The toughest spaceship we’ve ever built

Future missions to Venus need tough new technologies – and an old one.

It’s been a long time since anyone tried landing on Venus, one of the most hostile environments in the Solar System. Covered in sulphuric acid clouds, the surface temperatures approach 460 C (860 F) with atmospheric pressure 90 times that of Earth. Lead, zinc and tin are liquids, and the weight of the carbon dioxide air is roughly equal to that found a kilometre under the ocean – enough to crush a submarine.

Yet the planet is getting renewed attention – Japan’s Akatsuki mission successfully entered orbit in December 2015 and there are new missions planned from Nasa and Esa in the 2020s. Even Russia plans a follow up to their highly successful Venera and Vega missions of the 1970s and 80s. All of these missions involve orbiters, and will study the planet's atmosphere, magnetic field, and geography.

To really understand the planet one needs a lander. Landers can test the chemistry of the air and rocks at the surface, and do seismology to reveal what the interior of the planet looks like. The Venera D has a lander, but it has a mission lifetime of three hours based on the batteries it can carry. The previous record for surviving on the surface was set by the Soviet Union’s Venera 13 lander, which touched down in 1982. It lasted 127 minutes in this toxic, hazardous environment.

To make a probe that lasts longer periods – a day, perhaps, or more – requires tough electronics that can handle the high temperatures, or a way to cool a probe that will, effectively, be sitting in an oven, or both. It has to work without solar panels, which are less efficient on a planet that is always overcast. Batteries don’t last as long and can’t generate the kind of power necessary.

For the electronics, Nasa scientists are looking at new materials for computer chips that will keep working at high temperatures. “Near 500C (932 F) the game has to be different,” says Gary Hunter, a research engineer at Nasa. “We need different insulators and different contacts…  We have to reinvent how those circuits come together.”

The problem, Hunter says, is that at high temperatures, many materials start to behave differently. For example, the silicon is a semiconductor, but at high temperatures – about 300C (572 F) – it starts to become a conductor, and thus less useful for electronics. Another problem is that even if the silicon circuits themselves survive, it’s difficult to come up with materials for the connections between the circuits that will not fail when sitting in Venus’s hot atmosphere.

Hunter says Nasa is looking at silicon carbide-based electronics, which can operate for longer periods at the kind of temperatures that a lander is likely to see on Venus's surface. The downside is that so far it seems the kind of chips one could make would be less powerful than modern computers. According to a 2014 presentation from Nasa’s Venus Exploration Analysis Group, such electronics are about as powerful as computers from the 1960s. “We’re not talking Pentiums here,” Hunter says. But with some creative design, it may be enough to get the pictures and data from a probe and transmit it to an orbiter which can relay it to Earth.

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The goal, Hunter said, is to get to thousands of hours of operations – enough to last a whole (Venusian) day, which is as long as 117 days on Earth. 

As for power systems, Timothy Miller and Michael Paul at Pennsylvania State University, with Steven Oleson at Nasa's Glenn Research Center, have proposed using a Stirling engine.

Stirling engines start with a working fluid inside a “cold” chamber (cold just means that the temperature is lower, rather than frigid). The fluid is compressed by a piston and moves to a second chamber, where it is heated. The heated fluid expands, moving a second piston, linked to the first via a wheel or arm. As the second piston moves the first it draws the fluid back to the cold side, where its temperature drops, and the cycle begins again. As long as there is a heat source, the engine keeps working. Stirling engines are used today in some refrigeration systems, and even in submarines (the Swedish Navy's Gotland-class boats use them for underwater propulsion).

The technology has been around since 1816, when they were invented by Scottish clergyman Robert Stirling. Miller and Paul believe this old idea could be used on future spacecraft, and published their idea in the journal Acta Astronautica. Nasa have already funded some initial tests.

A Stirling engine, Miller said, can provide enough power to both cool the electronics and supply electricity for instruments, so they can operate for longer than on batteries. The working fluid would probably be helium, as it transmits heat efficiently compared to other gases and isn’t reactive.

But power wasn't the only consideration: a Stirling engine needs fuel. Miller and his team decided on lithium, which can burn in an atmosphere of carbon dioxide and nitrogen. (Nitrogen makes up about 4% of the air on Venus). Lithium also melts at 180 C (356 F), which makes it effectively a liquid fuel on Venus, and as such much easier to “burn.”

This minimizes the weight of the spacecraft at launch – all it needs to take along is the lithium. A 50kg (110lb) combined engine-and-fuel setup could power a space probe for two days, according to Miller’s research.  

The engine would be configured as a kind of single-piston system, with one side the cold and the other the hot, which would push an alternator back and forth, generating electricity. Thus far Miller and his team have been able to do small-scale tests, at four to five atmospheres; the group is looking for further funding to do experiments under the kind of conditions a lander would encounter on Venus.

Also, lithium doesn’t pollute. That might seem an odd consideration on an uninhabited planet, but it’s important when trying to do science. “What we want to do is have a system that if we’re to land somewhere and execute its mission for however many days, we didn’t want outgassing that will contaminate the surrounding environment,” Miller said.

When lithium burns in a CO2 atmosphere it becomes lithium carbonate. That means that a lander, testing the atmosphere, won’t have its readings put out of kilter by exhaust gases.

If the team can show their combustion system will work at 90 atmospheres, that will move the technology to a level that gives it a better chance to fly. “If we can demonstrate something that works for a week,” Miller said.

Venus and Earth are similar in many respects.  Their radii are within a few percent of each other, and Venus has 81% the mass of Earth. When the planets formed they were in nearby parts of the solar nebula, so the bulk composition should be similar.  Technologies enabling a long-lived lander may prove crucial in solving the mystery of how one planet became a home for life and the other became what Carl Sagan called “the closest thing to hell”.

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