EnVision Aerobraking in Venus Atmosphere

EnVision Mission: Preparing Spacecraft To Surf Venus’ Hot, Thick Atmosphere

EnVision aerobraking in Venus atmosphere. Credit: ESA/ Paris Observatory / VR2Planets / Damia Bouic

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

Venus, the second planet from the sun, is named after the Roman goddess of love and beauty. After the moon, it is the second-brightest natural object in the night sky. Its rotation (243 Earth days) takes longer than its orbit of the Sun (224.7 Earth days). It is sometimes called Earth’s "sister planet" because of their similar composition, size, mass, and proximity to the Sun. It has no natural satellites.

” data-gt-translate-attributes=”[{” attribute=””>Venus that will perform optical, spectral and radar mapping of Earth’s sister planet. However, before getting down to work the van-sized spacecraft needs to ‘aerobrake’ – lowering its orbit with thousands of passages through the planet’s hot, thick atmosphere for up to two years. A unique ESA facility is currently testing candidate spacecraft materials to verify if they can safely withstand this challenging process of atmospheric surfing.

“EnVision as currently conceived cannot take place without this lengthy phase of aerobraking,” explains ESA’s EnVision study manager Thomas Voirin.

EnVision Spacecraft

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.”

ESA EnVision

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 for Simulated Aerobraking

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 Space Shuttle Aglow With Atomic Oxygen

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.”

Atomic Oxygen Generator

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 Samples Exposed to Atomic Oxygen

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 Samples Seen in Infrared

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

A range of materials and coatings from different parts of the EnVision spacecraft, including multi-layer insulation, antenna parts and star tracker elements are placed within a plate to be exposed to the purple-glowing LEOX beam. At the same time this plate is being heated to mimic the expected thermal flux, up to 350°C 662°F).

Thomas adds: “We want to check that these parts are resistant to being eroded, and also maintain their optical properties – meaning they do not degrade or darken, which might have knock-on effects in terms of their thermal behavior, because we have delicate scientific instruments that must maintain a set temperature. We also need to avoid flaking or outgassing, which lead to contamination.”

This current test campaign is part of a larger panel looking into EnVision aerobraking, including the use of a Venus climate database developed from previous mission results to estimate the local variability of the planet’s atmosphere to set safe margins for the spacecraft.

The results of this test campaign are expected at the end of this year.

ESA EnVision Mission

The EnVision mission to Venus will explore why Earth’s closest neighbor is so different. Credit: NASA / JAXA / ISAS / DARTS / Damia Bouic / VR2Planets

About EnVision

EnVision is an ESA-led mission in partnership with
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