An ESA project, EnVision, is a mission to Venus that will map the Earth's sister planets optical, spectral and radar . Before starting work, however, the van-sized spacecraft needed to " air brake " - lower its orbit, and make thousands of flights through the planet's high-temperature and heavy atmosphere over a period of up to two years. A unique ESA facility is currently testing candidate spacecraft materials to verify that they can safely withstand this challenging atmospheric "surfing" process.
European Space EnVision Research Manager Thomas "Without this long air braking phase, the envisioned EnVision would not be possible at present," Voirin explained. "
" spacecraft would enter Venus orbit at very high altitudes, about 250,000 kilometers (about 150,000 miles), and then we would need to descend to a polar orbit at an altitude of 500 kilometers (about 300 miles) for scientific operations. Flying on the Ariane 62 rocket, we could not afford all the additional propellant needed to lower the orbit. Instead, we would slow ourselves down by repeatedly traversing the upper atmosphere of Venus, as low as 130 kilometers (80 miles) from the surface."
EnVision's predecessor spacecraft, Venus Express , conducted experimental air braking in the last months of the 2014 mission, gathering valuable data on the technology. In 2017, ESA's ExoMars Trace Gas Orbiter (TGO) used air brakes for the first time, reducing its orbit around the Red Planet for 11 months.
Thomas notes: "The air braking around Venus will be much greater than the challenge of TGO. First, Venus' gravity is about 10 times higher than that of Mars . This means that the spacecraft will be twice as fast as TGO when passing through the atmosphere. Therefore, EnVision must target a lower air braking regime, thus double the time of the air braking phase."
" excluding In addition, we will be closer to the sun, experiencing about twice the intensity of the Earth's sun, and the thick white clouds in the atmosphere reflect a lot of sunlight directly into space, which also needs to be taken into account. Then above all of this, we realize that we have to calculate another factor in the thousands of orbits we envisioned, which has only been experienced in low-Earth orbits: highly erosive atomic oxygen. "
This particular phenomenon remains unknown in the first few decades of the space age. It was not until the early 1980s when the early shuttle flight returned from low orbit that engineers were hit: the spacecraft's heat shield had been severely eroded.
The culprit turned out to be the highly reactive atomic oxygen - a single oxygen atom at the edge of the atmosphere, which is the result of the decomposition of the standard oxygen molecule found on the ground by powerful ultraviolet radiation from the sun. Today, all tasks below about 1,000 kilometers (about 620 miles) need to be designed to resist atomic oxygen, such as the Copernican sentinel that European observes the Earth or any hardware built for International Space Station .
The spectral observations of gas light above the planet by the Venus Orbiter of the past confirmed that atomic oxygen is also common at the top of the Venus atmosphere , which is more than 90 times thicker than the air around the Earth. "The concentration is quite high, and it doesn't matter at a time, but after thousands of times it starts to accumulate and eventually reaches the level of atomic oxygen flux that we have to consider, equivalent to what we experienced in low Earth orbits, but with higher temperatures," said Thomas. The
EnVision team turned to a unique European facility built specifically by the ESA to simulate atomic oxygen on orbits. The Low Earth Orbit Facility, LEOX, is part of the ESA Materials and Electrical Components Laboratory, located at the ESA ESTEC Technology Centre in , the Netherlands .
ESA materials engineer Adrian Tighe explained: "LEOX generates atom oxygen at an energy level equivalent to orbital speed. The purified molecular oxygen is injected into a vacuum chamber and a pulsed laser is focused on it. This converts oxygen into a hot plasma, which rapidly expands along a conical nozzle and is directed. It then dissociates to form a high-energy atomic oxygen."
" To work reliably, the laser timing must be kept accurate to milliseconds and the directional accuracy reaches one thousandth of a millimeter over the four months of the current test activity. "
" This is not the first time the facility has been used to simulate extraterrestrial orbital environments - we have previously conducted atomic oxygen tests on candidate solar cell array materials for ESA's Juice mission, because telescopic observations show that atomic oxygen will be found in the atmospheres of Europa and Europa . However, for EnVision, the high temperatures during air braking present additional challenges, so the facility has been tuned to simulate this more extreme Venus environment. ”
A range of materials and coatings from different parts of the EnVision spacecraft, including multi-layer insulation , antenna components and interstellar tracker elements, are placed in a board exposed to a purple glowing LEOX beam. Meanwhile, the board is heated to simulate the expected heat flux up to 350°C.
Thomas added: “We want to check if these parts are resistant to erosion while maintaining their optical properties – meaning they do not degrade or darken, which may have a chain reaction to their thermal behavior, as we have exquisite scientific instruments that must maintain a set temperature.” We also need to avoid peeling or deflation, which leads to contamination. "
The current test activity is part of a larger team studying EnVision air braking, including the use of Venus climate database developed from previous mission results to estimate local changes in the planet's atmosphere in order to set safety margins for the spacecraft. The results of this test activity are expected to be released by the end of this year.
