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NASA has a mantra to prepare spaceships to launch: "Test in flight." The idea is to test the entire spacecraft, fully assembled, in the same environment and the same configuration that it will see in orbit.
Parker Solar Probe, whose launch is scheduled for August 11, is not an ordinary spacecraft (SN Online: 7/5/2018) . And he is heading for an unusual environment. Parker will sweep the torrid atmosphere of the sun for the first close encounter of humanity with the star in the center of the solar system
"Solar Probe is a bit special", says the physicist of plasmas Stuart Bale spatial from the University of California, Berkeley. It was deemed impossible to install the entire kit and caboodle in an environment simulating the energetic particles of the sun, the intense light and the burning heat. Scientists had to be creative to test the technology that will affect the sun. reams of paper.
Taking Heat
The first order of the day was to find materials that could withstand heat. The atmosphere of the sun, or the crown, sizzles at millions of degrees Celsius – but it is so diffuse that it does not pose much threat to the spacecraft (SN Online: 20/08 / 17) . However, direct sunlight can heat exposed components to around 1370 ° Celsius. Two of the spacecraft's scientific instruments, plus parts of its solar panels and its revolutionary heat shield, will be exposed to daylight.
"Normal things … would melt," says Kelly Korreck, solar physicist of the Smithsonian Astrophysical. Korreck is working on the Solar Wind Electrons Alphas and Protons instrument, known by its acronym SWEAP, which will capture the charged particles of the solar wind with a sensor called a Faraday cut (SN Online: 18/08/17) . "It sticks around the heat shield and will be able to touch the sun," says Korreck. "This cup is special.
To build the cup and other instruments that will see the sun directly, the engineers settled on three main materials – a niobium alloy called C103 that is used in rocket motors, a titanium alloy , some zirconium and molybdenum called TZM and tungsten.Some cables feeding the SWEAP cup are also lined with sapphire, a good high temperature insulator, and the thermal screen of the probe is made up of two types of carbon-based materials.
It was difficult to understand how each of these materials would behave in the space: the engineers could not simply use an oven to test the metals that, in heat, can react with oxygen to rust or corrode. The team had to test instrument parts in airless vacuum chambers.
"It's easier to heat things on Earth than you think," says Elizabeth Congdon of the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. the chief engineer for the heat shie "It's hard to heat things on Earth in a vacuum."
The Parker team used sunlight as a way to mimic the heat of the sun. Engineers took samples of materials from the world's largest solar furnace, the PROMES facility in Odeillo, France. A series of 63 mirrors built on the hillside redirect sunlight onto a huge concave mirror on the side of an eight-story building. This mirror then concentrates the sunlight in a beam of at most 80 centimeters wide that heats the materials at 3000 ° C in a small vacuum chamber in a laboratory on stilts. -four and swing through the beam, and it burns right away, "says Bale." Just a flash of smoke and it's gone. "Bale runs another of the probe experiments called FIELDS that also need FIELDS is composed of five long antennas, four of which will be exposed to the sun, which will measure electric and magnetic fields in the crown.
The SWEAP team needed an even more simulator Realistic, which would produce intense light in the sun, angles that Parker will know.They found an unlikely solution in the IMAX film projectors, which emit light in a range of wavelengths similar to the sun.
"It took a fully customized test bench," says astrophysicist Anthony Case of the Smithsonian Astrophysical Observatory, also working on the SWEAP instrument.With his colleagues, he has run four IMAX projectors so that the lamps concentrate the light in a small vacuum chamber, rather than spreading it on a large screen. This gave the & # 39; s team & # 39; light intensity right and angles to test their instrument
Biting the Dust
Solar heat is not the only threat to the Parker solar probe.Scientists do not know exactly which The amount of dust they can expect, but it is likely that they are moving almost as fast as the spacecraft, about 170 kilometers per second
.Great worry for Parker's twin telescopes, called together the wide-field imager for solar sensor, or WISPR.Un telescopes will be facing the direction that Parker is traveling, so that he will head straight into the dust storm. US Naval Research Laboratory, Washington, DC
Dust particles reaching the telescope lens leave tiny craters: only 0.6% of the lens should be stitched at the end of the annual mission, according to computer models. dust in the internal solar system. But even some wells can skew the data, so the team wanted to minimize the damage by choosing the right glass.
