ESA's EnVision Mission Doesn't Have Much Fuel, So Will Aerobrake in Venus' Atmosphere

Venus has almost become a “forgotten planet”, with only one space mission carried out there in the last 30 years. But the recent resurgence of interest in Earth’s closest neighbor led NASA and ESA to commit to three new missions to Venus, all set to launch in the early 2030s.

ESA’s EnVision Venus mission is scheduled to take high-resolution optical, spectral and radar images of the planet’s surface. But to do so, the van-sized spacecraft would need to perform a special maneuver called aerobraking to gradually slow down and lower its orbit through the planet’s hot, thick atmosphere. Aerobraking uses atmospheric drag to slow the spacecraft and EnVision will make thousands of passes through Venus’ atmosphere over about two years.

Aerobraking maneuvers are a necessity for the mission.

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“EnVision as it is currently understood could not have occurred without this lengthy aerobraking phase,” said EnVision study manager Thomas Voirin. “The spacecraft will be injected into the orbit of Venus at a very high altitude, about 250,000 km, then we need to descend into a polar orbit of 500 km altitude for science operations. Flying the Ariane 62, we couldn’t afford all the extra propellant needed to lower our orbit. Instead, we will slow ourselves down through a repeated trajectory through Venus’ upper atmosphere, coming as low as 130 km from the surface.”

Aerobraking has been carried out by several spacecraft on Mars, such as the Mars Reconnaissance Orbiter and the ExoMars Trace Gas Orbiter, to gradually slow down the spacecraft to place the spacecraft in the correct orbit for the mission parameters. But because of Venus’ very thick atmosphere, ESA says they are currently testing candidate spacecraft materials to “check whether they can safely withstand this challenging process of atmospheric surfing.”

However, this is not the first time a spacecraft has used aerobraking on Venus. ESA’s Venus Express, performing experimental aerobraking during the final months of its mission in 2014, gathered valuable data on the technique. The Venus Express mission was supposed to last 500 days, but the powerful spacecraft ended up spending eight years orbiting Venus before running out of fuel. It begins to descend in a controlled manner, further into Venus’ atmosphere, while using the onboard accelerometer to measure its own deceleration.

Voirin said aerobraking around Venus is a challenge because Venus’ gravity is about 10 times higher than that of Mars. This means the speed is about twice as high as on Mars, the spacecraft passes through the atmosphere – and heat is generated as a cube of velocity. Therefore, EnVision had to target a lower aerobraking regime, resulting in a twice as long aerobraking phase.

Artist’s impression of ESA’s EnVision mission on Venus. Credits: ESA/VR2Planets/Damia Bouic

“In addition, we will also be closer to the Sun, experiencing about twice the intensity of Earth’s sun, with thick white clouds in the atmosphere reflecting a lot of direct sunlight back into space, which is also something to watch out for. taken into account,” said Voirin. “Then on top of that, we realized we had to factor in another factor for the thousands of orbits we envisioned, which had previously only been experienced in low-Earth orbits: highly erosive atomic oxygen.”

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

The culprit turned out to be highly reactive atomic oxygen – individual oxygen atoms in the periphery of the atmosphere, the result of standard oxygen molecules of the kind found just above the ground being broken up by the Sun’s intense ultraviolet radiation. Currently, all missions below about 1,000 km need to be designed to contain atomic oxygen.

The tail of the Space Shuttle Endeavor glows with atomic oxygen, as seen during the STS-99 mission in February 2000. The highly erosive atomic oxygen apparently eats away at the unprotected thermal blanket during the initial Shuttle mission, until precautions are taken. Credit: NASA

Spectral observations by past Venus orbiters of light above the planet confirm that oxygen atoms are also widely dispersed in the upper atmosphere of Venus, which is more than 90 times thicker than Earth’s.

Thomas says: “The concentration is quite high, with one pass it doesn’t really matter but over thousands of times it starts to build up and ends up with a level of influence of atomic oxygen that we have to take into account, equivalent to what we experience in low Earth orbit, but at a higher temperature. tall.”

ESA said material test results are expected by the end of this year.

EnVision will use a series of instruments to conduct comprehensive observations of Venus from the inner core to the upper atmosphere to better understand how Venus and Earth evolved so differently.

Other upcoming Venus missions are DAVINCI+, a mission to understand the evolution of Venus’ atmosphere, and VERITAS, a mission to better map the surface and subsurface of Venus. Both missions aim to launch between 2028 and 2030.

Further reading: ESA press release
EnVision mission fact sheet

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