Getting a probe safely to the surface of Mars is not easy: Numerous landing attempts have ended in a crash. Sufficiently slowing a lander in the thin air requires plenty of sophisticated gear, including designer heat shields, powerful retrorockets, and, sometimes, giant airbags. But the European-Russian Exo-Mars mission is struggling with a piece of 18th century technology: parachutes.
High-altitude tests earlier this year revealed that ExoMars's chutes were tearing as they were pulled from their bags. The European Space Agency (ESA) has turned to NASA colleagues for help, and this week, a joint team began tests to see whether redesigned bags and chutes now work, and if not, why. It could be their last chance to fix the problem and preserve a launch set for next summer—or face 2 years of delay. "It's a race against time," says David Parker, ESA's director of human and robotic exploration in Noordwijk, the Netherlands.
The ExoMars mission, Europe's largest ever planetary mission and first Mars rover, has been in gestation since 2001. The golf cart–size rover, named after British DNA pioneer Rosalind Franklin, will look for signs of life. It is just one-third of the size of NASA's Mars 2020 rover, which will gather rocks for eventual return to Earth. But uniquely, it has a drill that can burrow 2 meters below ground to sample material that has been shielded from the harsh radiation that bombards Mars's surface. "It's a hostile place for life, so you have to dig deeper," says Andrew Coates of University College London, principal investigator of the rover's science camera system.
About 10 years ago, ESA teamed up with NASA to transport its rover in a U.S. lander. But in 2012, when the U.S. side pulled out for budgetary reasons, ESA had to find a new landing craft. The agency teamed up with its Russian counterpart, Roscosmos. Russia's landing system does not have retro-rockets as powerful as NASA's, so ESA had to devise larger parachutes to slow the craft for landing, says Francois Spoto, ExoMars project manager in Noordwijk.
The parachutes are just one step in a hair-raising descent. But because they unfurl and inflate in unpredictable ways, they are considered risky. When the approaching spacecraft first hits the thin martian atmosphere, drag on its heat shield slows it from 21,000 to 1700 kilometers per hour. Then, a 15-meter-wide parachute, with a large, ring-shaped gap to cope with the supersonic air-flow, slows the craft to 400 kilometers per hour. That chute is jettisoned and followed by a 35-meter parachute, the largest ever deployed in a descent to Mars, with multiple ring-shaped slots that create higher drag at low speeds. When that chute has done its work, it, too, is jettisoned, and at an altitude of 1 kilometer retrorockets take over to lower the 2000-kilogram lander to the surface. "It'll be a real finger-biting time," Coates says.
ESA's 2016 lander, Schiaparelli, tested the 15-meter chute. Although the chute did its job, a software error caused the craft to think it had landed when it was still 4 kilometers high. The parachute was released, thrusters were turned off, and Schiaparelli crashed. "Let's hope this goes better this time," says Francesca Esposito of the Astronomical Observatory of Capodimonte in Naples, Italy, whose instrument DREAMS was destroyed in the crash. Her new instrument on the ExoMars lander, called Micro-MED, will study how martian dust forms and how it affects the atmosphere.
In May, engineers tested the two-chute system in Sweden, dropping it from a balloon at an altitude of 30 kilometers, better to mimic the thin air of Mars. Both the 15-meter and 35-meter chutes tore while being pulled from their bags. ESA made adjustments, reinforcing the parachutes and lining the bags with Teflon to make them more slippery. But in an August test, the chutes tore catastrophically.
ESA sought help from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, which has designed parachutes for many successful Mars landings. In September, ESA staff traveled to JPL for a 3-day workshop. "It was very open. We received some critical comments on the design," Spoto says. Two JPL engineers traveled to Europe to inspect the damaged chutes and suggested a different lacing system for the bags, which ESA has now adopted.
This week, tests on these new chutes will begin at JPL. The chutes will be hydraulically pulled from the bags in a series of tests, which will gradually ramp up to full extraction speeds of 60 meters per second. If the tests are successful, engineers will perform more high-altitude drop tests in Oregon in February and March 2020. That would leave just enough time for the redesigned chutes to be installed on the spacecraft in Cannes, France, before it is shipped to the Baikonur Cosmodrome in Kazakhstan for its summer launch. "It's very challenging, but it's doable," Parker says.
A final check would come in late April 2020, when the parachute system must pass a qualification review by senior ESA experts. If the parachutes miss that deadline, "it's bye-bye for 2 years," Spoto says—until the next time orbital mechanics bring Mars into line for a suitable launch window. Esposito, who has already seen one of her dreams dashed, would rather be safe than sorry. "I'll be happy to wait," she says.