying the sun has never been hard to make. The limiting factor since the dawn of the space age has been the ability of a probe to endure the hellishly hostile near-sun environment. But this obstacle has recently given way to some new technologies, which we and others at the Johns Hopkins University Applied Physics Laboratory have built into a spacecraft called the Parker Solar Probe. That probe (named after astrophysicist Eugene Parker) was launched last August and has just recently completed its second close pass by the sun.
At the probe’s closest approach, about 6 million km from the sun’s surface, solar irradiance is almost 500 times as much as it is at Earth’s orbital distance—a whopping 650 kilowatts per square meter. Along with that immense intensity come extreme ultraviolet rays, which degrade materials rapidly. But ultraviolet radiation is not the only challenge. Sun-grazing comets are torn apart by the sun’s intense gravity, leaving the near-sun environment filled with dust that speeds along at upwards of 300 km/s, an order of magnitude faster than such particles travel in the vicinity of Earth.
Close Encounters: The Parker Solar Probe made its first close pass of the sun three months after launch in 2018. It will make a series of such passes as it continues to orbit the sun and will approach within a distance of 10 solar radii in 2024. Illustration: NASA
For the Parker Solar Probe to complete its mission, the spacecraft will need to survive this barrage of dust and intense radiation for at least seven years while making scientific measurements autonomously. The technologies required to accomplish that feat simply did not exist until the late 1990s, when high-temperature carbon-composite foams became available. At that point it became possible to fabricate a heat shield that was lightweight and sufficiently stiff, one that could provide the needed shade for the rest of the spacecraft. Before that, the refractory materials needed to construct an adequate heat shield would have been too heavy to fly.
It took some years for the Parker probe team to adapt these materials for use on spacecraft. And the design of a mission to the sun went through a decade of revisions before NASA settled on the current strategy. The probe won’t get quite as close to the sun as some scientists had wanted, but this compromise simplified the construction of the spacecraft and allows it to study the sun during multiple close passes, rather than just one or two.
Although the heat shield is absolutely critical to the mission’s success, the technology behind it has been well covered already, so here we will instead describe the development of the probe’s equally important solar array and the system used to keep it cool. This cooling system is found on no other spacecraft, meaning that to design and build it our Johns Hopkins engineering team had to, well, boldly go where no one had gone before.
Every spacecraft needs a source of electrical power. Some probes sent to the far reaches of the solar system use radioisotope thermoelectric generators to produce electricity. The Apollo command module used fuel cells. But more typically, electrical power comes from photovoltaic panels.
Because the Parker Solar Probe would be operating close to the sun, you might think the engineers designing the spacecraft’s power-generating solar panels had it easy. Not so. Our team had to figure out how to manage as many as 13 watts of heating in the panels for every watt of electrical power generated. This is almost 500 times as much heat as a satellite circling Earth experiences, so the standard method of cooling panels passively doesn’t apply.
A Dress Rehearsal: The solar array cooling system for the Parker Solar Probe spacecraft is shown here undergoing thermal testing. In the foreground (lower right) is a rectangular surface on which solar cells will be mounted, stretched out as it will be while the spacecraft is far from the sun. Photo: Ed Whitman/Johns Hopkins APL/NASA
The probe’s solar array consists of two wings. Each is about two-thirds of a meter wide and a little more than a meter long. The wings house solar cells that were designed to work well at high irradiance levels. But sufficient electrical output was not the issue—the challenge was keeping them cool.
In large part, that’s done by controlling the geometry of the probe’s solar panels. As the spacecraft nears the sun, the wings with these panels are rotated back toward the body of the spacecraft so that the sun’s rays impinge on them at a shallow angle. This decreases the amount of sunlight falling on the panels and puts more of the wings into the shadow or partial shadow of the 11-centimeter-thick heat shield mounted on the sun-facing side of the spacecraft.
