When will HESSI finally make it into Earth orbit? Credit: NASA

Its launch has been delayed 19 months by six different concerns, but finally all appears ready for NASA's HESSI satellite to soar into space Tuesday to study the most powerful explosions in the solar system.

The High Energy Solar Spectroscopic Imager spacecraft will ride to a 373-mile-high perch above the planet aboard an air-launched Orbital Sciences Pegasus XL rocket.

The launcher will be flown 75 miles off the coast of Palm Bay, Florida, and released from the belly of a modified Lockheed L-1011 jetliner. The launch window extends from 3:21 to 5:21 p.m. EST with a targeted 3:26 p.m. EST drop of Pegasus to begin the nine-and-a-half minute flight.

Air Force meteorologists are calling for a 70 percent chance of acceptable conditions.

The $85 million HESSI mission was designed for NASA by the University of California-Berkeley to explore what causes solar flares to erupt from the Sun and how they release their energy. Such flares can damage orbiting satellites, threaten the health of astronauts, disrupt communications and knock out power grids on Earth.

Launch was originally scheduled for July 2000, placing the craft in orbit to observe the furry of solar flares during the peak of the 11-year cycle of activity on the Sun, called the Solar Maximum.

But in March 2000, as HESSI was undergoing vibration testing at the Jet Propulsion Laboratory to simulate the conditions the satellite would encounter during launch, the mechanism shaking the spacecraft malfunctioned. HESSI was subjected to extreme vibrations, severely damaging parts of the craft.

Called a "shaker," the device delivered approximately 20 G's, ten times the appropriate level for the test, to the spacecraft. The lack of maintenance and checks of the test mechanism was blamed.

HESSI after it was repaired. Credit: University of California-Berkeley

NASA officials decided HESSI was worth rebuilding, and so the mission lived on. Launch was reset for mid-November 2000.

However, that launch date was lost when a so-called "Red Team", dispatched to review NASA missions to ensure no unnecessary risks were being taken, ordered all of HESSI's electronics boxes be opened up so internal boards could be inspected, said Peter Harvey, the HESSI project manager at the University of California-Berkeley.

That work would postpone the launch until March 28, 2001, marking the second and final time issues with HESSI itself would force a delay.

In October 2000 a Pegasus rocket launched NASA's HETE-2 satellite to search for gamma-ray bursts. The flight appeared normal and the spacecraft was successfully delivered into orbit.

During post-launch analysis, however, engineers discovered that the interstage portion of the rocket -- the cylindrical barrel that connects the first and second stages -- didn't separate properly because the explosive charge failed to create a clean cut for jettisoning.

Although the incident didn't doom the launch of HETE-2, Orbital Sciences and NASA set out to redesign the separation system to prevent a reoccurrence in the future.

As a result of the rocket work, HESSI's launch date again slipped, from late-March to June 7, 2001.

By late spring the Pegasus was fixed and everything was looking favorable for the HESSI team. The 645-pound satellite was bolted to the rocket at Orbital's processing facility at Vandenberg Air Force Base in California, and then the completed launcher was mounted to the L-1011 carrier jet.

The X-43A launch goes bad just after the Pegasus first stage began to fire, sending the vehicle out of control. Photo: NASA-DFRC

On June 2 the duo was flown across the country to Cape Canaveral Air Force Station in Florida where the launch will originate. But as the ferry flight was about half-way to the East Coast that afternoon, a NASA launch of the X-43A hypersonic test craft mounted on the end of a modified Pegasus rocket first stage flew out of control moments after launch over the Pacific Ocean near Southern California.

The L-1011 touched down in Florida with an obvious question mark hanging over it -- would the X-43A launch failure cause HESSI to be delayed again?

The rocket stage used in the launch of the X-43A vehicle was fundamentally the same as the first stage of the HESSI booster, although there were some differences -- including the guidance system, avionics and thermal protections.

