A proposed NASA mission to send a probe outside the solar system to study the Sun’s interstellar environment and the heliosphere, the vast bubble-like region marking the extent of the solar wind, is gaining momentum at the Johns Hopkins Applied Physics Laboratory.
Now in the final year of a four-year concept study, engineers envision launching the probe into the inner solar system for an extremely close flyby of the Sun, a gravitational slingshot that will provide the kick needed to boost the craft on a relatively quick trajectory out of the heliosphere and into interstellar space.
But flying within a 1.6 million kilometres (1 million miles) of the Sun will expose the craft to temperatures at high as 2,480 degrees Celsius (4,500 Fahrenheit), hot enough to melt most spacecraft hardware.
But Jason Benkoski, a materials scientist at the Applied Physics Lab, says the team has come up with a possible solution: a heat shield incorporating tiny channels filled with hydrogen. As the probe races around the Sun, the gas will heat up, expand and shoot out an exhaust nozzle, providing cooling and additional thrust.
“The idea is to absorb all this heat with hydrogen and shoot it out the back of the probe,” Benkoski said. “It’s like hitting two birds with one stone.”
Solar thermal propulsion is not a new idea, but the APL team may be the first to build and test a prototype. Using a solar simulator originally built to test the Parker Solar Probe, banks of LEDs shining with 20 times the intensity of the noonday Sun heated metal tiles incorporating small tubes, or channels, using helium as a stand in for hydrogen. The heated helium expanded and rushed through the channels as expected, producing thrust.
With the proof-of-concept test behind them, engineers now plan to built more sophisticated prototypes using lighter materials, more intense heating and, eventually, hydrogen coolant.
If approved by NASA for actual development, the probe could launch in the 2030s and reach the edge of the heliosphere about 15 years later. The goal is to study the heliosphere from a vantage point at least 50 billion miles out, about 10 times farther than the twin Voyager spacecraft have traveled since launch in the 1970s.
Voyager 1 crossed the outer edge of the heliosphere and entered interstellar space in 2012 while Voyager 2 reached that milestone in 2018. But the probes, built for a “grand tour” of the outer planets, were not designed for interstellar observations and in any case, both are nearing the end of their lifetimes.
“The Interstellar Probe will go to the unknown local interstellar space, where humanity has never reached before,” said Elena Provornikova, the Interstellar Probe heliophysics lead at APL “For the first time, we will take a picture of our vast heliosphere from the outside to see what our solar system home looks like.”