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Planetary News: Dawn (2007)Earth's Weather Forces Dawn Launch Delay to SeptemberArrival Date at Vesta UnchangedBy Emily LakdawallaJuly 9, 2007 After a series of delays with rocket assembly decreased the July launch period to an uncomfortably short five days, weather problems in both Florida and the South Atlantic Ocean stymied NASA's hopes of launching the Dawn spacecraft to Vesta and Ceres this month. Fortunately, another launch period of five to six weeks' duration is available in September for the mission. The spacecraft will now be removed from its launcher temporarily to make way for the launch of the Phoenix mission to Mars. Jim Green, the director of NASA's Planetary Science Division, stated in a press briefing this morning that accepting a delay for Dawn to a September launch has no effect on the mission's primary or secondary science objectives; the only negative effect of the delay is financial. He said that the $20 to $25 million cost of maintaining the "standing army" of personnel necessary to launch and check out Dawn can be accommodated within the current fiscal year's budget without any effect on the selection of the next Discovery mission, still planned for next year, or on the recently selected Missions of Opportunity that will extend two other Discovery program missions, Deep Impact and Stardust. The decision to delay involved a cascade of factors, but weather was the ultimate cause, as Green explained. "In late June, the Dawn [launch period] was from July 7 to July 19. The daily launch window started at about 3:30 to 4:00 each day for about 20 to 30 minutes." [Note: the term "launch window" refers to the time on a given day that a spacecraft may launch, whereas the term "launch period" refers to a multi-day stretch of time that usually contains many launch windows. During the press briefing Green used "window" for both; his usage has been corrected here for clarity.] Green showed a graph that explained the availability of two different vehicles that NASA uses for tracking rockets launched from the Cape during their second and third stage burns. "Big Crow is a plane, and OTTR is a ship. For these assets to work, they must be stationed in the southeast Atlantic, just off the coast of Africa. As we got close to the beginning of the launch period, we had several problems. First OTTR had mechanical problems, and that delayed it getting started towards its station. As we began early in July to decide to fuel [Dawn's Delta II launcher] second stage, we had problems with the local weather at the Cape that prevented us doing it on successive days. This also eroded the launch period. And finally, OTTR, as it made its journey into the southeast Atlantic, ran in to 12-foot seas. This further delayed it getting in to station, reducing our launch period to only four or five days."
Green continued, "When these events occurred -- bad weather at the Cape preventing us from fueling, bad weather in the south Atlantic preventing us from getting our tracking assets into station -- we took a good look at our second launch period for Dawn, which begins in September and goes all the way into mid-October, a period of a good five to six weeks. The advantages of the new period we saw immediately. It enabled us to begin our launch window on a daily basis at 9:00 in the morning. [The September launch windows are] still 20 to 30 minutes in duration, but well before the afternoon thunderstorms that are anticipated at this time of the year. "Furthermore, launch within that second period enables us to meet all our prime mission objectives and enables us to get to Vesta approximately two months earlier." By this Green meant that the Earth-to-Vesta cruise duration is shortened by two months with the two-month delay to a September launch, resulting in an arrival date at Vesta that is the same -- October 2011 -- regardless of whether Dawn launches in July or September. "So this made the decision easy. Move out of the way of our Phoenix launch, and get ready for our second window, starting in September, for Dawn." The Phoenix mission is scheduled to launch to Mars in a launch period opening August 3 and closing August 24. Phoenix has much tighter launch windows than Dawn, lasting only minutes each day, and a delay past the August 24 end of its launch period would result in a delay to the mission of more than two years, until the next time Earth and Mars align properly for an Earth-to-Mars trajectory. Phoenix is departing from pad 17A at Cape Canaveral Air Force Station. Dawn's rocket was being assembled on pad 17B. A launch cannot occur from one pad while the other is occupied by a spacecraft. One way or another, Dawn had to make way for Phoenix -- either by launching before Phoenix, no later than July 19, or by the delay of the launch period and the destacking and storage of the spacecraft. NASA tried to launch Dawn before Phoenix, but in the end had no choice but to get out of the way and try again in September.
The cost of the delay will be $20 to $25 million and is primarily due to the cost of destacking the spacecraft and maintaining the already-ramped-up staffing for Dawn's launch, checkout, and early orbit phase through the length of the September launch period. Some cost savings can still be achieved if Dawn launches earlier rather than later. The dates have not yet been fixed for the launch period, but Green said that they "anticipate the early part of September" for its opening date and that "it extends well into October, around the 15th or 17th. The period is five or six weeks long." If the September launch period is such a good opportunity, it seems a fair question to ask why NASA did not choose this delay earlier, before incurring the costs and risk of stacking the spacecraft and launcher on the pad. Indeed, on June 25, NASA seemed close to this action when they changed Dawn's scheduled June 26 prelaunch press conference from the standard format of an explanation of the science goals of the mission by the science team to an "update" on "issues that need to be resolved" by Headquarters staff. At that press conference, they explained that by pressing forward for a July launch they hoped to avoid incurring the extra $20 million cost of the delay, and pointed out that September and October are during the Atlantic hurricane season. Despite the length of the September to October launch window, there's no guarantee that weather will permit Dawn to launch then -- or that the problems that have plagued Dawn's launch preparations will not recur. For now, Dawn will be removed from its launch vehicle and stored for two months, making way for the launch of the Phoenix mission to Mars from the adjacent pad at Cape Canaveral Air Force Station. In two months, Dawn will have its final opportunity to get to space. |
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