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An Aerobraking Swan Dive for Venus Express

On May 16th, European Space Agency officials announced that Venus Express would begin a new mission: the testing of new aerobraking techniques.

Humanity’s lone sentinel at Venus was launched in 2005 and arrived at Venus in April 2006. In the eight years since, the probe has made some major discoveries. Among others, it made the first detection of water in Venus’s atmosphere and detected evidence of active volcanic activity on the planet. Despite these discoveries, it has largely remained out of the public eye.

Currently, Venus Express maintains a highly elliptical orbit that takes around 24 hours to complete. At its lowest, it skims the atmosphere only 197km (122 mi) above the north pole. At the other end, it hovers nearly 66,000km (41,000 miles) above the planet’s south pole. Over time, a combination of atmospheric drag and precessional effects similar to those experienced by MESSENGER at Mercury slowly cause the orbit to decay. To counter this, the probe periodically needs to reboost itself to higher altitudes.

Unfortunately, the propellant needed to reboost Venus Express has almost run out. Rather than leave Venus Express to its fate, the ESA sees this as an opportunity to conduct experiments in aerobraking.  Although Mars missions commonly use aerobraking to slow down into their operational orbits, these are usually one and done affairs. Venus Express will take multiple deep passes through the Venusian atmosphere, allowing the ESA to refine its aerobraking techniques.

In an ESA press release, head of mission operations Paolo Ferri said, “The campaign…provides the opportunity to develop and practise the critical operations techniques required for aerobraking, an experience that will be precious for the preparation of future planetary missions that may require it operationally.”

The aerobraking campaign also provides a unique opportunity to test the engineering limits of the spacecraft. Although Venus Express has dipped its toes into the Venusian atmosphere before, flying as low as 165km (103 miles), the aerobraking campaign will bring Venus Express to within 130km (81 miles) of the surface. The low altitude could prove fatal for the spacecraft, as the repeated heating by atmospheric drag could degrade or destroy the spacecraft’s electrical components. Drag forces may also damage the Venus Express’s high-gain antenna, the probe’s main communications line with Earth.

This chart shows how Venus Express's lowest altitude will change over the course of the aerobraking campaign. Credit: ESA

This chart shows how Venus Express’s lowest altitude will change over the course of the aerobraking campaign. Credit: ESA

Here’s how the aerobraking campaign will unfold:

  • From now until June 18, Venus Express will be in what mission operators call the “walk-in phase”. The lowest point of the spacecraft’s orbit will be allowed to decay from its current 197km (122 mi) to 130km (81 mi), the starting altitude of the aerobraking campaign. On May 20, mission control will perform its first test of Venus Express in “aerobraking mode”, a special orientation of the spacecraft designed to keep Venus Express stable during its passes through the atmosphere.
  • Beginning on June 18, the aerobraking campaign will begin in earnest. At least once a day, the spacecraft will swoop within 130km (81 mi) of the Venusian surface, collecting diagnostic data. The probe’s scientific payload may also be able to collect atmospheric data during several passes. Due to the orientation of Venus Express in aerobraking mode, mission control will have to wait two hours after each dip to learn if the probe survived.
  • On July 25, if the probe is still operational, it will reboost itself to an altitude of 475km (295 mi), beginning the “bonus science” phase of the mission. After this reboost, the spacecraft’s orbit will be allowed to decay naturally, bringing the mission to a close sometime before the year’s end.

Venus Express won’t be the first spacecraft to practice aerobraking at Venus. That honor belongs to NASA’s Magellan spacecraft, which orbited Venus from 1990 to 1994. During the final two years of its stay at Venus, Magellan practiced the first aerobraking manuevers ever performed. Originally placed into an elliptical orbit 295 x 7762km (183 x 4823 mi), Magellan used aerobraking to circularize its orbit to 180 x 540km (112 x 336 mi) during the middle of 1993. Then, in September 1994, NASA directed the spacecraft to enter deeper into the atmosphere to perform another round of aerobraking experiments. These experiments, known as the “windmill experiments”, measured how the atmosphere interacted with Magellan’s solar panels.

The Magellan mission came to an end when the spacecraft entered the Venusian atmosphere on October 13, 1994. Status reports from the spacecraft shortly before entry provide a glimpse of the conditions that Venus Express could encounter during the aerobraking campaign. During its final days in orbit, Magellan’s lowest altitude was just under 140km (87 mi). At this altitude, the atmospheric friction heated Magellan’s solar panels to 126 degrees Celsius (258F). Although Magellan continued transmitting data to Earth up until it deorbited, a number of factors could prevent Venus Express from being as hardy.

The ESA plans to keep the public up to date with the adventures of Venus Express during this phase of its mission through a series of mini-updates at their Rocket Science blog and through their twitter account @esaoperations once the campaign begins. Be sure to check in there for updates!

Image Credit: ESA/C. Carreau

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About The Author
Justin Cowart
Justin Cowart is a geologist interested in Earth and Solar System history. As a geologist, he spends hist time looking at the ground, but in his free time he looks to the skies as an amateur astronomer.

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