Bigger isn't always better: Japan's newest rocket, scheduled for its maiden voyage this week, is designed to be a smaller, cheaper way to get science satellites into space. Advances in pre-flight automation mean that the rocket, dubbed Epsilon, can be ready to lift off in about a week with fewer people in mission control, helping to slash costs to about $38 million per launch – much cheaper than its heavier, labour-intensive predecessors.
But perhaps the most exciting aspect of the inaugural launch will be Epsilon's cargo: the world's first space telescope designed to study the planets from afar. The Spectroscopic Planet Observatory for Recognition of Interaction of Atmosphere, or Sprint-A, will look at Venus and Mars to find out why some worlds lose their atmospheres while others manage to keep a grip on their gases.
This will in turn help exoplanet hunters figure out which distant worlds are capable of hosting atmospheres that might support life. Sprint-A will also peer at Jupiter's moon, Io, the most volcanically active body in the solar system, to see how the tiny moon influences Jupiter's mighty auroras.
Extreme ultraviolet
If all goes to plan, the Epsilon rocket will launch from Japan's Uchinoura Space Center on 27 August at 0445 UTC. It will deploy Sprint-A into low Earth orbit, where the spacecraft will take aim at the planets using cameras and sensors that record extreme-ultraviolet light.
"Extreme UV is a range of light suitable for observing planetary atmospheres," says Shujiro Sawai of the Japan Aerospace Exploration Agency (JAXA). Extreme UV from the sun gets bent at the boundary where a planet's atmosphere meets space, and the way it is redirected can reveal the atmospheric composition.
"But extreme UV radiation coming from space is absorbed by the Earth's atmosphere, so it is not observable from the ground," says Sawai. "Very little outer space observation with extreme UV has been done, so scientists are expecting new discoveries that no one has ever imagined before."
So far, our best clues to the original atmospheres of Mars and Venus come from the composition and structure of ancient rocks, either meteorites that made it to Earth from those planets, or rocks examined by rovers and orbiters. Based on the evidence, it seems that Mars, Earth and Venus probably had similar atmospheres long ago.
But we also know that the sun pumps out a constant stream of charged particles called the solar wind, which can ionise gases in a planet's upper atmosphere and pick up the newly charged particles, effectively sweeping them away. Earth is protected from the solar wind by a relatively strong global magnetic field, which repels charged particles from the sun, explains Nick Schneider of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, who has worked on Sprint-A.
Stripped by the sun
Still, the solar wind would have been much stronger when the sun was young and more active. Because Venus is closer to the sun, the solar wind might have stripped gaseous water from its early atmosphere, leaving a thick haze of mostly carbon dioxide that turned the planet's surface into a hellish desert. And while Mars is farther away, it has no global magnetic field. It is thought the solar wind thinned the Red Planet's atmosphere over time, making it cold and dry.
"It turns out that most atmospheres have lost a lot of gas over their lifetimes. On Mars it may be as much as 99 per cent. What drives the escaping is a big question," says Schneider. Solar stripping is a leading hypothesis, but it is not the only runner. For instance, others have suggested that Mars lost its atmosphere all of a sudden during a powerful collision with an asteroid or comet. A NASA probe called Maven, due to launch in November, will orbit Mars to study its atmosphere up close to try to solve the puzzle.
Sprint-A will help from afar by looking for the extreme UV radiation generated as the solar wind slams into the upper atmospheres of both Mars and Venus, says Sawai. "By observing this phenomenon, we will investigate how the solar wind affects the upper atmosphere of planets, and how the planetary atmosphere escapes into outer space."
Exoplanets and auroras
The results may add a new twist to the search for exoplanets that can support life, says Schneider. Until recently, a planet's habitability was largely defined by its distance from its star, which hints at whether its surface would be warm enough to support liquid water. But it is clear from our solar system that a lot of other factors come into play, says Schneider.
"Venus, Earth and Mars are extremely different, despite differing by less than a factor of two in their sizes and distance from the sun," he says. "If we can't explain these relatively small effects, how can we hope to evaluate the habitability of all those Earths, super-Earths and even mini-Earths?"
Sprint-A will also peer at Io, a wildly volcanic moon of Jupiter. Gases spewing from Io feed a ring of charged particles around Jupiter that might influence the giant planet's atmosphere, including its huge auroral displays.
"It turns out that everything in the Jovian system – from Io's volcanoes, to its atmosphere, to the plasma, to Jupiter's aurora – are all wildly variable," says Schneider. "And they're all connected. But nobody knows how. Sprint-A hopes to observe all these phenomena and sort out the cause-and-effect relationships."
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