An ESA facility is currently testing candidate materials for spacecraft to see if they can safely withstand the difficult aerobraking process the EnVision mission must perform: navigating its atmosphere.
“EnVision as it is currently designed cannot be done without this long phase of aerobraking“, explains Thomas Voirin, director of the ESA EnVision study, who adds that “the spacecraft will be injected into the orbit of Venus at a very high altitude, around 250,000 km, then you have to descend on a polar orbit of 500 km altitude for scientific operations. Flying in an Ariane 62, we can’t afford all the extra propellant it would take to lower our orbit. Instead, we will slow down with repeated passes through Venus’ upper atmosphere, reaching up to 130 km from the surface.”
EnVision’s predecessor spacecraft, Venus Express, performed experimental aerobraking during the last months of his mission in 2014, collecting valuable data on the technique. “Airbraking around Venus is going to be much more difficult than for TGO. For starters, Venus’ gravity is about 10 times that of Mars. This means the spacecraft will experience speeds about twice as high as for TGO. as it passes through the atmosphere, and heat is generated as a cube of velocity, so EnVision should aim for a lower aerobraking rate, resulting in a twice longer aerobraking phase,” said added Thomas.
“Besides that, we will also be much closer to the Sun, knowing about twice the solar intensity of the Earth, the thick white clouds in the atmosphere reflecting a large amount of sunlight directly into space, must also be taken into account. Then, on top of all that, we realized we had to consider another factor on the thousands of orbits we envision, previously only experienced in low Earth orbit: highly erosive atomic oxygen.”
Spectral observations of the airglow above the planet by previous Venus orbiters confirm that atomic oxygen is also prevalent in the upper part of the atmosphere of Venus, which is more than 90 times thicker than the air that surrounds the Earth. “The concentration is quite high, with one pass it doesn’t matter much, but after thousands of times it starts to build up and ends in an atomic oxygen fluence level that we have to being aware, equivalent to what you experience in low Earth orbit, but at higher temperatures,” explains ESA’s EnVision study director.
The EnVision team used a unique European facility built specifically by ESA to simulate atomic oxygen in orbit. The low Earth orbit facility, LEOX, is part of the Agency’s Electrical Components and Materials Laboratory, based at ESA’s ESTEC Technical Center in the Netherlands.
ESA Materials Engineer Adrian Tighe explains that “LEOX generates atomic oxygen at energy levels equivalent to orbital velocity. Purified molecular oxygen is injected into a vacuum chamber with a pulsed laser beam focused on it. This transforms the oxygen into a hot plasma whose rapid expansion is channeled along a conical nozzle. It then dissociates to form a highly energetic beam of atomic oxygen. To operate reliably, laser timing must remain millisecond-scale accurate and focused with precision measured in thousandths of a millimeter, throughout the four months of this current test campaign.
“This is not the first time the facility has been used to simulate an extraterrestrial orbital environment. We have previously performed atomic oxygen tests on candidate solar panel materials for ESA’s Juice mission, as telescopic observations suggest that atomic oxygen will be found in the atmospheres of Europa and Ganymede. However, for EnVision, the high temperature during aerobraking poses an additional challenge, so the setup has been adapted to simulate this more extreme Venusian environment,” he added.
The results of this test campaign are expected by the end of this year.