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Most pictures of the Venus surface are synthetic, like this view of a volcanic region called Eistla, created from Magellan orbital radar data. The SAGE lander would take actual photos from ground level. (NASA/JPL)

Forbidden Planet

We’ve been to the moon. Mars is easy. But landing on Venus? That’s tough.

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The dangers start more than 80 miles above the planet’s surface, after the lander is dropped off by a carrier spacecraft, and get worse as the lander descends through the atmosphere. The engineers designing SAGE have had to think up a different solution for each hazard. Thick clouds of sulfuric acid at 40 miles up explain why the planet’s surface is invisible from Earth. As the lander parachutes down, it also has to contend with lightning and withstand winds up to 200 mph.

At an altitude of about nine miles, well below the cloud decks, SAGE finally begins to take pictures of the surface. So few photographs exist of Venus (all taken by the Venera landers) that every image of the surface will be precious. But aside from showing impressive volcanoes, the landscape shots might be pretty blasé. There are no vistas of lakes or forests, and the air will be hazy—a dreary low light like the fifth rainy morning in a row. The thick atmosphere absorbs high-frequency blue light, so the resulting colors, or rather color, of the surface will be both dull (a kind of rusty yellow) and intense (relentless, unbroken).

At eight miles above the surface, according to JPL planetary scientist Suzanne Smrekar, the carbon dioxide in Venus’ atmosphere becomes so dense that it turns “supercritical.” Supercritical carbon dioxide is a gas-liquid mix that can eat through metal, and SAGE is designed to keep this nasty stuff from entering the sealed vessel.

For protection from the crushing atmospheric pressure—1,300 pounds per square inch—the lander will be roughly spherical, the strongest geometric shape. SAGE’s core—where the computer circuits are housed—will be surrounded by an inner titanium pressure vessel. The one redeeming quality of the heavy atmosphere is that it cushions the lander’s descent. Terminal velocity on Venus is a leisurely 25 mph—so slow that the parachute is no longer needed after the spacecraft is 42 miles above the surface.

There is no upside to the broiling heat, however. “Temperature is the thing that will kill you the quickest,” says Smrekar. To protect circuits and batteries, she and others are testing advanced insulation materials, like lithium nitrate, a kind of salt that absorbs heat as it melts from a solid to a liquid. This “phase changing” material could be sandwiched between layers of other insulators for extra protection.

As for where SAGE might land on Venus, in some ways it doesn’t matter. Four-fifths of the planet is volcanic plain, with weather more uniform than the Sahara. Venus has no axial tilt, so its poles get little light, and it rotates slower than most people walk, just once every 243 Earth days, exposing whole hemispheres to the sun for weeks. The forecast for Venus is always the same, day or night, equator or antipodes, on the sunny side or on the side facing black space: 850 degrees Fahrenheit, high pressure.

Nevertheless, scientists have diligently scouted landing sites, because wherever SAGE touches down, it will remain. Unlike the recent Mars rovers, the Venus lander will have no wheels or locomotive apparatus of any kind—a mobile explorer would have been too expensive and wouldn’t have gotten far in just a few hours anyway.

The SAGE team has proposed landing on the slopes of Mielikki Mons, a volcano 200 miles wide but just 4,800 feet high. That low grade is typical of Venusian volcanoes, which are like Hawaiian volcanoes in that they ooze lava rather than erupt explosively, building up a massive mountain over time. Based on recent imaging of the region around Mielikki Mons by Europe’s Venus Express orbiter, scientists think the volcano might have recently been active.

Lava flows have shaped the Venusian landscape the way plate tectonics have shaped Earth’s, and the lander will spend much of its short life sampling the terrain for clues about Venus’ past. Lasers from two portholes above the sphere’s waist will zap the soil, vaporizing small patches of ground. SAGE will also blast a neutron pulse at the soil, then examine the resulting gamma ray spectrum. Both experiments will tell scientists what minerals and elements the soil contains. Cameras looking out four other portholes will take panoramic pictures and microscopic shots, while other instruments sample the atmosphere.

Perhaps most important of all, SAGE will drill into the surface “weather rind” to get at the virgin soil underneath. This might prove the trickiest part of the mission, since the drilling arm will be exposed to Venus’ melting heat.

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