Venus has almost been “the forgotten planet”, with only one space mission going there in the last 30 years. But the recent resurgence of interest in Earth’s closest neighbor has NASA and ESA committing to three new missions to Venus, all slated for launch in the early 2030s.
ESA’s EnVision Venus mission is programmed to take high-resolution optical, spectral and radar images of the planet’s surface. But to do so, the van-sized spacecraft will have to perform a special maneuver called aerobraking to gradually slow down and lower its orbit through the planet’s hot, thick atmosphere. Aerobrake uses atmospheric drag to slow a spacecraft, and EnVision will make thousands of passes through Venus’ atmosphere over about two years.
The airbrake maneuver is a mission necessity.
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“EnVision, as currently conceived, cannot take place without this long aerobraking phase,” said Thomas Voirin, director of studies at EnVision. “The spacecraft will be injected into the orbit of Venus at a very high altitude, about 250,000 km, then we have to descend to a polar orbit of 500 km altitude for scientific operations. Flying in an Ariane 62, we can’t afford all the extra propellant that would be needed to lower our orbit.Instead, we’ll slow down through repeated passes through Venus’ upper atmosphere, reaching as low as 130 km from the surface.
Aerobraking has been performed by several spacecraft on Mars, including the Mars Reconnaissance Orbiter and the ExoMars Trace Gas Orbiter, to gradually slow the spacecraft to place it in the correct orbit for the mission parameters . But because of Venus’ ultra-thick atmosphere, ESA said they are currently testing candidate spacecraft materials to “check that they can safely withstand this challenging atmospheric surfing process.”
However, this won’t be the first time a spacecraft has used aerobraking on Venus. ESA’s Venus Express performed an experimental aerobraking during the final months of its mission in 2014, gathering valuable data on the technique. The Venus Express mission was supposed to last 500 days, but the sturdy spacecraft eventually spent eight years orbiting Venus before running out of fuel. It began a controlled descent, plunging deeper and deeper into Venus’ atmosphere, while using on-board accelerometers to measure its own deceleration.
Voirin said aerobraking around Venus is challenging because Venus’ gravity is about 10 times that of Mars. This means that the velocities are about twice as high as on Mars, the spacecraft passes through the atmosphere and the heat is generated as a speed cube. Consequently, EnVision must target a lower aerobraking regime, resulting in an aerobraking phase that is twice as long.
Artist’s impression of ESA’s EnVision mission to Venus. Credit: ESA/VR2Planets/Damia Bouic
“On top of that, we’ll also be much closer to the Sun, experiencing about twice the solar intensity of Earth, with the thick white clouds in the atmosphere reflecting a lot of sunlight directly into space, which it must also be taken into account,” said Voirin. “Then, on top of all that, we realized that we had to take into account another factor over the thousands of orbits we’re considering, which had previously only been experienced in low Earth orbit: highly atomic oxygen erosive”.
This is a phenomenon that remained unknown during the early decades of the space age. It was only when the first space shuttle flights returned from low orbit in the early 1980s that engineers got a shock: the spacecraft’s thermal blankets had been 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 torn apart by a powerful ultraviolet radiation from the Sun. Today, all missions below about 1,000 km must be designed to withstand 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 proved to consume unprotected thermal blankets during the shuttle’s early missions, until countermeasures were put in place. Credit: NASA
Spectral observations of past air-bright Venus orbiters above the planet confirm that atomic oxygen is also widespread in the upper part of the Venusian atmosphere, which is more than 90 times thicker than Earth’s.
Thomas says: “The concentration is quite high, with 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 fluence that we have to take into account, equivalent to what we have. experience in low Earth orbit, but at higher temperatures.”
ESA says the results of a materials test are expected later this year.
EnVision will use a series of instruments to take 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. Both of these missions are targeted for launch between 2028 and 2030.
Further reading: ESA EnVision mission briefing press release
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