LONDON — After almost 20 years of development, the European Space Agency has finally unveiled the BepiColombo Mercury orbiters and confirmed the mission is on track for an October 2018 launch.

The 1.65-billion-euro mission, a joint venture between ESA and the Japan Aerospace Exploration Agency (JAXA), is Europe’s first attempt to enter the orbit around scorching Mercury, the closest planet to the sun.

Marcus Bauer, ESA’s communications officer, said 70 percent of the orbiters’ technology had to be developed specifically for the mission due to the challenging environment around the planet.

Speaking to reporters during a media briefing at the European Space Research and Technology Centre (ESTEC), in Noordwijk, the Netherlands, ESA Bepi Colombo project manager Ulrich Reininghaus, compared orbiting Mercury to flying into a pizza oven.

He said that challenges in the design of solar panels, the thermal control systems and protective coatings, were the main cause of the lengthy development and several delays.

“When we tested the thermal structure of the Mercury Planetary Orbiter [one of the two orbiters that form the mission], we learned that our thermal coverage is inadequate,” Reininghous said. “The heat intake into the structure was such that we would not have covered it, reducing life time significantly. We had to completely rethink the system.”

The 4,100-kilogram BepiColombo consists of two orbiters that will launch together — the ESA-managed Mercury Planetary Orbiter (MPO) and the JAXA-owned Mercury Magnetospheric Orbiter (MMO). The two spacecraft will be delivered to the orbit around Mercury stacked on top of each other by the Mercury Transfer Module (MTM). During the seven-year journey, the MMO will be shielded from the sun by the MMO Sunshield and Interface Structure (MOSIF), which will also serve as a mechanical and electrical interface between the two orbiters.

“MPO focuses on the planet, the surface and the interior size,” said Reininghous. “The orbit is a polar one — 480km times approximately 1500km — a little bit elliptical but extremely close to the planet as such with a return period of 2.3 hours. The data return is estimated at 1.5 gigabit per year.”

The MMO will focus on the planetary environment including the planet’s atmosphere, according to Reininghous.

“The orbit is also polar but far more elliptical — 590 km times approximately 11,700 km. It has a period of 9.3 hours. The data return is approximately 10 percent of what we expect from the MPO.”

ESA covers about 1.3 billion euros of the overall budget. JAXA representatives said the Japanese agency contributes about one-tenth of this amount. The rest is paid by nations responsible for building individual scientific instruments – 11 for MPO and five for MMO.

According to ESA Director of Science Alvaro Gimenez, over 30 companies from 12 ESA member states are collaborating on the project. In addition to Japan, the United States and Russia have also built instruments for Bepi Colombo.

Airbus leads the consortium together with Thales Alenia Space.

Airbus led the work on BepiColombo’s solar arrays, which according to Airbus program manager Markus Schelkle, took “almost as long as the whole spacecraft” to build due to the extreme heat and solar radiation levels in Mercury’s orbit.

“The MPO solar array itself is a master piece of technology,” said Schelkle.

“It’s 8 square meters in size and produces with 3,000 solar cells up to nearly 2 [kilowatts] power.”

The 7.5-meter-long solar array must be kept at temperatures below 215 degrees Celsius using a sophisticated tilting mechanism that protects the cells against direct sun exposure.

“We have to survive 10 times the solar radiation we are experiencing on Earth plus surface temperatures of up to 430 degrees Celsius,” Reininghaus commented.

The craft also has to be protected against the infrared radiation coming from the planet itself.

ESA said BepiColombo had passed its final test in launch configuration, including the acoustic and vibration test. Engineers will now separate individual modules to see whether the technology survived the testing unharmed.

“We still have to pass some reviews,” said Reininghaus. “We merged the system qualification review with the flight acceptance review. We have started with an internal review on the project level but have the big one coming in October and this should complete on [March 8] next year.”

The day after that, Reininghaus said, the team will start packing the craft for a journey to Kourou, French Guiana, to get ready for the October 2018 launch atop Ariane 5.

“The launch window opens on Oct. 5, 2018 and closes approximately on Nov. 28,” Reininghaus said.

“We will have more than 7 years cruise duration to Mercury, mainly supported by electric propulsion and the gravity assists — one Earth, two Venus and six times Mercury.”

BepiColombo will cover some 8.9 billion kilometers on its journey to Mercury, which it is expected to reach in December 2025.

ESA said Bepi Colombo features a more complex design than NASA’s Messenger that orbited Mercury between 2011 and 2015. Bepi Colombo is expected to provide higher-resolution imagery than Messenger, ESA said. However, its mission is only expected to last one year with a possible one-year extension.

The mission was almost cancelled in 2004 when it was revealed the challenges of the environment would require a much bigger and costlier spacecraft than had been originally estimated.