The planned European mission to explore Venus will depend on a challenging air-stop procedure to lower its orbit, which will test the thermal resistance of the spacecraft’s materials to their limits.
The EnVision missionwhich is expected to launch in the early 2030s, will study the geology and atmosphere of Venusthe hellish planet there could ever be it looked a lot like Earth but turned into a charred hostile world due to escape greenhouse effect.
To get EnVision to its target orbit, 310 miles (500 kilometers) above the surface of Venus (which is so hot it would melt lead), it will take thousands of passes through the dense atmosphere of the planet for a period of two years, the European Space Agency (ESA) announced in a statement (opens in new tab).
“EnVision, as currently conceived, cannot be realized without this extended air-stop phase,” ESA’s EnVision study lead, Thomas Voarin, said in the statement.
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The van-sized spacecraft that will launch Europe’s future Ariana 6 rocket, will not be able to carry enough fuel to decelerate to Venus orbit using on-board propulsion. Instead, it will use the air-stop procedure and follow a highly elliptical orbit that will periodically take it to within 80 miles (130 km) of the surface of Venus at its closest point and about 155,000 miles (250,000 km) from the planet at its farthest point.
The ESA previously used air braking to slow down ExoMars Trace Gas Orbiter before entering its science orbit Mars. But the atmosphere of Mars is much thinner than that of Venus and its gravity is much lower, which affects the speed of the orbiting spacecraft.
“Aerostopping around Venus will be much more challenging than for the Trace Gas Orbiter,” Voirin said. “The gravity of Venus is about 10 times that of Mars. This means that velocities about twice those for TGO will be experienced by the spacecraft when passing through the atmosphere, and heat is generated as the cube of the velocity. “
ESA briefly tested the airlock around Venus in the last months of Venus Express a mission that eventually spiraled toward the planet and burned up in the atmosphere in 2014. Since Venus Express was already at the end of its mission, spacecraft controllers weren’t concerned about heat damage to the spacecraft. EnVision, on the other hand, will be expected to explore Venus for at least four years.
Engineers are already busy developing the right materials that would allow EnVision to withstand the extreme conditions. In addition to the heat experienced during the air-stop procedure, the spacecraft will also be exposed to very high concentrations of highly reactive atomic oxygen. Atomic oxygen is a form of oxygen present in the upper layers of Earth’s atmosphere, which consists of a single oxygen atom. Atomic oxygen, enemy of all low Earth orbiting spacecraft, burned several NASA thermal blankets space shuttle missions in the 1980s.
Observations from previous Venus missions have shown that atomic oxygen is present in the upper atmosphere of Venus in concentrations similar to those around Earth.
“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,” Voirin said.
ESA is currently testing materials for their ability to withstand both the heat and concentration of atomic oxygen expected during EnVision’s air-stop, and hopes to have some candidate materials selected by the end of this year.
“We want to check that these parts are resistant to erosion and also retain their optical properties —meaning they don’t degrade or darken, which can have side effects in terms of their thermal behavior, because we have delicate scientific instruments that they have to maintain a set temperature,” Voarin said. “We also need to avoid flaking or off-gassing, which leads to pollution.”
Venus, sometimes thought of as Earth’s twin because of their similar sizes, has been a bit sidelined lately solar system researchers, as the potentially more habitable Mars (which is more likely to harbor signs of life) has become a favorite. But a 2020 study that found molecules that could be traces of living organisms in the planet’s sulfur-rich clouds sparked a new surge of interest in Venus.
In addition to Europa, NASA has plans to send orbiters to the hot planet: The DAVINCI+ and VERITAS missionswhich are expected to be launched between 2028 and 2030. Currently a lone spacecraft, Japan’s Akatsukiorbits Venus, studying its dense atmosphere in an attempt to unravel the mysteries of its harsh climate.
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