The space agency kept hope alive the HESSI would be delayed only a matter of a couple weeks. But facing the expiration of batteries inside the Pegasus, the L-1011 was forced to return the rocket and HESSI to Vandenberg for servicing. NASA decided HESSI wouldn't launch until the X-43A investigation panel could determine what went wrong, ensuring there wasn't a generic problem that would doom the Pegasus.

As the X-43A inquiry dragged on through the summer and fall, HESSI sat covered inside the Pegasus hangar at Vandenberg, just waiting for a chance to fly.

"Starting with X-43, we've had to go back and look at what we knew our margins for our flight control on the vehicle were," said NASA Launch Manager Omar Baez. "We've been able to refine some of those margins via getting better models.

"We also went in the wind tunnel and did some additional wind tunnel testing. We didn't get any results that did us any good but we reaffirmed that what we had as far as wind tunnel data was accurate.

"Basically it boils down to margin. It boils down to knowing what you're flying a little bit better and modeling that product as accurately as you can using the computer simulations."

The Taurus rocket veers off course moments into its second stage firing. Photo: Air Force video/Spaceflight Now

Adding to the mix, an Orbital Sciences Taurus rocket launched from Vandenberg on September 21 failed to place a commercial Earth-imaging satellite and NASA ozone monitoring craft into space after a stuck steering mechanism sent the vehicle into a wild gyration in the sky.

Since the Pegasus and Taurus are similar, although Taurus is ground-launched, the mishap became the fifth reason on HESSI's list of issues keeping it from launching.

"The concern there was really more on now Orbital had lost two vehicles. What was going on there? And we looked at Orbital from a systems approach to make sure things weren't slipping through their systems. It appears we've got that under control," Baez said.

The Taurus investigation did turn up a concern with part of HESSI's Pegasus rocket, Baez said.

"In reviewing some of the pedigree paper as a result of Taurus (failure) we found that there was some acceptance test data from a rudder that was not quite up to spec. We decided to change out that whole aft section as a whole unit. It's easier for us to do that than to change a rudder motor and assembly."

By the end of 2001, engineers had concluded that the X-43A and Taurus failures were not concerns for the Pegasus launch of HESSI. Liftoff was set for January 24.

A GEM rocket booster is seen here being readied for a national missile defense test. Photo: Boeing

On December 13 a tiny booster was launched from a silo at Vandenberg as a missile defense test. But seconds into flight it malfunctioned. The missile's main stage was made up of a single Graphite Epoxy Motor derived from the strap-on solid rocket booster used by Boeing Delta 2 rockets.

At first the failure appeared to have no ties to HESSI. The spacecraft was re-attached to the Pegasus upper stage on December 20 as preparations were underway for a January 24 launch date.

But just after the holidays, NASA learned that the military panel investigating the mishap was focused on the GEM motor itself, which is built by Alliant Techsystems, the same company that builds stages of the Pegasus.

"Although they are of different sizes, a lot of the manufacturing techniques, a lot of the materials they use, are common," Baez said.

HESSI's launch was delayed again, for the sixth time.

Through the month of January engineers looked at how the Alliant-built Pegasus motors are manufactured, transported, processed and how Orbital Sciences takes possession and processes them for flight.

"We looked at all that data and got comfortable with it," Baez said. "We got really good hardware and we are confident the thing was built correctly and that nothing was missed."

So last Friday HESSI was flown back to Cape Canaveral for Tuesday's launch attempt. And with any luck at all, the satellite will finally head to space to do what it was designed to do.

"There isn't anything else we can do to make that (launch) any safer," Baez said.

"We were aiming for the peak of solar activity in mid-2000, so we have been lucky that, with the slip of our launch date by a year and a half, solar activity has continued to stay high," said Robert Lin, the HESSI principal investigator from University of California-Berkeley.

"The Sun had another peak of activity at the end of 2001. We still think we will image around a thousand solar flares, though whatever we see will be new and interesting."