NASA still expects

its asteroid-belt-bound Dawn spacecraft to blast off by mid

-July, but cautioned that a combination of bad weather, range conflicts, and last-second concerns about the probe’s

Delta 2 launcher

could delay the launch until September or even October.

The U.S. space agency is targeting July 7 to launch Dawn on an eight-year $450 million mission to visit Ceres and Vesta, two of the solar system’s largest embryonic planets, or


Slipping the launch until fall would add $25 million to the price tag, but NASA officials said they do not expect that to be necessary.

Todd May, deputy associate administrator for programs in

NASA’s Science Mission Directorate

, said during a June 26

teleconference with reporters that a July liftoff remains likely

despite several

unresolved issues

. He said he expects those issues to be cleared by

July 3 when NASA conducts Dawn’s flight readiness review.

“There is no doubt we are pushing full steam ahead toward a July 7 launch readiness date,” May said.

NASA’s chief concern heading into Dawn’s roughly five-day July launch window is the weather at Florida’s Cape Canaveral Air Force Station. Dawn’s approximately 30-minute daily flight window opens

around 4:00 p.m. local time – prime time for summer thunderstorms

in the near-tropical environment.

If NASA and Delta 2 provider United Launch Alliance

cannot get Dawn off the ground by July 11, the mission starts pushing up against launch preparations for NASA’s Mars-bound Phoenix lander. The Phoenix mission

must lift off during its August window or wait another two years for a favorable planetary alignment.

“In our terminology, it’s a hard window constraint to get that thing off by Aug. 25,” May said. “So what we are trying to do is manage Dawn to make sure we are ready to go at the beginning of the launch window so we can get it off and out of the way before the Phoenix window opens on Aug. 3.”

Further complicating matters, NASA aims to launch the Space Shuttle Endeavor

Aug. 7, putting additional demands on the Florida launch range.

In addition to keeping an eye on the weather forecast and competing range demands, NASA is

tracking a couple of last-minute technical issues.

Bill Wrobel, NASA assistant associate administrator for launch services, said one area of concern is the Delta 2’s forward attach points for its strap-on solid-rocket motors.

NASA and United Launch Alliance engineers, he said, are busy doing additional loads analysis to reconcile what they are seeing during pre-launch checkout with what they expected to see on the attach points.

“Every one of these missions has its quirks and this one certainly has its quirks as well,” Wrobel said. “One of the biggest ones we’re probably facing is downrange assets.”

NASA needs to collect data on Dawn’s launch all the way through the spacecraft’s separation from the launch vehicle.

Wrobel said the tracking ship that will help NASA accomplish that

was delayed

getting through the Panama Canal and then encountered rough weather on its way to Puerto Rico for some necessary pre-mission repairs. Wrobel said NASA is trying to line up


as a substitute should the tracking ship prove

unable to

reach its destination in time to support the Dawn’s launch.

The Dawn team already has had to work through its share of glitches and setbacks on the way to the launch pad. Late delivery of some rocket hardware followed by a crane malfunction at the Cape delayed final

assembly of the Delta 2, prompting NASA to postpone a scheduled

June 20 launch date, according to May.

The spacecraft

itself sustained some minor damage June 11 during preparations for a pre-launch spin test when a technician’s tool inadvertently struck the back of a solar array panel. May said Dawn has been fully repaired and was on its way to the launch pad June 27.

As confident as May and Wrobel were that Dawn will

be ready to go by July 7, they said NASA would not decide for sure until the July 3 Flight Readiness Review.

If NASA decides to press ahead with the launch, Denver-based United Launch Alliance would proceed to fill the Delta 2’s second stage with corrosive hydrazine fuel, committing NASA to either launch the rocket during the July window or stand down until October to give United Launch Alliance enough time to replace the rocket’s second-stage tank.

If NASA decides

there are too

many outstanding issues to commit to a July launch, May said, the agency


return the spacecraft to a storage

hangar, concentrate on getting Phoenix on its way, then try to launch Dawn in September.

But May emphasized that pressing ahead with a July launch is NASA’s preferred and most probable approach.

“If we were to jump over the Phoenix launch at this point, first of all you get into hurricane season, but second of all it’s a fairly large budget hit to do that. And frankly there’s no guarantee to get off at that time either,” he said.