The Dawn spacecraft, advertised by NASA as "a journey to the beginning of the solar system", will explore the two largest objects in the asteroid belt in an effort to answer questions about the formation of Earth’s solar system

One of the primary reasons to delay the mission to September was the busy launch schedule for NASA in July. An upcoming Phoenix Mars Lander mission is scheduled for liftoff on August 3, 2007 (for a landing on the planet Mars in May 2008). It has a very tight launch window between August 3 and 24—if missed, another launch is not possible until 2009.

In addition, Space Shuttle Endeavour is due to launch with its STS-118 crew on August 7, 2007 to deliver and install the starboard S5 truss segment to the International Space Station, along with the External Stowage Platform 3. NASA is trying its best to stay on schedule after the STS-117 mission, the first shuttle mission of 2007, was delayed for three months by a freak hailstorm that damaged the external fuel tank (ET).

Third, a ship, which was to replace a mechanically ailing airplane, had not reached its correct Atlantic Ocean destination in order to clear the launch of the Dawn asteroid mission from NASA’s base in Florida. Either the airplane or the ship is needed in order to track the spacecraft’s telemetry (remote positional data coming off the spacecraft) from second and third stage firings off the west-central coast of Africa.

For the Dawn mission itself, there was only a limited number of days to launch in July. With the last day to launch fast approaching—July 19th—NASA officials decided not to rush its liftoff. They opted to wait for the September-to-late-October launch window.

Also complicating the situation is the nitrogen tetroxide and hydrazine fuel that is loaded into the Delta rocket to propel Dawn into space. It is loaded two days before launch. Once the propellant is loaded, however, it must be used within 40 days before becoming unusable. Thus, the short launch window in July made it more likely that with any weather or technical delays, the mission would be forced to launch in September and the rocket would have to be replaced, which adds additional costs and delays to the mission. NASA officials opted to go with the longer launch window in September-October.

Earlier in the week, the mission had problems due to lightning storms over the skies of east-central Florida. In addition, the fueling of the second-stage rocket had been delayed due to hot conditions on the exterior payload walls of the rocket (its fairing), which prevented the super-cold oxidizer propellant from being loaded.

With a launch in September NASA officials state that all the scientific goals of the mission can be met. By the end of October the two bodies that the Dawn spacecraft will visit—dwarf planet Ceres and asteroid Vesta—will begin to drift apart. It will be much more difficult to rendezvous with both bodies if the launch of the mission is delayed this long. Ceres and Vesta will not be this close together for another fifteen years.

However, it will not be impossible to rendesvous with both asteroids if launched in October because the spacecraft uses an ion-drive propulsion system. The high-tech propulsion system can be stopped and restarted frequently, which allows for much more flexible launch windows than if spacecraft are propelled by chemical rocket systems, which are only started a few times for major course changes.

With this two-month delay, Dawn will arrive at the asteroid Vesta in December 2011 in order to investigate its lava flows, which may help answer how it was initially formed. Later, in April 2015, Dawn will rendezvous with the dwarf planet (and largest of the asteroids) Ceres.

For more information on NASA’s ion powered Dawn mission to Ceres and Vesta, go to the iTWire article titled “NASA’s Dawn mission to asteroids powered with high-tech ion drive”.

The NASA Dawn mission homepage is: http://dawn.jpl.nasa.gov/.

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