The European Space Agency’s (ESA) EnVision mission to Venus will perform optical, spectroscopic and radar maps of our sister planet. But before getting down to business, the truck-sized spacecraft needs to “airbrake” – lowering its orbit by thousands of passes through the planet’s thick, hot atmosphere for up to two years. A unique ESA facility is currently testing candidate spacecraft materials to verify that they can safely withstand this challenging process of atmospheric surfing.
“EnVision as it is currently understood cannot occur without this long phase of atmospheric braking,” explains Thomas Voeren, director of the EnVision study at ESA.
“The spacecraft will be injected into the orbit of Venus at a very high altitude, approximately 250,000 km, and then we need to descend into a polar orbit 500 km in height for scientific operations. When flying an Ariane 62, we cannot afford all the additional fuel needed to lower our orbit. Instead, we will slow down our speed through frequent passes through Venus’ upper atmosphere, as high as 130 kilometers from the surface.”
EnVision’s previous spacecraft, Venus Express, performed an experimental air brake during the final months of its mission in 2014, gathering valuable data on the technology. Aerobraking was first used operationally in 2017 by the ExoMars Trace Gas Orbiter (TGO) to lower its orbit around the Red Planet over an 11-month period.
Thomas notes: “Atmospheric braking around Venus would be much more difficult than TGO. First of all, the gravity of Venus is about 10 times higher than that of Mars. This means that speeds twice the speeds of TGO the spacecraft will experience when passing through the atmosphere – Heat is generated as a cube of velocity.Accordingly, EnVision must target a lower air-brake system, resulting in an air-braking phase twice its length.
Moreover, we will also be much closer to the sun, seeing twice the density of the sun on Earth, with the thick white clouds of the atmosphere reflecting a lot of sunlight directly into space, which also needs to be taken into account. Above all, we realized that we had to take into account another factor on the thousands of orbits we envision, and which we had previously only seen in low Earth orbit: the highly corrosive atomic oxygen. “
This is a phenomenon that remained unknown during the first decades of the space age. Only when the early space shuttle flights returned from low orbit in the early 1980s did the engineers receive a shock: The spacecraft’s thermal blankets had been severely eroded.
The culprit turned out to be highly reactive atomic oxygen – single atoms of oxygen on the fringes of the atmosphere, as a result of standard above-ground-type oxygen molecules broken down by strong ultraviolet rays from the sun. Today, all missions less than 1,000 km must be designed to withstand atomic oxygen, such as Earth observation in Europe’s Copernnicus Sentinels or any instrument built for the International Space Station.
Spectroscopic observations by Venus’ pre-orbiters of the atmospheric flare above the planet confirm that atomic oxygen is diffused in the upper part of Venus’s atmosphere as well, which is more than 90 times thicker than the air around Earth.
“The concentration is very high, with one pass it doesn’t matter much but over thousands of times it starts accumulating and ends up with a level of atomic oxygen flux that we have to take into account, which is equivalent to an experiment in low Earth orbit, but at higher temperatures,” says Thomas. .”
The EnVision team turned to 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 Materials and Components Laboratory, based at ESA’s ESA Technical Center in the Netherlands.
Adrian Tighe, ESA Materials Engineer, explains: “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 converts oxygen into hot plasma whose rapid expansion is directed along a conical nozzle. Then it separates to form a high-energy beam of atomic oxygen.
To work reliably, the laser’s timing must remain millisecond accurate, and directed with an accuracy of one thousandth of a millimeter, throughout the four-month period of the current test campaign.
“This isn’t the first time the facility has been used to simulate an extraterrestrial orbital environment – we’ve previously performed atomic oxygen testing on candidate solar array materials for the European Space Agency’s Juice mission, because telescopic observations indicate that atomic oxygen will be found in Europa’s atmosphere. However, for EnVision, the high temperature during air braking poses an additional challenge, so the facility has been adapted to simulate the more extreme environment of Venus.”
A range of materials and coatings from various parts of the EnVision spacecraft, including multi-layer insulation, antenna parts and star-tracking elements, are placed inside a panel to be exposed to a glowing purple LEOX beam. At the same time this plate is heated to mimic the expected heat flow, up to 350°C.
Thomas adds: “We want to verify that these parts are resistant to corrosion, and also maintain their optical properties – meaning that they do not degrade or darken, which could have indirect effects in terms of their thermal behavior, because we have sensitive scientific instruments that must maintain a temperature Specific. We also need to avoid precipitation or outgassing, which leads to pollution.”
This current test campaign is part of a larger panel looking at EnVision aerobraking, including using a Venus climate database developed from results from a previous mission to estimate the local variability of the planet’s atmosphere to map safe margins for the spacecraft.
The results of this test campaign are expected at the end of this year.
EnVision is a mission led by the European Space Agency in partnership with NASA, providing the Synthetic Aperture Radar instrument, VenSAR and deep space network support for mission critical phases. EnVision will use a suite of tools to make comprehensive observations of Venus from its inner core to the upper atmosphere to better understand how Earth’s closest neighbors in the Solar System evolved differently.
EnVision has been selected by the ESA Science Program Committee as the fifth intermediate mission in the agency’s cosmic vision plan, targeting launch in the early 2030s.
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