“Getting things hot on Earth in vacuum is difficult.”
One way the Parker team mimicked the sun’s
heat was by using actual sunlight. Engineers took
material samples to the world’s largest solar furnace, the PROMES facility in Odeillo, France. A
series of 63 mirrors built on a hillside redirects
sunlight onto an enormous concave mirror on the
side of an eight-story building. That mirror then
focuses the sunlight into a beam no more than
80 centimeters wide that heats materials to 3000° C
inside a small vacuum chamber in a laboratory
that’s on stilts to reach the height of the beam.
The beam is so hot, “you can take a two-by-four
and swing it through the beam, and [the wood]
burns right off,” Bale says. “Just a flash of smoke.”
Bale leads another of the probe’s experiments,
called FIELDS, that also needed heat testing.
FIELDS is comprised of five long antennas, four
of which will be exposed to the sun, that will measure electric and magnetic fields in the corona.
The SWEAP team needed a simulator that would
also deliver intense sunlight at the same angles
that Parker will experience. The engineers found
an unlikely solution in IMAX film projectors,
which emit light in a similar range of wavelengths
to the sun.
“It took a completely custom test facility,” says
Biting the dust
Anthony Case, an astrophysicist at the Smithsonian
Astrophysical Observatory who works on the
SWEAP instrument. He, Korreck and colleagues
turned four IMAX projectors around so the lamps
focused light into a small vacuum chamber, rather
than spreading it across a huge screen. That setup
gave the SWEAP team the right light intensity and
angles to test the particle-catching cup.
Solar heat isn’t the only threat to the Parker Solar
Probe. The region around the sun that the spacecraft will explore is expected to be full of dust.
Scientists don’t know exactly how much dust to
expect, but it’s likely to be moving almost as fast as
the spacecraft, about 170 kilometers per second.
That dust is a big worry for Parker’s main telescopes, together called the Wide-field Imager for
Solar Probe, or WISPR. One of the telescopes will
be facing the direction that Parker is traveling, so
the telescope will be heading directly into the dust
storm. “It can’t be protected,” says astrophysicist
Russell Howard of the U.S. Naval Research Laboratory in Washington, D. C., and WISPR team leader.
Dust particles hitting the telescope’s lens will
leave it pocked with little craters. Only 0.6 percent
of the lens should be pitted by the end of Parker’s
seven-year mission, according to computer simulations of dust in the inner solar system. But even
a few pits can obscure the real observations, so the
WISPR team wanted to minimize the damage by
choosing the right type of glass.
Howard and colleagues tested three possible
materials for the lens 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 micrometers wide, to speeds from half
a kilometer per second to 8 km/s — fast enough for
the scientists to extrapolate up to the dust speeds
Parker might experience.
Sapphire withstood the barrage best, but it
was unclear how it would behave as a lens. 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. Regular, uncoated
BK7 was the best option.
Running hot and cold
Aside from SWEAP and FIELDS, most of the
spacecraft will be tucked behind the all-important
heat shield and thus protected from the dust and
the sun’s extreme heat.
That 2.5-meter-wide heat shield is made of carbon foam sandwiched between two carbon sheets.
The whole thing is just 11. 5 centimeters thick and
coated on the sun-facing side with white ceramic
paint to reflect as much sunlight as possible. Even
This building-sized curved mirror focuses sunlight into
a tight beam and sends it into a vacuum chamber, where
the Parker probe’s parts were tested for heat tolerance.
A trip fit for Icarus
Each of the Parker Solar
Probe’s 24 orbits will
bring it closer to the sun.
The last three orbits will
take the probe within
6. 2 million kilometers of
the solar surface.
Earth to the sun
approach to the