For BepiColombo, ESA has had to extend the limits of existing design standards and develop altogether new design concepts as well. How to begin building a spacecraft that needs to endure sunlight 10 times more intense than in Earth orbit, with surfaces hotter than a kitchen hot plate - high enough, in fact, to melt lead?

Back in late 2000, when the mission was first selected, no-one knew for sure.

1. Achieving a close-up view of Mercury

BepiColombo will be the third mission to visit the innermost planet after NASA's Mariner 10 in the 1970s and the current Messenger.

It is three spacecraft in one: ESA's Mercury Planetary Orbiter (MPO), Japan's Mercury Magnetosphere Orbiter (MMO) plus ESA's additional Mercury Transfer Module (MTM) to convey the other two across interplanetary space.

But BepiColombo will be taking a much closer look than its predecessors: Mariner 10 only flew past while Messenger has entered a highly-elliptical Mercury orbit. While MMO will also follow an elliptical orbit, the planet-mapping MPO will orbit much more tightly, coming as close as 400 x 1500 km from Mercury's heat-radiating surface.

In certain orbital positions, when the orbiter comes between the Sun on one side and Mercury on the other, it will have to endure temperatures as high as 450 degrees C.

2. Technology making the mission possible

"A considerable team of researchers was involved in making the mission feasible," comments Jan van Casteren, BepiColombo Project Manager. "An exceptional amount of technology development and demonstrations has been needed across a variety of fields." Just reaching Mercury presents a major challenge: a new generation of highly efficient electric propulsion was required, capable of achieving the tens of thousands of hours of thrust needed to enter orbit.

3. How the spacecraft keeps its cool

Then comes the problem of thermal management, which drives the spacecraft design. Slice through MPO and you would see a complex labyrinth of heat pipes. Previously used on a variety of missions, these sealed pipes work like a closed-loop version of human sweat glands, containing liquid whose evaporation carries excess heat from MPO's sunward-side to radiating plates facing deep space. The liquid then condenses, allowing the process to begin anew. The heat pipe concept helps keep MPO's interior within room temperature. What was new was the 2 x 3.6 m size of the radiator, and the operational constraints it faced: "Its radiating plates must remain cold and shaded for it to work," explains Ulrich Reininghaus, BepiColombo Spacecraft Development Manager.

"If they ever come into sustained contact with sunlight, or the infrared radiation emitted from Mercury's surface then they would stop working."

The mission had to develop a unique set of coated louvres that prevent the radiator 'seeing' the hot planet below while not preventing its own radiation escaping to cold deep space.

4. Searching for material solutions

Expelling internal heat only goes so far, however; much better if it never makes it inside the spacecraft at all. The real technical challenge has been finding new materials for everything on the outside of the spacecraft in particular - including antennas, the solar array and its associated Sun-tracking sensors and mechanisms and again the radiator and protective multi-layer insulation (MLI) - which would be able to withstand the Sun's tenfold increase in brightness and associated temperature extremes.

"We began a critical materials technology programme for BepiColombo at the start of 2001," comments Christopher Semprimoschnig, head of the Materials Space Evaluation and Radiation Effects Section of ESA's Materials and Components Technology Division.

"We've kept busy for approaching a decade, gradually qualifying materials. It's been a huge challenge because we had no previous experience of such a harsh environment. The closest we ever came was with Venus Express, though that meant handling two solar constants rather than 10."

5. Testing, testing...

ESA's Materials and Processes engineers were involved because they had a good understanding of what materials could be candidates, as well as of related fields that might offer useful 'spin-in' technologies, such as protective coatings on jet engine turbines.

It took years to develop the laboratory facilities required for testing, however, adapting existing facilities wherever possible. "When you increase the light and heat intensities you are operating with 10 or 20 times compared to before, then failures can happen," Christopher says. "We had to deal with melted lamp holders, melted reflectors, but we gradually managed to build some representative simulation chambers like our Synergistic Temperature Accelerated Radiation (STAR) facility."

6. A life in the Sun ESA's materials experts needed to predict the end-of-life condition of all the materials in question. How might specific mission-critical properties change after years of intense solar glare? Would reflective coatings discolour, MLI crack, solar arrays lose electrical performance or thermal emittance - the crucial ability to radiate away heat?

"Total exposure will be something like 100 000 equivalent Sun hours," explains Christopher. "Traditionally we boost illumination levels for accelerated lifetime testing. But a move up from 11 solar constants to 30 or 40 is not so easy. "The accuracy is uncertain, due to non-linear effects - the materials might unexpectedly fail for some unknown reason."