Howard and his colleagues tested three possible materials for glass in a dust acceleration tunnel at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. The tunnel accelerated charged iron particles, ranging from half a micrometer to 3 microns wide, at speeds ranging from half a kilometer to 8 kilometers per second – fast enough for scientists to extrapolate up to the dust speeds that Parker might know.
Sapphire is more dam resistant, but we do not know how he behaves like a goal. The team also rejected diamond coated BK7 glass, commonly used for space telescopes, after the coating separated from the glass and left an extra ring around the impact spot.
What's New
The Parker Solar Probe will use four sets of scientific instruments and innovative self-protection measures to explore the near-sun environment. Take a tour of spacecraft technology
Press or click to explore technology
H. Thompson, E. Otwell, T. Tibbitts
Oscillation of Temperatures
Most spacecraft will not have to worry about dust or the extreme heat of the sun. Apart from SWEAP and FIELDS, almost everything is hidden behind the very important heat shield.
This 2.5 meter wide heat shield is made of carbon foam sandwiched between two sheets of carbon. The set is only 11.5 centimeters thick and is coated on the sun-facing side of a white ceramic paint to reflect as much light as possible. Even then, this side could reach 1370 ° C. But behind him, most of the spacecraft will cool down to an average temperature of only 30 ° C (about 85 ° Fahrenheit) "
" We are hiding in the l 39; Shadow, "explains solar physicist Eric Christian of NASA's Goddard Space Flight Center in Greenbelt, Maryland. He is the Senior Assistant Investigator of the Integrated Science Investigation of the Sun experiment, which will measure solar particles across a wide range of energies. His team was able to build with ordinary materials and pass the rigorous heat tests. "We are lucky. "
But Parker will not always be near the sun, the orbit of the craft will bring it as far from the sun as Venus, where temperatures at around -270 ° C. At this distance, the spacecraft that will touch the sun needs heaters on board to keep it at 20 ° C. so Parker had to undergo cold and extreme temperature testing. We are worried about hot then cold then hot then cold, "says Congdon
In January 2018, the entire vessel was sunk into a vacuum chamber at NASA Goddard for two months. tests, 12 meters high and 8 meters wide, was cooled to -180 ° C. A radiator shining at around 315 ° C accounted for the heat coming from the back of the heat shield – but most this heat has never reached the scientific instruments since a titanium beam maintains the thermal screen at a safe distance from the main body of the spacecraft. cycled through the hot and cold several times to simulate what Parker will experience.
Another challenge was to keep the solar panels of the probe cool. "You think, obviously, you go to the sun, solar energy has the most meaning," says Congdon. "But solar panels do not like to be hot." The panels are thus threaded with veins that carry water to cool them. The water absorbs the heat from the panels and transports it to the radiators that release the heat into the space.
The solar panels are also located on a shoulder joint, so that they can slip behind the heat shield. Only the last row of cells will see the sun then. "This single row of cells can produce the same amount of power as the full wing when we are near the Earth," says solar physicist Nicola Fox of the Johns Hopkins Applied Physics Laboratory, the probe project scientist. .
Before and After
Before Parker could scrutinize the secrets of the sun, however, he must survive the journey into space
The Launch of the device will make testing time difficult for scientists, even if they have tested all the parts in an acoustic vibration chamber. See the SWEAP vibration test "Makes me swear" Korreck says, 10 years later, the flop continues to quiver more and more. "
His team had to face an unusual challenge in making sure that Parker was ready to click. SWEAP team has hand twisted the thin niobium wire to attach hundreds of screws so that, if one comes off, the others hold it back.
The launch can be a time of high pressure for the spaceship, Engineers first thought that the launch of Parker aboard a powerf Delta IV H rocket eavy, would subject the heat shield to a force 20 times greater than the gravity of the Earth, although later the team realized that the launch force would not be so severe. To prevent the 72.5-kilogram shield from bending or breaking, the team will stack 1,360 kilograms of paper over it.
Once the final test of launch and deployment has been successfully completed, Parker's first scientific data should begin to recover. Earth in December. These missives will allow scientists to take the first step to discover the secrets of the overheated atmosphere of the sun and its energy winds.
"It's like being a proud parent. I'm worried that something could happen, but I'm not afraid we have not prepared or tested it, "says Fox. "I just hope she's writing at home everyday with nice data."
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