Why won't NASA's Parker Solar Probe melt? (Lead)
Washington : With NASA launching a historic Parker Solar Probe deeper into the solar atmosphere than any mission before it, a moot question arises: Why won't it melt?
Inside the solar atmosphere -- a region known as the corona -- the probe will provide observations of what drives the wide range of particles, energy and heat that course through the region.
The spacecraft will travel through material with temperatures greater than several million degrees Celsius while being bombarded with intense sunlight.
According to the US space agency, Parker Solar Probe has been designed to withstand the extreme conditions and temperature fluctuations for the mission.
"The key lies in its custom heat shield and an autonomous system that helps protect the mission from the Sun's intense light emission, but does allow the coronal material to 'touch' the spacecraft," NASA said in a statement.
While the Parker Solar Probe will travel through a space with temperatures of several million degrees, the surface of the heat shield that faces the Sun will only get heated to about 1,400 degree Celsius.
This is because "in space, the temperature can be thousands of degrees without providing significant heat to a given object or feeling hot. Since space is mostly empty, there are very few particles that can transfer energy to the spacecraft".
The corona through which the Parker Solar Probe flies, for example, has an extremely high temperature but very low density.
The probe makes use of a heat shield known as the Thermal Protection System, or TPS, which is eight feet in diameter and 4.5 inches thick.
Those few inches of protection mean that just on the other side of the shield, the spacecraft body will sit at a comfortable 30 degrees Celsius.
The TPS was designed by the Johns Hopkins Applied Physics Laboratory, and was built at Carbon-Carbon Advanced Technologies, using a carbon composite foam sandwiched between two carbon plates.
This lightweight insulation will be accompanied by a finishing touch of white ceramic paint on the sun-facing plate, to reflect as much heat as possible.
"Tested to withstand up to 1,650 degrees Celsius, the TPS can handle any heat the Sun can send its way, keeping almost all instrumentation safe," said NASA.
Another challenge came in the form of the electronic wiring -- most cables would melt from exposure to heat radiation at such close proximity to the Sun.
To solve this problem, the team grew sapphire crystal tubes to suspend the wiring, and made the wires from the chemical element niobium.
Several other designs on the spacecraft keep Parker Solar Probe sheltered from the heat.
Without protection, the solar panels -- which use energy from the very star being studied to power the spacecraft -- can overheat.
At each approach to the Sun, the solar arrays retract behind the heat shield's shadow, leaving only a small segment exposed to the Sun's intense rays.
The solar arrays have a surprisingly simple cooling system: a heated tank that keeps the coolant from freezing during launch, two radiators that will keep the coolant from freezing, aluminium fins to maximise the cooling surface, and pumps to circulate the coolant.
The spacecraft, launched from Cape Canaveral Air Force Station in Florida on August 12, will transmit its first scientific observations in December.