To make this strategy work better, we designed each wing to have two distinct sections. Most of the area of a wing, the primary section, serves as the source of power most of the time. But when the spacecraft swoops close to the sun, the wings are angled so that their primary sections are entirely in the shadow of the heat shield. Only the small secondary sections at the tip of each wing are exposed to the sun’s rays.
Early on, we decided to mount the secondary sections so that they would be a few degrees closer to facing the sun than the primary sections are. Our objective was to ensure that the angle of the sunlight striking the surface was never less than 10 degrees. If this angle were to become any more acute, small variations in that angle would cause large changes in power output, which could be difficult to control.
We modeled performance very carefully as we were designing this solar array. That modeling was especially tricky for this mission because we couldn’t just treat the sun as a distant point source of radiation: We had to take into account that when the probe approached the sun, the partial shading from the heat shield would cause the solar cells to receive only rays coming from the outer fringes of the solar disk, what astronomers call the limb.
The limb is redder and dimmer than the sun as a whole. That’s because the sunlight coming from the limb has to pass through more of the sun’s lower atmosphere, so the only light that escapes comes from higher up, where temperatures are lower. To be able to predict the electrical output of the solar panels, we had to calculate how the color of the light falling on them would change with the probe’s varying distances from the sun.
Our biggest worry while we were designing the probe’s photovoltaic array was not that it wouldn’t work correctly after launch, but rather that it would degrade over time faster than expected. That’s what had happened to some previous space probes with trajectories that brought them close to the sun.
For instance, the solar array of the MESSENGER mission to Mercury (which was also designed, built, and operated by the Johns Hopkins Applied Physics Lab) was predicted to degrade by 10 percent over the course of its 11-year mission. In fact, its output diminished by 50 percent, for reasons nobody could quite understand.
MESSENGER was nevertheless successful because it stayed relatively close to the sun, where sunlight was intense enough for the craft to work properly even with an array that wasn’t performing as desired. This wouldn’t be the case, though, for the Parker Solar Probe. Its highly elliptical orbit periodically takes it outside the orbit of Venus. At those distances from the sun, it couldn’t function if its solar panels started providing much less energy than expected.
When the Parker probe was first conceived, we decided to design its solar array so it would still be able to do the job even if it suffered as much as MESSENGER’s did. But as the probe’s design matured, we and others at the Applied Physics Lab decided to take a different tack. We resolved to figure out what had caused the anomalous degradation of the solar panels on earlier spacecraft so that we could avoid whatever the problem was.
Initially, we suspected that two issues might be at play. One possibility could be outgassing from the adhesives used in the construction of these panels. Such outgassing would, we reasoned, deposit a film of adhesive material on the top of the cells, which would then turn brown after exposure to ultraviolet light. All solar arrays outgas to some extent, but the effect is worsened by high heat and radiation.
Another possibility was that the transparent adhesive used to attach the glass covers to the solar cells had turned brown, again because of exposure to ultraviolet light. Through extensive testing, we discovered that this process indeed accounted for most of the degradation.
We soon figured out that we could reduce this darkening by driving out the more volatile components in the cover-glass adhesive. Doing so involved heating the array under vacuum while exposing it to intense ultraviolet light, provided by light-emitting diodes. Some degradation of the solar panels would still occur, just as it does for other sorts of electronic components when they are “burned in,” but this would be a small price to pay for avoiding dangerous amounts of darkening later during the mission.
Of course, we needed to be sure our strategy would actually work. To test the theory, we had to place the arrays in an environment that resembled what they would experience close to the sun. And such conditions are not so easy to come by.
To meet that objective, we used 80 sets of eight mirrors. Each set could be individually rotated to reflect sunlight on a fixed target, even as the sun moves. Such devices, called heliostats, are sometimes used to provide daylight in places that would otherwise remain in the shadows. Large numbers of them are employed at solar-thermal energy plants to focus sunlight on a central tower. We set these heliostats up at a facility in New Mexico that was built specifically for this testing, where the solar panels could be held under vacuum while they were exposed to concentrated sunlight.
