The project's research director on behalf of the European Space Agency: "The flight on Ariane 6 will not allow all the extra propellant that would be necessary to lower the orbit. Instead we will slow ourselves down by making repeated passes through the upper atmosphere of Venus, lowering to 130 km from the face of the planet"
Venus has almost become the "forgotten planet", being the target of only one space mission in the last thirty years. But because of renewed interest in Earth's closest neighbor, NASA and the European Space Agency (ESA) have committed to three new missions to Venus, all to be launched in the early 1930s.
ESA's Envision mission is supposed to take high-resolution optical, spectral and radar images of the planet's surface. But to do so, the cargo-sized spacecraft will need to perform a special maneuver called atmospheric braking to gradually slow and lower its path through the hot, thick atmosphere of Venus Atmospheric braking uses atmospheric drag to slow a spacecraft and Envision will perform thousands transits through the atmosphere of Venus for about two years to achieve this.
The atmospheric braking maneuver is essential to the mission.
"Envision as it is now conceived could not happen without this long phase of atmospheric containment," said Envision's director of research, Thomas Warin. "The spacecraft will be thrown into orbit around Venus at a very high altitude, about 250,000 km, and then we will have to descend to a polar orbit at an altitude of 500 km for scientific activity. The flight on Ariane 6 will not allow all the extra propellant that will be needed to lower the orbit. Instead We will slow ourselves down by making repeated passes through the upper atmosphere of Venus, and will descend to 130 km from the planet".
Several spacecraft have done atmospheric braking on Mars, including the Mars Survey Orbit and the Exo-Mars Gas Remnant Orbit, to gradually slow themselves down to settle into the correct orbit for the mission parameters. But because of Venus' very thick atmosphere, ESA said they are now testing potential materials for the spacecraft to "test that they can safely withstand this challenging process of atmospheric drift."
This would not be the first time a spacecraft has used atmospheric braking on Venus. ESA's Venus Express performed atmospheric braking experiments 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 spacecraft ended up staying in orbit around Venus for eight years before running out of fuel. It began a controlled descent and descended further into Venus' atmosphere, using accelerometers on its return to measure its deceleration.
Warin said atmospheric containment around Venus is challenging because the planet's gravity is about ten times that of Mars. This means that the speeds are roughly twice as high as on Mars when the spacecraft passes through the atmosphere - and the heat generated is proportional to the speed to the third power. Accordingly, Envision should aim for a lower regime of atmospheric braking, which would prolong the braking phase twice as long.
"In addition, we will also be much closer to the sun and the intensity of the sun will be roughly twice as high as in Israel, when the thick white clouds of the atmosphere reflect a lot of sunlight back into space, and that should also be taken into account," Warin said.
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