The L-1011 aircraft "Stargazer" takes off from Cape Canaveral to launch the Pegasus rocket with HESSI. Photo: NASA

After a 19-month delay, NASA's HESSI spacecraft was finally lofted into orbit Tuesday aboard an air-launched Pegasus rocket, but not without a hiccup in the countdown that forced officials to abort one attempt before succeeding a second time around.

Severely damaged in a testing mishap in March 2000 and grounded by four rocket-related concerns, the High Energy Solar Spectroscopic Imager was able to shake off its tough luck with a successful launch to begin a two-to-three year mission to observe the most powerful explosions in the solar system.

A Lockheed L-1011 carrier jet, called Stargazer, took off from Cape Canaveral Air Force Station, Florida, with the Orbital Sciences-built Pegasus mounted to its belly, heading over the Atlantic Ocean on a preset flight path -- dubbed "the race track" -- where the rocket would be dropped to soar into space.

The countdown was proceeding smoothly for launch at 3:28 p.m. EST when communications between the ground team and crew aboard the aircraft were lost in the final minutes. The link was restored with about three minutes until the planned drop time, but officials decided to abort the launch attempt as a precaution.

The L-1011 crew turned the plane around, making a U-turn in effect to get back into position to launch the Pegasus again.

The second try was the charm.

The Pegasus is dropped from the L-1011 and launches to space. Photos: NASA TV/Spaceflight Now

With a final "go" from the ground, pilot Rodney Boone in the cockpit of Stargazer pushed the drop button and the 52,000 pound Pegasus rocket was released at 3:58 p.m. EST, free falling for five seconds to clear the plane before the first stage motor ignited.

Nine minutes and 42 seconds after the drop, HESSI was successfully deployed into space from the rocket.

"It looks like we had an excellent flight," said NASA Launch Manager Omar Baez. "(On) our initial attempt we lost communications with our L-1011 aircraft at a crucial point in the countdown. We decided to abort and recycle and go back into the 'race track' for a second attempt. We were able to pull that off and get HESSI up successfully into space."

Bryan Baldwin, Orbital Sciences' Pegasus launch vehicle program manager, said it was the first time a Pegasus countdown had been aborted only to have the countdown reset and carrier jet circle around to launch the rocket during a second try on the same day.

The launch team and scientists alike celebrated the delivery of HESSI into space after a long, sometimes frustrating road. The craft was supposed to fly in July 2000.

"For us at NASA, it is a tremendous load off our shoulders to get this mission behind us and move on and fly the rest of the Pegasus missions," Baez said.

"Terrific," said Robert Lin, HESSI's principal investigator from the University of California-Berkeley. "We couldn't have asked for a better launch."

HESSI's $83 million mission, $13 million more than originally planned because of the delays, will observe about 1,000 solar flares -- the massive eruptions on the sun. Flares can damage orbiting satellites, threaten the health of astronauts, disrupt communications and knock out power grids on Earth.

An artist's concept of HESSI studying the sun. Credit: NASA

"What we are interested in studying are solar flares, which are the most powerful explosions in the solar system. They release about a billion megatons' equivalent of TNT in the time scale of a few seconds to a few minutes," Lin explained.

Scientists will use HESSI to study the mysterious fundamental basics of solar flares -- where they are born, what triggers them and how they generate huge energy releases.

"HESSI is going to study the workings of the explosion itself," said Bill Wagner, the mission's program scientist from NASA Headquarters. "It is going to be making movies in X-ray light and gamma ray light, which has been impossible to do until this team from Berkeley came along and conceived and developed the HESSI mission."

A summer 2000 launch would have allowed HESSI to observe the flurry of solar flares during the peak of the 11-year cycle of activity on the sun, called the Solar Maximum.

Although the solar activity began to subside after peaking in mid-2000, another peak occurred late last year and early this year, giving scientists renewed hope of a successful mission.

Lin said larger solar flares are preferred for studying and they tend to erupt on the descending side of the solar peak, which is happening now.

The HESSI team is negotiating a six-month mission extension to see more flares, making up for those lost in the delays. Such an extension would cost $2 million. Officials say HESSI would have seen about 2,000 flares had it been launched as planned.

Tuesday's launch was the 31st for the Pegasus rocket since debuting in 1990, the 21st in the XL configuration and the third to occur off Florida. Other launch sites used are in California, Virginia, the Kwajalein Atoll in the central Pacific Ocean and the Canary Islands.

As many as four more Pegasus missions may occur this year: NASA's Galaxy Evolution Explorer (GALEX) in July; the commercial OrbView-3 Earth-imaging spacecraft in September; NASA's Solar Radiation and Climate Experiment (SORCE) satellite in October; and the Canadian SciSat-1 ozone research probe in December.