With only one space mission having visited Venus in the previous 30 years, it was near “the forgotten planet.” However, due to the current upsurge of interest in Earth’s nearest neighbor, NASA and ESA have committed to three new missions to Venus, all of which are scheduled to launch by the early 2030s.
The EnVision mission of the European Space Agency Venus is planned to take high-resolution optical, spectral, and radar photographs of the planet’s surface. However, in order to accomplish this, the van-sized spacecraft will need to engage in a unique maneuver known as aerobraking to gradually slow down and lower its orbit through the planet’s scorching, dense atmosphere. EnVision will travel through Venus’ atmosphere tens of thousands of times over the course of around two years using aerobraking to slow down the spacecraft.
According to EnVision study manager Thomas Voirin, “EnVision as currently designed cannot take place without this significant aerobraking phase. “The spacecraft will be launched into Venus orbit at a very high altitude, over 250,000 km, before descending to a polar orbit at a 500 km altitude for science operations. We cannot afford to use all the additional propellant necessary to decrease our orbit while flying on an Ariane 62. Instead, we will gradually slow down by making multiple passes through Venus’ upper atmosphere, going as close to the planet as 130 km.
Numerous Mars-based spacecraft, including the Mars Reconnaissance Orbiter and the ExoMars Trace Gas Orbiter, have used aerobraking to gradually slow them down and set them in the ideal orbit for their mission objectives. However, because of Venus’ extraordinarily thick atmosphere, the ESA reported that they are now investigating potential spacecraft materials to “ensure they can safely resist this demanding process of atmospheric surfing.”
A spacecraft has already employed aerobraking at Venus, though not this one. During the latter months of its mission in 2014, ESA’s Venus Express engaged in experimental aerobraking, collecting insightful data on the process. The Venus Express mission was only meant to last 500 days, but the hardy spacecraft ended up orbiting Venus for eight years before running out of fuel. Using onboard accelerometers to gauge its own deceleration, it started a controlled descent and descended deeper and deeper into Venus’ atmosphere.
The gravity of Venus is nearly ten times greater than that of Mars, according to Voirin, making aerobraking around Venus difficult. This indicates that the spacecraft travels at speeds that are roughly twice as high as those on Mars as it passes through the atmosphere, where heat is produced as a function of velocity. EnVision is forced to aim for a lower aerobraking regime as a result, lengthening the aerobraking phase by two times.
The dense white clouds of the atmosphere reflect a lot of sunlight directly back to space, which is another factor that must be considered, added Voirin. “On top of that, we are also going to be much closer to the Sun, experiencing about double the solar strength of Earth,” he added. Then, on top of everything else, we found we had to take into account another element throughout the thousands of orbits we envision: highly-erosive atomic oxygen. This factor was previously only experienced in low Earth orbit.
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 with a range of fewer than 1,000 kilometers must be built to withstand atomic oxygen.
Atomic oxygen is also widely distributed above the top of Venus’ atmosphere, which is over 90 times thicker than Earth’s. This is confirmed by spectral analyses of airglow made by previous Venus orbiters of the planet.
“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 up with a level of atomic oxygen flux we have to take into account, equal to what we encounter in low-Earth orbit, but at higher temperatures,” explains Thomas.
The findings of a material test, according to ESA, are anticipated towards the end of this year.
In order to better understand why Venus and Earth evolved in such distinct ways, EnVision will use a variety of sensors to conduct thorough examinations of Venus from its deep core to the upper atmosphere.
DAVINCI+, a mission to comprehend Venus’ atmospheric development, and VERITAS, a mission to more accurately map Venus’ surface and subsurface, are the other forthcoming Venus missions. Between 2028 and 2030 are the target launch dates for those two missions.
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