Venus will be mapped using optical, spectral, and radar data from the EnVision mission to Venus. However, the van-sized spacecraft must first “aerobrake,” or reduce its orbit, by passing through the planet’s hot, dense atmosphere hundreds of times for up to two years. Candidates for spacecraft materials are presently being tested at a unique ESA laboratory to see if they can safely resist the difficult process of atmospheric surfing. According to Thomas Voirin, research manager for EnVision at ESA, “EnVision as currently planned cannot take place without this lengthy phase of aerobraking.”
The spacecraft will enter Venus’ orbit at a very high height, around 250 000 km, before being lowered to a polar orbit at a 500 km altitude for science operations. We are utilising an Ariane 62, thus we are unable to purchase all the additional propellant necessary to reduce our orbit. Instead, we’ll slow down by repeatedly entering Venus’ upper atmosphere, flying as close to the planet as 130 km.
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During the closing months of its mission in 2014, Venus Express, EnVision’s forerunner spacecraft, conducted experimental aerobraking and gathered important data on the process. The ExoMars Trace Gas Orbiter (TGO) of the European Space Agency (ESA) lowered its orbit around the Red Planet over an 11-month period by employing aerobraking for the first time. Aerobraking around Venus will be significantly more difficult than for TGO, Thomas observes. First off, Venus has a gravity that is roughly ten times greater than Mars’.
Because heat is produced as a cube of velocity, the spacecraft will experience velocities through the atmosphere that are almost twice as high as for TGO. As a result, EnVision must aim for a lower aerobraking regime, which lengthens the aerobraking phase by two. “On top of that, we will be much closer to the Sun than Earth and will experience solar activity that is about twice as intense. In addition, the atmosphere’s dense white clouds will reflect a significant amount of sunlight back into space, which must also be considered. Then, on top of everything else, we realised we had to take into account another aspect throughout the millions of orbits we envision: highly erosive atomic oxygen.
In the early years of the space era, nobody was aware of this phenomenon. Engineers weren’t shocked to discover that the spacecraft’s thermal blankets had been substantially damaged until early Space Shuttle flights started returning from low orbit in the early 1980s. Highly reactive atomic oxygen, or individual oxygen atoms at the edges of the atmosphere, was revealed to be the culprit. These atoms are the end product of ordinary oxygen molecules of the type found just above the ground being split apart by potent ultraviolet radiation from the Sun.
Today, all missions below around 1 000 km must be engineered to withstand atomic oxygen, including any hardware created for the International Space Station or Europe’s Copernicus Sentinels, which monitor the Earth. Atomic oxygen is widely distributed at the top of the Venusian atmosphere, which is more than 90 times thicker than the air around the Earth, according to spectral measurements of airglow above the planet by previous Venus orbiters. “The concentration is fairly high; with one pass it doesn’t matter so much but with thousands of passes it starts to collect and ends with a level of atomic oxygen flux we have to take into account, equal to what we encounter in low-Earth orbit,” explains Thomas.
The EnVision team turned to a special European facility that the ESA had constructed especially to model atomic oxygen in orbit. The Agency’s Materials and Electrical Components Laboratory, located at ESA’s ESTEC technical centre in the Netherlands, houses the Low Earth Orbit Facility, or LEOX. “LEOX creates atomic oxygen at energy levels that are similar to orbital speed,” says ESA materials expert Adrian Tighe. Purified molecular oxygen is introduced into a vacuum chamber while being targeted by a pulsed laser beam.
As a result, oxygen is transformed into a heated plasma whose quick expansion is directed along a conical nozzle. Then it splits apart into an extremely powerful beam of atomic oxygen. “Over the four-month course of this present test campaign, the laser timing must remain accurate to millisecond scale and directed to an accuracy measured in thousandths of a millimetre.
The facility has previously been used to simulate an extraterrestrial orbital environment, and we tested potential solar array materials for the ESA’s Juice mission using atomic oxygen because telescopic observations indicate that atomic oxygen will be present in the atmospheres of Europa and Ganymede. The facility has been modified to imitate this more harsh Venusian environment because EnVision finds that the increased temperature during aerobraking is an extra challenge.
Materials and coatings from various EnVision spacecraft components, such as multi-layer insulation, antenna components, and star tracker elements, are arranged inside a plate to be exposed to the LEOX beam, which emits a purple glow. This plate is heated up to 350°C at the same time to simulate the anticipated thermal flux. Thomas continues, “We want to make sure that these parts are not only resistant to eroding, but also maintain their optical properties, which means they don’t deteriorate or darken, as this could affect how they behave thermally because we have sensitive scientific instruments that need to maintain a certain temperature.
Additionally, we must prevent flaking and outgassing because they can cause contamination. This test campaign is a component of a larger investigation into EnVision aerobraking, which also includes the use of a climate database for Venus created from data from earlier missions to determine the local variability of the planet’s atmosphere and determine safe operating distances for the spacecraft.
At the end of this year, the test campaign’s results are anticipated.
The Synthetic Aperture Radar instrument, VenSAR, and Deep Space Network help EnVision, a NASA and ESA-led mission, during key mission phases. EnVision will observe Venus holistically, from its inner core to its high atmosphere, using a variety of instruments in order to better understand why Earth’s nearest neighbour in the Solar System evolved so differently. EnVision, which aims to launch in the early 2030s, has been chosen as the fifth Medium-class mission in the Cosmic Vision programme of the ESA by its Science Programme Committee.