ESA's Venus Express Probe Is On The Rise One Final Time
July 14, 2014

ESA’s Venus Express Probe Is On The Rise One Final Time

John P. Millis, Ph.D. for - Your Universe Online

The planet Venus has often been referred to as Earth’s Twin, or Earth’s Sister, because in many ways it bears a striking similarity to the world we call home. But while it is roughly the same size, similar mass, possesses a magnetic field, and maintains an atmosphere, there are also some very important differences. Perhaps the most significant departure is the exact nature of Venus’s atmosphere.

Surrounding our planetary neighbor is a layer of thick gas, dominated by carbon dioxide (96.5 percent) and nitrogen (3.5 percent) with only minute traces of other elements and molecules. Because the atmosphere is so dense the surface temperature averages around 872 degrees Fahrenheit, driven by an intense greenhouse effect. Additionally, the weight of the gas pushes down on the surface with a barometric pressure more than 90 times what we experience on Earth.

This makes exploring the surface incredibly difficult, even with robotic probes. More than a dozen previous efforts have found the surface so inhospitable that missions never last more than mere hours before succumbing to the intense heat and pressure. With the immense cost of designing, building, and launching a planetary probe, planetary researchers have to be clever in planning missions to the Venusian surface.

However, because of the other similarities with Earth, researchers are keen to unlock the planet’s secrets. In 2006, the European Space Agency (ESA) launched a mission known as Venus Express, an exploration probe that was able to take advantage of the planet’s thick atmosphere and “ride” the gas, skimming along the clouds at various depths to better characterize the atmosphere.

Venus express spent the vast majority of its mission at altitudes between 155 miles and 41,000 miles above the Venusian surface, following elliptical orbits that lasted about 24 hours each. On May 15 of this year, the probe completed its official duties. But since fuel remained, the ESA took on a series of maneuvers designed to accumulate further data on the Venusian atmosphere, but also to test the limits of the probe itself. The information accumulated will inform the design and construction of future probes to Venus and other worlds.

Once the designed mission was completed, ESA scientists allowed the probe to drop through the atmosphere under the influence of gravity and atmospheric drag. Venus Express reached an altitude of around 81 miles, where it has been “surfing” for nearly a month. Researchers are now using the onboard thrusters to drive the probe even deeper into the carbon dioxide cloud, pushing it to about 80 miles.

"We have explored uncharted territory, diving deeper into the atmosphere than ever before," says Håkan Svedhem, ESA's Venus Express project scientist. "We've measured the effects of atmospheric drag on the spacecraft, which will teach us how the density of the atmosphere varies on local and global scales."

As the spacecraft descended from 102 miles to 80 miles, the research team noted incredible increases in stress on the craft. The relatively small altitude change yielded a thousand fold increase in pressure, also causing significant heating as Venus Express soared along at roughly 22,400 miles per hour.

"During several of the 100-second long passages through the atmosphere, the solar panel temperature sensor reading increased by over 100°C," notes Adam Williams, ESA's spacecraft operations manager for Venus Express. “Analyzing the spacecraft's response to such rapid heating will be useful for planning future spacecraft systems and subsystem design."

The ESA team is now preparing for one final climb through the clouds before the probe exhausts its fuel. Venus Express has already begun a series of 15 maneuvers that will send it to a height above 285 miles  by July 26. Once there the craft will be released to fall under the influence of gravity through the atmosphere, ending its mission once and for all.

However, a challenge still remains; because the probe is operating on limited fuel, there is a chance that it may expel its reserves before climbing above the 285-mile height. If this happens, the probe will lose its ability to communicate with Earth. As a result, the probe will start its plunge toward the Venusian surface before expected. Worse, any data that could have been accumulated as it passed through the various cloud layers would be lost.

"We have already gained valuable experience in operating a spacecraft in these challenging conditions that will be important for future missions that may require it. Once we have completed the orbit raise, we look forward to processing and analyzing the scientific data collected on the atmosphere," says Patrick Martin, the Mission Manager for the ESA's Venus Express mission.