After Kevin was Jacque Blamont of CNES talking about sending balloons to Venus, Mars, and Titan. The principle of flying balloons has been known since 1783 - but they're not a particularly efficient way of carrying a payload around. The Vega balloons in 1984 had a science payload of just 7 kilograms for a total spacecraft mass of 160 kilograms. The proposed Mars '94 balloons were a 30-kilogram payload for a mass of 300 kilograms. Flying a balloon around Venus is also a short experiment - even at 60 kilometers altitude, the winds are 100 meters per second and thus rotate around the entire planet in about 5 Earth days. So if you deploy a high pressure balloon on the night side - 2 days later it's local noon, and gets massive heating, causing it to burst.
One interesting technique is to use a phase change liquid in the balloon to control its altitude. It turns out that water is perfect for this. The balloon would drop to an altitude of 42 kilometers at which point the water would vaporize inside the balloon, inflate it - and the balloon would rise. Four or so hours later, it would reach 60 kilometers, the water would re-condense, the balloon would drop back to 42 kilometers again - and the process would repeat. You could explore that entire range of atmosphere with a period of around 6 to 8 hours, up and down, using water to autonomously control the altitude.
Jacque was rushed towards the end of his presentation, but he described how a 45-meter-tall balloon on Mars with a 30-kilogram gondola would last a long time oscilating between dragging a tether along the ground at night, and reaching as high as 3 kilometers during the day....and Titan...Titan is perfect for ballooning. Jacques first proposed a Titan balloon in 1978. Using a 80-kilogram RTG you could heat a 30-meter-wide hot-air balloon internally augmented with a smaller hydrogen balloon and fly around with 20 kilograms of instruments at a height of 10 kilometers for ever.
Artist's concept of a Titan balloon
Following Jacque was I. Ekonomov from the Space Research Institute in Russia. He proposed a long-life lander for Venus - Venera
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