On an average day, some 100 million tons of carbon dioxide is liberated from oil and coal by combustion, wafting into the air. The gas traps heat in the atmosphere, resulting in the gradual warming that has alarmed scientists and much of the public.

But only half of the carbon dioxide stays up there; the other half falls back to earth. While scientists know what happens to half of that half — it dissolves into the oceans — the rest is a continuing puzzle. It is taken up by growing plants, but nobody knows exactly where and how. “Somewhere on earth, on land, one-quarter of all our carbon emissions released through fossil fuel emissions is disappearing,” said David Crisp, a senior research scientist at NASA’s Jet Propulsion Laboratory. “We can’t identify the processes responsible for this. Wouldn’t it be nice to know where?”

Now NASA is launching a satellite to help solve the puzzle.

The satellite, the Orbiting Carbon Observatory-2, is scheduled to lift off Tuesday morning from Vandenberg Air Force Base in California. Passing over the North and South Poles at an altitude of 438 miles, it will observe the same spots every 16 days as the earth rotates beneath.

These repeated measurements will allow scientists to observe the rise and fall of carbon dioxide with the seasons. They may also figure out how the balance changes with droughts or floods.