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The chances of acceptable weather for Sunday's sunset launch of the Deep Space Climate Observatory, or DSCOVR, are more than 90 percent — but SpaceX says the chances of pulling off an unprecedented at-sea landing of the Falcon 9 rocket's first stage, minutes after the launch, are just 50-50.

DSCOVR — a $340 million mission supported by NASA, the National Oceanic and Atmospheric Administration and the U.S. Air Force — is designed to monitor solar storms and provide full-disk Earth imagery from a gravitational balance point a million miles away. The satellite, originally known as Triana or GoreSat, is due to go up from Cape Canaveral Air Force Station in Florida at 6:10 p.m. ET Sunday after languishing in storage since the Clinton administration.

Launch coverage will be streamed online by NASA and SpaceX.

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The most interesting part of the launch is expected to take place after the Falcon 9's second stage and the satellite leave the first stage behind. The 14-story-tall first stage is programmed to flip 180 degrees, relight its rocket engines at an altitude of 80 miles (130 kilometers) and slow itself down from supersonic speeds to land on the deck of a drone ship in the Atlantic Ocean. SpaceX tried the maneuver during a Falcon 9 launch last month, but it didn't quite work. The hydraulic fluid for a fin stabilization system ran out just before the landing, and the rocket stage crashed and burned.

Before that attempt, SpaceX founder Elon Musk estimated the odds of success at no more than 50 percent. For this attempt, SpaceX has supplied the rocket with lots more hydraulic fluid. However, the flight trajectory is more challenging. "The speed of the stage coming in to the entry is actually higher. ... I believe the dynamic pressure is twice what it was before," Hans Koenigsmann, SpaceX's vice president of mission assurance, told reporters at NASA's Kennedy Space Center in Florida on Saturday.

So how likely is it that the maneuver will succeed? "I think I want to stick with 50 percent, after careful deliberation," Koenigsmann said, only half-seriously.

If the landing maneuver becomes routine, that would represent a huge step toward rocket reusability and lower-cost access to outer space. Musk has said the economics could eventually make trips to Mars affordable.

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Tune in tomorrow for the launch of the Deep Space Climate Observatory! http://t.co/YWQHIpV3vr #DSCOVR — Al Gore (@algore) February 7, 2015

View a timeline of the Falcon 9 rocket’s launch with NOAA's Deep Space Climate Observatory: http://t.co/A633jEG8du pic.twitter.com/7grCSrnLQx — Spaceflight Now (@SpaceflightNow) February 7, 2015

— Alan Boyle