As the ESA’s Venus Express mission came to a close, scientists pushed the spacecraft to its limits. In its final months, the spacecraft dipped into Venus’ atmosphere and experienced aerobraking. This is atmospheric drag that slows the spacecraft down.

“We were able to use the accelerometer measurements to explore the density of Venus’ atmosphere,” said Ingo Müller-Wodarg of Imperial College London, UK and lead author of the study. Plans to use aerobraking weren’t even in the works when the mission launched in 2005. It wasn’t until right after launch did the team realize how much more science they could accomplish with Venus Express.

They just had to wait until the mission was coming to a close. And they waited a while. The original mission was only scheduled to last for 500 days. Venus Express, like many space missions, blew past the expectations and lasted eight years.

When the time finally came to study Venus’ atmosphere, the spacecraft was positioned between 130 and 140 kilometers above Venus’ polar regions. An area of the atmosphere never studied from directly within. All we knew about Venus’ polar atmosphere is what we inferred from data gathered by NASA’s Pioneer Venus probe in the late 1970s. That spacecraft studied the atmosphere near the equator. Scientists took that data and extrapolated it to the poles in order to create a complete model.

Armed with the new data, researchers could see just how accurate the reference model is in the polar regions. The research team found several surprises waiting for them. For one, the polar atmosphere is much colder than expected – up to 70 degrees. The average temperature comes out to a chill -157 degrees Celsius. That’s -250 degrees Fahrenheit for us Americans.

Müller-Wodarg and his colleagues also found the atmosphere in this region was less dense. At 130 kilometers, the atmosphere is 22% less dense than predicted in the previous atmospheric model. At 140 kilometers, it was 40% less dense than predicted.

“This is in-line with our temperature findings, and shows that the existing model paints an overly simplistic picture of Venus’ upper atmosphere,” said Müller-Wodarg. “These lower densities could be at least partly due to Venus’ polar vortices, which are strong wind systems sitting near the planet’s poles. Atmospheric winds may be making the density structure both more complicated and more interesting!”

Did You Know: One of the most variable and unstable vortexes in the entire solar system calls Venus’ south pole home. The structure of the vortex often resembles an ‘S’ or figure 8, and changes daily.

venus vortex

Venus’ polar regions are also home to strong atmospheric waves. This isn’t that surprising. Atmospheric waves are thought to be present in all planetary atmospheres. Studying them is tough, though. You have to be inside the atmosphere to accurately measure them.

Venus Express measures atmospheric waves

What are atmospheric waves? You ever have particularly rough flight through turbulence? Atmospheric waves can be responsible.

“Venus Express experienced them as a kind of turbulence, a bit like the vibrations you feel when an aeroplane flies through a rough patch,” said co-author Sean Bruinsma. “If we flew through Venus’ atmosphere at those heights we wouldn’t feel them because the atmosphere just isn’t dense enough, but Venus Express’ instruments were sensitive enough to detect them.”

Venus Express’ aerobraking maneuver and the data collected by the researchers are paving the way for future missions. NASA has been using a similar technique for years, but it’s the first time an ESA mission has successfully done it. And they’ll be doing it again soon.

The ExoMars Trace Gas Orbiter launched earlier this year and one of its mission objectives will be to aerobrake through Mars atmosphere to gather data. Because Mars’ atmosphere is much denser, the maneuver will last for nearly a year. That means scientists will be able to gather a much more complete picture of Mars’ atmosphere and how it varies by season.

Images via ESA