Readying Spacecraft To Surf Venus’ Hot, Thick Atmosphere

EnVision, a project of the European Space Agency (ESA), is a mission to

Artist impression of ESA’s EnVision mission. Credit: ESA/VR2Planets/Damia Bouic

“The spacecraft will be injected into Venus orbit at a very high altitude, at approximately 250,000 km (~150,000 miles), then we need to get down to a 500 km (~300 mile) altitude polar orbit for science operations. Flying on an Ariane 62 rocket, we cannot afford all the extra propellant it would take to lower our orbit. Instead we will slow ourselves down through repeated passes through the upper atmosphere of Venus, coming as low as 130 km (80 miles) from the surface.”

Artist impression of ESA’s EnVision mission at Venus. EnVision needs to ‘aerobrake’ through Venus’ atmosphere. Credit: ESA/VR2Planets/DamiaBouic

EnVision’s predecessor spacecraft, Venus Express, performed experimental aerobraking during the final months of its mission in 2014, gathering valuable data on the technique. Aerobraking was used operationally for the first time in 2017 by ESA’s ExoMars Trace Gas Orbiter (TGO) to lower its orbit around the Red Planet over an 11 month period.

Samples of candidate materials for different parts of the EnVision spacecraft were subjected to simulated aerobraking conditions including orbital-velocity atomic oxygen and heat flux using ESA’s LEOX facility. Credit: ESA

Thomas notes: “Aerobraking around Venus is going to be much more challenging than for TGO. To begin with, the gravity of Venus is about 10 times higher than that of

This particular phenomenon remained unknown during the first decades of the space age. It was only when early Space Shuttle flights returned from low orbit in the early 1980s that engineers received a shock: the spacecraft’s thermal blankets had been severely eroded.

The culprit turned out to be highly reactive atomic oxygen – individual atoms of oxygen at the fringes of the atmosphere, the result of standard oxygen molecules of the kind found just above the ground being broken apart by powerful ultraviolet radiation from the Sun. Today, all missions below about 1,000 km (~620 miles) need to be designed to resist atomic oxygen, such as Europe’s Earth-watching Copernicus Sentinels or any hardware built for the International Space Station.

Space Shuttle Endeavour’s tail aglow with atomic oxygen, as seen during the STS-99 mission in February 2000. Highly erosive atomic oxygen turned out to eat away at unprotected thermal blankets during early Shuttle missions, until countermeasures were put in place. Credit: NASA

Spectral observations by past Venus orbiters of airglow above the planet confirm that atomic oxygen is widespread at the top of the Venusian atmosphere too, which is more than 90 times thicker than Earth’s surrounding air.

Thomas says: “The concentration is quite high, with one pass it doesn’t matter so much but over thousands of times it starts to accumulate and ends up with a level of atomic oxygen fluence we have to take account of, equivalent to what we experience in low-Earth orbit, but at higher temperatures.”

ESA’s new LEOX, Low Earth Orbit Facility, being fired for the first time in April 2017. This new simulator that fires a laser to generate ‘atomic oxygen’ normally encountered only in low orbits – and known to eat away at satellite surfaces. LEOX generates atomic oxygen at energy levels that are equivalent to orbital speed – 7.8 km/s – to simulate the space environment as closely as possible. It can also test at a higher flow, saving time and money for testing. Purified molecular oxygen is injected into a vacuum chamber with a pulsing laser beam focused onto it. With a purple flash each time the laser is fired, the oxygen is converted 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. The new facility is housed in the Materials and Electrical Components Laboratory, one of a suite of labs at ESA’s technical center in the Netherlands, devoted to simulating every aspect of the space environment. Credit: ESA, CC BY-SA 3.0 IGO

The EnVision team turned to a unique European facility specifically built by ESA to simulate atomic oxygen in orbit. The Low Earth Orbit Facility, LEOX, is part of the Agency’s Materials and Electrical Components Laboratory, based at ESA’s ESTEC technical center in the Netherlands.

ESA materials engineer Adrian Tighe explains: “LEOX generates atomic oxygen at energy levels that are equivalent to orbital speed. Purified molecular oxygen is injected into a vacuum chamber with a pulsing laser beam focused onto it. This converts the oxygen into a hot

EnVision candidate materials samples exposed to atomic oxygen in ESA’s LEOX generator. Credit: ESA

“To work reliably, the laser timing must stay precise to millisecond scale, and directed to an

EnVision candidate materials sample observed by infrared camera. The samples are also heated as they are exposed to atomic oxygen by the LEOX generator to better simulate aerobraking through the atmosphere of Venus. Credit: ESA