9

increased flux did not cross the baseline threshold. The Perseids are lower in velocity than the Leonids (50km/second vs. 70km/second), but they still pose an ESD threat. For this year, the MEO changed the manner of computing stream fluxes which removed bias introduced by different stream velocities. For the Perseids, this resulted in estimating larger numbers when compared to the Leonid forecasts of previous years. The expected benefit of this new, and still current, methodology is a more accurate flux estimate4. Fluxes were normalized to a kinetic-energy equivelant of a 10µg particle moving at the velocity of a sporadic particle (20km/second). Another added feature of this analysis is a direct comparison of flux and fluence with sporadic background levels.

Because of the low level of predicted activity for this particular stream, no special actions were planned and Chandra passed through without event.

Starting in 2006 the MEO began delivering annual activity reports which included all streams. There are approximately 50 identified streams orbiting the sun, each with different characteristics. Streams have different particle mass and diameter distribution, velocity, spatial density, and duration, and are all normalized for an entire year of activity with respect to Earth’s orbit. Flux predictions continued to use the methodology from the 2004 Perseid report, but the reported particle sizes were chosen in accordance with International Space Station and space shuttle penetration fluxes. Basically, a particle of 1mm in diameter traveling at sporadic velocity can penetrate a Reinforced Carbon Composite (RCC) panel. It seemed reasonable that Chandra should also base risk management on a penetrating flux of particles at least 1mm in diameter, since the GFRP panels used on Chandra are analagous to RCC panels.

The annual reports were visually screened for potential threats by selecting any event with 1mm, and larger, fluxes exceeding sporadic levels. This would prompt a request for a detailed report of the given streams. Fortunately, the next several years were also suppressed in terms of meteoroid activity and did not direct risk mitigation schedules.

E. Preparation for 2011 Draconids

As early as 2009, the MEO warned the program of a 2011 Draconid outburst. The Draconids are a slower particle stream at 20km/second (i.e., the same velocity as sporadics) compared to the Leonids. At this velocity, ESD is not a threat, but spacecraft motion does cause an aberration in the radiant direction. Additionally, with an angular extent of approximately 3 degrees, the stream is more widely dispersed than previously managed storms. These differences had to be considered when calculating the actual impact threat.

With the long lead time to develop a handling plan, the basic restrictions and thresholds used for previous events were re-evaluated. For most of the mission, the CARD thresholds were defined by the 1999 Leonids, but this was an objectively arbitrary stream. The 1999 stream provided a convenient baseline since the vehicle successfully passed through the stream without impact. Using that same logic, the thresholds would have been re-baselined with the greater fluence of the 2001 event. An even more appropriate set of thresholds should be based on the design of the vehicle (i.e., establish the number of impacts the mission could survive, then translate into a probability of impact). The design documentation does define a survivability requirement for a 5 year mission, but that requirement could be modified to account for the extended mission. Significant effort would be required to complete the necessary engineering analysis to establish a new recommended threshold for taking action, and there was no guarantee it would be completed by the storm. After discussion within the program, the decision was made to keep the thresholds as they were for at least the 2011 planning. The next step was to determine if the 1mm particles were still the appropriate threshold size. Increasing the size of concern would lower the predicted flux for a given stream, since smaller particles would be ignored. Without scaling the impact thresholds within the CARD, fluxes of larger particles may never trigger meteor handling response. Eliminating small particles from consideration would also allow telescope pointing at a stream radiant, as long as the large particle flux was below threshold. The program decided not to accept this risk, since a 1mm particle could cause measurable damage with a direct impact on a detector.

The requirement to power down the instruments during peak activity was also revisited. A key operational change occurred during 2010 which modified onboard radiation detection. The radiation environment monitoring (RADMON) process was reconfigured to include inputs from the HRC anti-coincidence shield, which operates at an elevated voltage during available science time, to mitigate degraded capability of the original radiation detector. With the new configuration, powering down the HRC high voltage within the science orbit could cripple the vehicle’s response time to safely stow the instruments during periods of heightened radiation. So, if the HRC high voltage was powered down, the SIs would have to be stowed in a radiation safe configuration, eliminating the possibility of observing during the storm and violating the requirement to put the SIM at the ACIS position.