The scientist leading NASA's Dawn spacecraft on a three-billion-mile reconnaissance mission to explore a massive asteroid and a "dwarf planet" believed to harbor water has been designing the project for more than a decade. Liftoff was supposed to happen this weekend, yet troubles interfered. And officials Saturday ordered another launch delay -- all the way to September.

Chris Russell, professor of geophysics and space physics at the UCLA, first proposed the mission in 1994. He's been waiting a long time to see the robotic probe, powered by exotic ion thrusters, travel into the asteroid belt where it will orbit the rocky body Vesta, then venture out further to the small world called Ceres and also orbit that tantilizing object.

But getting the spacecraft built and launched has been beleaguered by setbacks.

"The spacecraft will spend much less time in space than we put in preparing for the mission," Russell said recently. "I want to get this spacecraft up in space, where it belongs. I'm really confident about the spacecraft. We've been testing and retesting."

Russell's team hoped to see the instrument-laden space probe leave Earth in the coming days. Now, everyone must wait a bit longer.

A 12-story United Launch Alliance Delta 2-Heavy rocket with Dawn nestled inside the vehicle's nose cone was supposed to blast off from Cape Canaveral's pad 17B on Saturday afternoon. But stormy weather prevented the rocket's second stage from being fueled on Thursday, forcing the liftoff to be delayed from Saturday to Sunday.

NASA decided early Friday morning to slip the launch another 24 hours - to Monday - because of troubles with a telemetry-relay aircraft downrange. Then a decision was made late Friday to retarget the launch for no earlier than Sunday, July 15.

Problems with the tracking plane and delays getting a substitute ship into the Atlantic Ocean region has been a source of headaches for the launch officials. Either the aircraft or the instrumented ship is required to receive telemetry from the rocket during the second and third stage firings off the west-central coast of Africa. Without a mobile tracking asset in place, engineers would have no insight or data while those critical events of the launch occur.

NASA has been racing against the calendar because Dawn's current launch opportunity closes July 19, giving just a few days left to get the spacecraft on the required trajectory to fly past Mars for a sling-shot maneuver and then into the asteroid belt for its rendezvous with Vesta and Ceres over the next eight years.

Missing this launch period forces a wait until the next window, which opens in September and extends through late October.

With the dwindling dates left to fly and the looming August 3 launch of the Mars lander Phoenix aboard another Delta 2 rocket from the neighboring Cape Canaveral pad, NASA management on Saturday opted to call an end to Dawn's liftoff chances in July.

"The decision was made Saturday to move the launch to September after careful review by NASA's Science Mission Directorate officials, working with Dawn mission managers, the Dawn principal investigator, and with the concurrence of the NASA Administrator," an agency spokesman said.

"Primary reasons for the move were a combination of highly limited launch opportunities for Dawn in July and the potential impact to launch preparations for the upcoming Phoenix Mars Lander mission, set for early August."

The alignment of the planets dictates a tight August 3 to August 24 window for the Phoenix liftoff to happen. If the craft doesn't launch within those three weeks, the next shot at Mars won't come until 2009.

"A September launch for Dawn maintains all of the science mission goals a July launch would have provided," the NASA spokesman said.

An additional complicating factor for Dawn is its launcher's second stage. A propellant blend of nitrogen tetroxide and hydrazine fuel must be loaded into the Delta vehicle two days prior to liftoff. But the corrosive nature of the rocket fuel limits the amount of time the stage remains suitable for flight -- roughly 40 days -- after the propellant is pumped aboard. Proceeding with the fueling for a launch next weekend and then possibly experiencing weather or technical delays that pushed the flight past the July 19 window cutoff date would have added further problems and cost to the Dawn mission.

The start of Dawn's adventure to examine Vesta and Ceres has experienced a number of hurdles, including outright cancellation of the project in March 2006. After a heated controversy, NASA restarted the mission less than a month later.

"It has been quite an emotional roller coaster," said Russell.

"There were some days I didn't think we were going to make it. But we never kept trying."

Recent plans called for the launch to happen June 20, but that date was scrapped because more time was needed to prepare the Delta rocket before on-pad assembly could start. Then a targeted June 30 launch day was doomed when the pad's crane developed a problem last month, causing a hiatus in attaching the solid-fuel boosters.

Those slips in the launch schedule coupled with this week's weather and tracking aircraft problems left Dawn with little of its window left prior to the high priority Mars lander liftoff.

Dawn's ion thrusters, derived from the engine successfully demonstrated on NASA's Deep Space 1 technology pathfinder craft, will propel the spacecraft during its eight-year, three-billion-mile mission, reaching Vesta in 2011 and Ceres in 2015. The spacecraft will orbit at increasingly lower altitudes above the objects to determine the composition, internal structure and evolutionary history of the bodies.

"I think of Dawn as two journeys," Russell said. "One is a journey into space. This is analogous to what ancient explorers did, who knew there was unexplored territory and wanted to discover what was there. We're going to explore a region for the first time to find out what the conditions are today.

"Dawn is also a journey back in time. Ceres and Vesta have been altered much less than other bodies. The Earth is changing all the time; the Earth hides its history, but we believe that Ceres and Vesta, formed more than 4.6 billion years ago, have preserved their early record. They're revealing information that was frozen into their ancient surfaces. By looking at the surface and how it was modified by the bombardment of meteoroids, we will get an idea of what the early conditions of Ceres and Vesta were and how they changed. So Dawn is a history trip too. We're going back in time to the early solar system."

Scientists suspect that Vesta is solid rock. The oval-shaped object has an average diameter of approximately 320 miles. On the other hand, Ceres could have water or ice beneath its rocky crust. This "baby planet" has an average diameter of about 600 miles.

"Why do we explore the solar system? Why did Lewis and Clark go across the U.S. at the start of the 19th century? We're not going to expand the human race off this planet for a long time, but discovering our origins and how the solar system evolved is valuable in itself. Mankind has always expanded horizons. Exploration is a human imperative."

Exactly when Dawn will blast off to begin its expedition wasn't immediately clear Saturday. NASA officials did not announce a target launch date for September.