Venus was almost the “forgotten planet” with only one space mission in the last 30 years. But a recent resurgence of interest in Earth’s nearest neighbor has led NASA and ESA to commit to three new missions to Venus, all due to launch by the early 2030s.
ESA’s Venus EnVision mission is planned to take high-resolution optical, spectral and radar images of the planet’s surface. But to do that, the van-sized spacecraft will have to perform a special maneuver called an air stall to gradually slow and lower its orbit through the planet’s hot, dense atmosphere. Aerbraking uses atmospheric drag to slow a spacecraft, and EnVision will make thousands of passes through Venus’ atmosphere over about two years.
The air stop maneuver is necessary for the mission.
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“EnVision, as currently conceived, could not be realized without this extended air suspension phase,” said EnVision’s head of research Thomas Voarin. “The spacecraft will be injected into Venus orbit at a very high altitude, approximately 250,000 km, then we need to descend to a 500 km high polar orbit for science operations. Flying Ariane 62, we can’t afford all the extra fuel needed to lower our orbit. Instead, we will slow down through multiple passes through the upper atmosphere of Venus, reaching up to 130 km from the surface.
Aerbraking has been performed by several Mars spacecraft, such as the Mars Reconnaissance Orbiter and the ExoMars Trace Gas Orbiter, to gradually decelerate the spacecraft to place the spacecraft in the correct orbit for the mission parameters. But because of Venus’ ultra-dense atmosphere, ESA said it is currently testing candidate spacecraft materials to “check that they can safely withstand this challenging atmospheric surfing process.”
However, this would not be the first time a spacecraft has used an air stop on Venus. ESA’s Venus Express performed an experimental airdrop in the final months of its 2014 mission, gathering valuable data on the technique. The Venus Express mission was supposed to last 500 days, but the healthy spacecraft ended up spending eight years in orbit around Venus before running out of fuel. It began a controlled descent, sinking deeper and deeper into Venus’ atmosphere while using onboard accelerometers to measure its own deceleration.
Voirin said air-stopping around Venus is challenging because Venus’ gravity is about 10 times that of Mars. This means that velocities are about twice as high as on Mars as the spacecraft passes through the atmosphere – and heat is generated as the cube of the velocity. Accordingly, EnVision must target a lower air-stop mode, resulting in twice as long of an air-stop phase.
An artist’s impression of ESA’s EnVision mission to Venus. Credit: ESA/VR2Planets/Damia Buick
“In addition, we will also be much closer to the Sun, experiencing about twice the solar intensity of Earth, with the thick white clouds of the atmosphere reflecting a lot of sunlight straight back into space, which should further be taken into account attention,” Voaren said. “Then, on top of that, we realized we had to account for one more factor over the thousands of orbits we predict, which had previously only been observed in low Earth orbit: highly erosive atomic oxygen.”
This is a phenomenon that remained unknown during the early decades of the space age. It wasn’t until early space shuttle flights returned from low orbit in the early 1980s that engineers got a shock: the spacecraft’s thermal blankets had severely eroded.
The culprit turned out to be highly reactive atomic oxygen — individual oxygen atoms at the fringes of the atmosphere, the result of standard oxygen molecules of the kind found just above the ground being broken into pieces by powerful ultraviolet radiation from the Sun. Today, all missions below about 1,000 km must be designed to withstand atomic oxygen.
The tail of Space Shuttle Endeavor glows with atomic oxygen as seen during the STS-99 mission in February 2000. Highly corrosive atomic oxygen was found to corrode unprotected thermal blankets during early shuttle missions until countermeasures were put in place . Credit: NASA
Spectral observations from past Venus orbiters of airglow above the planet confirm that atomic oxygen is also widespread in the upper atmosphere of Venus, which is more than 90 times thicker than Earth’s.
Thomas says: “The concentration is quite high, in one pass it doesn’t matter that much, but over thousands of times it starts to build up and ends up with a level of atomic oxygen flux that we have to consider, equivalent to what we experience in low earth orbit but at higher temperatures.
ESA says test results of the materials are expected later this year.
EnVision will use a suite of instruments to perform comprehensive observations of Venus from its inner core to its upper atmosphere to better understand how Venus and Earth evolved so differently.
The other upcoming Venus missions are DAVINCI+, a mission to understand the atmospheric evolution of Venus, and VERITAS, a mission to better map the surface and subsurface of Venus. These two missions aim to launch between 2028 and 2030.
Further reading: ESA press release EnVision mission fact sheet
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