The 3 months before launch saw an intense period of preparation by the Operations teams and Science Center. The Operations teams included the Shuttle Crew and Johnston Space Center Team, the Marshall Space Flight Center technical and management team, the TRW technical team, the Air Force team at Onizuka Air Force Base, as well as the CXC and IPI teams. The preparations included many days of simulation to train for a wide range of possible problems and contingencies.
The Shuttle launched from Cape Kennedy on 23 July, 1999, on the third attempt, and delivered Chandra and the IUS to the required orbit. Following two IUS burns, solar array deployment, IUS separation and a series of Integral Propulsion System burns, Chandra reached its final orbit on 7 August. The spacecraft and science instrument activation and checkout phase was completed as planned by 1 October. This phase included the excitement of taking the first focus image of the ``point source" quasar PKS 0637-752 and discovering that at Chandra's resolution the source showed a clear jet-like structure (see Fig. 2). The spectacular first light image of the supernova remnant Cas A was taken on August 19. A two month program of GTO observations began on 18 Sept followed by the start of combined GO/GTO observations on 26 November.
The spacecraft and ground system performed exceptionally well during the critical early mission phases and into the start of normal science operations. Major operational successes have included nominal power and thermal performance during the first two earth eclipse seasons, the uneventful passage through the Leonid meteor shower on November 17, and the nominal performance of the spacecraft during the 3 safe mode events. The safemodes resulted from separate ground triggers, and steps have been taken to avoid these triggers in the future. During a safemode, the spacecraft operates on a second set of electronics and hardware (the redundant ``B" side) so an important result from safemode entry and recovery is that we now know that all the B-side equipment is functioning well, and that the recovery procedures work properly on-orbit.
Of great concern during September was the detection of increasing degradation in the energy resolution of the front-side illuminated CCD chips within the ACIS instrument. The likely cause of the degradation has been traced to a flux of low-energy charged particles, probably protons, reflected by the mirrors onto ACIS at the focal point during radiation belt passage. Further degradation was halted by moving ACIS out of the focal plane during radiation belt passages. The ACIS engineering team are working hard to develop techniques to mitigate the effects of the degradation, and recent test results are encouraging.
As a result of the radiation damage to ACIS, the flight software was modified in December to ensure that the science instruments are placed in a safe position in the event of any spacecraft safing action. The software patch ensures that ACIS (or HRC) will not be at the telescope focus during radiation belt passages. A second flight software patch was uplinked in January to implement a new pointing mode called Continuous Dither. This mode enables the spacecraft to dither during slews which allows for the continuous collection of science data.
The mission transitioned from a real-time commanding mode to running from stored command loads developed from a mission schedule in late September. The pre-planned mission loads are now developed a week in advance and implement the science schedule. The team has been successful in interrupting the schedule to observe high priority Targets Of Opportunity. The approved targets have included a gamma ray burster detected by Beppo-Sax, the supernova in NGC 1637 and most recently Cygnus X-3 during an intense radio burst.
Overall the absolute observing efficiency since the start of GO observations has been approx 63% compared to a pre-launch goal of approx 70%. Contributors to the difference include larger than anticipated radiation belts and a conservative pad either side of the belts to ensure no further radiation damage to ACIS. Work is underway by the mission planning team to reduce the times for star acquisition after slews, and to allow observations to begin as early as is practical. These and other efforts in work will contribute to further increase the observing efficiency.
The processing team has been working very hard to process telemetry, generate science products, complete science Verification and Validation and make the products available to the GO and GTO's. Detailed metrics of all phases of the processing and data delivery are being tracked and the average time for data delivery has decreased from over a month at the start of the GO phase to approximately 3 weeks. The team is working to reduce the time further. In conjunction with the data distribution, the GO Grants program started in December with grants being awarded from the SAO Grants Office. Metrics have been established to track the time from data distribution to grant award and show the average reducing from 20 days at the start of the GO phase to less than 10 days.
The processing has been supported by a series of data system deliveries to fix problems found through early processing of on-orbit data, and to provide enhancements. The release of CIAO 1.1 in December for user analysis will be followed by CIAO 2.0 this summer which will include some key functions including access to an emission line data base and spectral identification tools. The second NRA has now been released with a proposal deadline of June 1, the peer review August 2-4 and the announcement of winners scheduled for August 18.
Finally, I wish to express some of the excitement of launch and activation and the thrill of seeing the first science results appear as press releases (21 since launch) and now science papers. The entire team has worked incredibly hard during the last year and are looking forward to the next year which promises to be one of great discovery.
- Roger Brissenden