Such a use of heliostats, to concentrate sunlight on a spacecraft’s solar array, was clearly an odd one. But it worked marvelously. By the time the probe was assembled, its photovoltaic panels had experienced conditions and exposure durations never before used to test a full-size solar array destined for space.
Light This Candle: The United Launch Alliance Delta IV Heavy rocket launched the Parker Solar Probe from Cape Canaveral Air Force Station, Fla., on 12 August 2018 (top). During launch, the probe was protected by a slender fairing, as can be seen in a photograph from a month earlier (bottom), taken while the fairing was being assembled at the Astrotech Space Operations facility in nearby Titusville. Photos: Bill Ingalls/NASA; Ed Whitman/Johns Hopkins APL/NASA
With all the precautions we took and all the testing we carried out, we were fully confident that the panels wouldn’t suffer from anomalous darkening. But we still needed to be sure that the cells in them would remain sufficiently cool.
A typical spacecraft solar array manages its temperature passively: The absorbed light heats the solar cells, which are typically mounted on one side of a graphite-epoxy composite panel. Heat conducts through the honeycomb mesh of the panel and radiates out into space from both the front and back sides. A typical solar panel in space operates at a temperature between –70 and 100 °C—low enough not to require any special materials or coatings.
The photovoltaic panels on the Parker Solar Probe are different. Their solar cells must be actively cooled. For that, the cells were mounted to sheets of titanium containing a large number of narrow channels through which cooling water flows. It’s not unlike the system used to keep the engine in your car from overheating. The cooling system on the Parker probe doesn’t use antifreeze, though. It uses ordinary deionized water, just as you might use in a steam iron. And just like the cooling system in your car, the system is pressurized to prevent the cooling fluid from boiling at high temperatures.
At the probe’s closest approach to the sun, the cooling system must be able to handle about 5,900 watts of heat load on the two solar wings. This heat is shed using four separate radiators, each about one meter square, located in the shadow of the probe’s heat shield. Those radiators, along with the coolant pumps and other critical components, all have backups on board, which can be activated if a primary component were to fail.
Although we focused most of our energies on how to combat the danger of overheating, there would be equally dire consequences if things ever got too cold. Because the spacecraft would not be exposed to the warming rays of the sun for almost an hour after launch, and would be in the shadow of Venus for a brief time later, we had to take care that the temperature of the water in the cooling system would not drop below freezing. As anyone who has experienced a burst pipe at home during a chilly winter night can tell you, water expands when frozen. So an ice plug in the probe’s cooling system could cause a rupture, which would end the mission. We clearly needed to avoid any chance of that.
At launch, most of the cooling system was evacuated, with the water stored in a reservoir that we call the accumulator. That water was heated before launch to about 50 °C—enough to prevent it from freezing should it encounter any cold spots in the spacecraft’s plumbing when the cooling system was initially flooded.
Before activation, the solar arrays were deployed and warmed by the sun. At the same time, two of the four radiators were similarly heated by turning the spacecraft so that they weren’t in the shadow of the heat shield. After these components became sufficiently warm, valves opened to allow the preheated water to flow from the accumulator. After half the cooling system was loaded with water, spacecraft operations commenced. Some weeks later, after the probe was closer to the sun, the other two radiators were warmed in the same manner and filled with coolant. This cooling system ensures that the highest temperature that any photovoltaic cell will ever reach is 120 °C.
The Parker probe, which has been in space for almost nine months now, has really just embarked on its multiyear mission to explore the sun’s mysterious corona, a part of the solar system previously considered too hostile for spacecraft to explore directly. That the probe is functioning well and deepening scientific interest in the workings of the sun is a tribute to the many men and women who have together contributed some 10 million hours to the immense engineering effort that was needed to make the mission a success.
Sure, the measurements taken during the first two of the probe’s 24 planned passes close to the sun are generating as many questions as they are answering. But that’s to be expected—it’s a hallmark of all the best scientific endeavors.
This article appears in the May 2019 print issue as “Journey to the Center of the Solar System.”