Venus has almost been “the forgotten planet”, with only one space mission in the last 30 years. But the recent resurgence of interest in Earth’s nearest neighbor has seen NASA and ESA commit to three new missions to Venus, all scheduled to launch in the early 2030s.
ESA’s EnVision Venus mission it 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 thick, hot atmosphere. Aerobraking uses atmospheric drag to slow a spacecraft, and EnVision will make thousands of passages through Venus’s atmosphere over about two years.
The aerobraking maneuver is a necessity for the mission.
“EnVision, as currently conceived, cannot be done without this long aerobraking phase,” said EnVision studio director Thomas Voirin. “The spacecraft will be injected into the orbit of Venus at a very high altitude, approximately 250,000 km, then we must go down to a polar orbit of 500 km altitude for science operations. Flying in an Ariane 62, we can’t afford all the extra propellant it would take to lower our orbit. Instead, we will slow down through repeated passes through Venus’s upper atmosphere, reaching as low as 130 km from the surface.”
Aerobraking has been performed by several spacecraft on Mars, such as the Mars Reconnaissance Orbiter and the ExoMars Trace Gas Orbiter, to gradually slow the spacecraft down and place it in the correct orbit for mission parameters. But due to the ultra-dense atmosphere of Venus, THAT said that they are currently testing candidate spacecraft materials to “verify that they can safely withstand this challenging process of atmospheric navigation.”
However, this will not be the first time a spacecraft has used aerobraking on Venus. ESA’s Venus Express performed experimental aerobraking during the final months of its mission in 2014, collecting valuable data on the technique. The Venus Express mission was supposed to last 500 days, but the robust craft ultimately spent eight years orbiting Venus before running out of fuel. It began a controlled descent, sinking deeper and deeper into Venus’s atmosphere, while using onboard 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 velocities are about twice as high as on Mars, the spacecraft passes through the atmosphere, and heat is generated as a velocity cube. Consequently, EnVision has to target a lower aerobraking rate, resulting in twice as long an aerobraking phase.
“In addition to that, 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 directly back into space, which must also be in space. account,” Voirin said. “Then, on top of all that, we realized we had to account for another factor over the thousands of orbits we envision, previously only experienced in low Earth orbit: highly erosive atomic oxygen.”
This is a phenomenon that remained unknown during the first decades of the space age. It was only when the first space shuttle flights returned from low orbit in the early 1980s that engineers received a surprise: 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 just above ground being broken apart by the sun’s powerful ultraviolet radiation. Today, all missions below 1,000 km must be designed to withstand atomic oxygen.
Spectral observations of the airglow over the planet by previous Venus orbiters confirm that atomic oxygen is also widespread in the upper part of Venus’s 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 as much, but after thousands of times it starts to build up and ends up with a level of atomic oxygen fluence that we need to be aware of, equivalent to what we experience in Earth orbit.” low, but at higher temperatures.
The ESA says the results of a materials test are expected later this year.
EnVision will use a variety of instruments to make comprehensive observations of Venus from its inner core to the 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. Those two missions are targeted for launch between 2028 and 2030.
Other readings: ESA press release
EnVision Mission Fact Sheet