NASA scrubbed its launch of an unmanned spacecraft on a nine-year voyage to Pluto for the second day in a row Wednesday, but this time weather in Maryland was to blame.

A storm in Laurel, Md., knocked out power at the John Hopkins University Applied Physics Laboratory, which is managing operations of the New Horizons spacecraft.

A decision on whether to try for a Thursday launch depended on whether backup power could be restored at the Maryland facility. That decision would be made late Wednesday, said Alan Stern, the mission's principal investigator.

"I ... was not comfortable with launching without backup power," Stern said. "I've been working on this for 17 years ... Two or three days doesn't mean a hill of beans."

The space agency has until Feb. 14 to launch the piano-sized spacecraft, but a launch in January would allow the spacecraft to use Jupiter's gravity to arrive by 2015. After Jan. 29, every few days' postponement adds another year to the Pluto arrival date.

The launch of the New Horizons probe had been called off Tuesday afternoon when winds at the launch pad in Cape Canaveral exceeded the space agency's 38-mph flight restriction.

"The winds picked up sooner than expected," said MIT scientist Richard Binzel, one of the mission's investigators. "Blame the meteorologists."

A successful journey to Pluto would complete an exploration of the planets that was started by NASA in the early 1960s with unmanned missions to observe Mars, Mercury and Venus.

Pluto is an oddball icy dwarf, unlike the rocky planets of Mercury, Venus, Earth and Mars and the gaseous planets of Jupiter, Saturn, Uranus and Neptune.

It also is the brightest body in a zone of the solar system known as the Kuiper Belt, made up of thousands of icy, rocky objects, including tiny planets whose development was stunted by unknown causes.

Scientists believe studying those "planetary embryos" can help them understand how planets were formed.

The New Horizons spacecraft will be launched on an Atlas V rocket that will accelerate away from Earth at 36,000 mph, the fastest launch speed on record.

The planned launch has drawn attention from opponents of nuclear power because the spacecraft is powered by 24 pounds of plutonium, whose natural radioactive decay will generate electricity for the probe's instruments.

NASA and the Department of Energy estimated the probability of a launch accident that could release plutonium at 1 in 350. As a precaution, the agencies brought in 16 mobile field teams that can detect radiation, plus air samplers and monitors.