TURIN, Italy — Managers of Europe’s ambitious BepiColombo mission to Mercury, which began development with cost overruns and schedule delays, said the program has now stabilized and is on track to meet its mid-2016 launch date.

The mission, which includes a European orbiter, a Japanese orbiter and a transfer module to carry them to Mercury orbit, is now expected to cost the European Space Agency about 1.2 billion euros ($1.6 billion). That sum includes 170 million euros for launch aboard a European Ariane 5 ECA rocket, a 7.5-year voyage to Mercury orbit and at least two years of operations.

To this figure would be added around $400 million to account for the Japanese orbiter, provided by the Japanese space agency, JAXA, and the contributions to the 11-experiment payload made by individual European nations from their national budgets.

BepiColombo is about two years late and ESA’s science directorate has used its cost growth as an example to assure that future programs’ technological challenges are not underestimated before an industrial contract is signed. BepiColombo originally was thought to be small enough to ride on a medium-lift Soyuz rocket, which is about one-half the cost of an Ariane 5.

In briefings here July 4 at the production plant of Thales Alenia Space, which is a major BepiColombo system provider to prime contractor Airbus Defence and Space, officials said the schedule is tight to retain the mid-July 2016 launch date, which if missed means waiting until the following January for the next opportunity.

A January 2017 launch would actually get the two orbiters to Mercury at an earlier date than if they launch in July 2016 because of better Earth-Mercury positioning. But it would also mean five months of keeping BepiColombo industrial teams active, weighing on the mission’s budget.

Among its other novelties, BepiColombo will be using an ion-electric propulsion system, provided by Qinetiq of Britain, for its long voyage to Mercury before reverting to conventional chemical propellant for maneuvers upon arrival in the vicinity of Mercury orbit in January 2024.

Ulrich Reininghaus, BepiColombo project manager at ESA, said the four ion-electric engines on the BepiColombo Mercury Transfer Module will require some 500 kilograms of xenon gas propellant — enough for the first 2.6 years of the journey — which may be a record amount of xenon on a space mission. To assure it has enough xenon for mission preparation, the BepiColombo team is purchasing 1,000 kilograms of the gas at a cost of about 1 million euros.

BepiColombo’s goal is to compile a map of Mercury that fills in the many blanks left by NASA’s Mariner 10 satellite in the 1970s and the Messenger satellite launched in 2004, which arrived at Mercury in 2011 and has since completed its primary mission.

“Messenger has made many unexpected discoveries,” said Johannes Benkhoff, ESA project scientist for BepiColombo. “What we hope to do is use the fact that Bepi is closer to the surface to close the gap in our knowledge of Mercury’s southern hemisphere.”

ESA’s Mercury Planetary Orbiter will be placed in an elliptical orbit of between 450 kilometers and 1,500 kilometers from Mercury’s surface to make a global three-dimensional map of the planet.

Thomas Passvogel, head of ESA’s science projects department, said the Japanese-built Mercury Magnetospheric Orbiter “is basically ready” and will be shipped to ESA’s European Space Research and Technology Centre (ESTEC) in Noorwijk, Netherlands, for testing with the European orbiter and the transfer module. Passvogel said the BepiColombo program has suffered no major issues in the past year, raising confidence that it will complete final testing with no delays.

Thermal vacuum testing is scheduled to start at ESTEC in October and be completed by the end of the year. The Mercury Transfer Module is scheduled to be tested by next spring.

Officials said one issue that slowed the program’s development was the unexpected consequences on multiple components of exposure to the high heat they will experience in Mercury orbit. Initially copper elements were used in the construction of the solar-array cells but they were found to cause short circuits during long exposure to temperatures higher than 300 degrees Celsius on the Mercury approach, with spikes to 400 degrees on certain elements.

Thermal blankets were mechanically sewn together to avoid heat-stress issues in screws. The 40-square-meter solar array assembly required specialized fabrication to prevent the arrays from pointing directly at the sun, remaining at a sharp angle to limit the heat exposure, requiring unusually precise satellite attitude and orbit control.

“BepiColombo is one of the most ambitious missions planned by ESA and has required the development of special technologies to deal with the severe conditions,” Thales Alenia Space Italy Chief Executive Elisio Prette said. 

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Peter B. de Selding was the Paris Bureau Chief for SpaceNews.