An unmanned Delta 2 rocket blasted off from California on Wednesday, carrying a NASA science satellite to survey where carbon dioxide, a greenhouse gas tied to climate change, is moving into and out of Earth's atmosphere, a NASA Television broadcast showed.

The 39-metre tall rocket lifted off at 2:56 a.m. PDT (5:56 a.m. EDT/0956 GMT) from Vandenberg Air Force Base, located about 240 km northwest of Los Angeles, and headed south over the Pacific Ocean.

The launch was timed so that NASA's Orbiting Carbon Observatory, or OCO, would end up at the front of a train of polar-orbiting environmental satellites that cross Earth's equator every afternoon.

A launch attempt on Tuesday was called off because of a problem with the launch pad's water system, needed to mitigate high temperatures and suppress acoustic vibrations of launch. Technicians replaced a failed valve, clearing rocket manufacturer United Launch Alliance, a partnership of Lockheed Martin and Boeing, for a second launch attempt.

Scientists have been waiting since 2009 for OCO to reach orbit. The original satellite was lost in a launch accident.

Understanding climate change

"OCO 2 is our first NASA mission dedicated to studying carbon dioxide," Betsy Edwards, program executive at NASA Headquarters in Washington, told reporters during a prelaunch news conference. "This makes it of critical importance to the scientists who are trying to understand the impact of humans on global change."

Every year about 40 billion tons of carbon end up in Earth's atmosphere, an amount that is increasing as the developing world modernizes, said atmospheric scientist Michael Gunson, with NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Roughly half of the carbon is re-absorbed by forests and the ocean, a process that is not well understood.

"Understanding the details of those processes will give us some insight into the future and what's likely to happen over the next decades, even if we continue to consume more and more fossil fuels and emit more and more carbon dioxide into the atmosphere," said OCO project manager Ralph Basilio, also with the Jet Propulsion Laboratory.

From its orbital perch 705 km above Earth, the spacecraft, built by Orbital Sciences Corp., will collect hundreds of thousands of measurements daily. Its path around the planet will take it over the same spot at the same time every 16 days, allowing scientists to detect patterns in carbon dioxide levels over weeks, months and years.

"It's really the fate of carbon dioxide once it's in the atmosphere that we're trying put our finger on," Basilio said.

The $468 million mission is designed to last at least two years.