End-of-life estimates for Venus Express offered a starting point: the five-year-old mission remains in good health, showing the team's original estimates had been broadly accurate.

6. A life in the Sun

ESA's materials experts needed to predict the end-of-life condition of all the materials in question. How might specific mission-critical properties change after years of intense solar glare? Would reflective coatings discolour, MLI crack, solar arrays lose electrical performance or thermal emittance - the crucial ability to radiate away heat?

"Total exposure will be something like 100 000 equivalent Sun hours," explains Christopher. "Traditionally we boost illumination levels for accelerated lifetime testing. But a move up from 11 solar constants to 30 or 40 is not so easy. "The accuracy is uncertain, due to non-linear effects - the materials might unexpectedly fail for some unknown reason."

End-of-life estimates for Venus Express offered a starting point: the five-year-old mission remains in good health, showing the team's original estimates had been broadly accurate.

7. Less air for more precise testing

In such an extreme environment, everything degrades, like plastic left out in the Sun. But precisely how critical properties degraded over time in orbit needed to be exactly understood.

For example, it was found that when test items were removed from their vacuum chamber then their subsequent exposure to air would induce chemical interactions with radicals within the material.

"These radicals are basically degradation products, so this alters the state of degradation," says Christopher. "A day later when a measurement is done, their condition could be very different. So if we extrapolate from these results we would get a performance curve, but the real curve would end up being much worse.

"So we set up a system to make measurements while still in vacuum, saving us time and letting us assess changes much more reliably."

8. Testing to breaking point and beyond

ESA needs to be sure that its chosen materials would function reliably for years on end.

"We are going to the limit of a material's performance, seeing what happens when it breaks down," Christopher adds. "The result is a wealth of information that could be of interest to many other industries as well."

The programme continues to qualify all the materials needed for the mission, currently standing at around 75% complete.

The MLI covering the bulk of the spacecraft's surface is foreseen to be a woven ceramic fabric. There are multiple layers kept apart by spacers, designed to be as light as possible - some of the layers have less than a tenth the thickness of printer paper, just 7.5 micrometres across.

"The result is much lighter than metallic foil but also more brittle," says Christopher. "Now we need to look at processing issues: how to stack it, what shapes can it fit around and how to handle it without damage and release of particles."

9. A way to save the solar arrays

Solar cells became the single most challenging material question. Dramatic degradation in solar cell performance was detected: just one simulated month saw a 20% power loss.

"This failure brought the mission to the brink of cancellation," recounts Christopher. The effect was due to a combination of material degradation from ultraviolet radiation and high temperatures driving down cell efficiency.

A combination of protective coatings and carefully solar array tilting offers a workable solution. If the solar arrays directly face the Sun then they would heat up and fail. So instead they stay tilted at an optimum angle. Their power production stays lower, but so does the temperature.

The main antenna also requires protective coatings, though for a different reason. It is made of thin titanium for maximum performance: it needs to perform highly accurate radio science experiments to determine how spacetime curves around the Sun.

By itself it would warm up like any other metal in the Sun - to as high as 700 degrees C. But temperature-driven deformations have to be prevented. A specially-tailored coating should keep its temperature 300 degrees C lower while allowing electromagnetic signals to pass through freely.

10. Spacecraft-level testing has begun

A test model of Japan's MMO arrived in the Netherlands in mid-September 2010 for testing in the Large Space Simulator (LSS) at the ESTEC Test Centre, the largest vacuum chamber in Europe. The LSS's Solar Simulator was carefully adjusted to attain 10 solar constants, its light beam being brought into much tighter focus.

"To safely remove the resulting heat from the chamber walls we installed an extra thermal shroud with a more than six times greater flow of liquid nitrogen than the existing system," explains Alexandre Popovitch, overseeing modifications. "That required around 5000 litres of liquid nitrogen per hour of each two-week test."

There were two sets of tests, one with MMO free-spinning - as it will operate during its active life - then one with an ESA sunshield that will keep it cool as it rides as a passenger to Mercury.

This summer, test models of Europe's BepiColombo spacecraft will go through the same experience. Follow-up versions incorporating any lessons learnt will be ready for evaluation in 2012, with the launch of BepiColombo scheduled for 2014.

The materials team, meanwhile, is looking forward to ESA's Solar Orbiter mission - destined to venture even closer to the Sun.

Tweet

Related Links ESA BepiColombo Space Technology News - Applications and Research