A publication of The Orbital Debris Program Office NASA Johnson Space Center Houston, Texas 77058 January 1999 Volume 4, Issue 1. Project Reviews International Space Station Debris Risk Assessment Process Dana M. Lear, Eric L. Christiansen element, conceptually a small, flat panel, which Since the first ISS M/OD assessments, Finite is used to define all of the outermost ISS The 8th International Space Station Integrated Element Models (FEMs) have been used to surfaces. The current ISS FEM consists of Meteoroid & Orbital Debris Threat Assessment define the physical shape of ISS surfaces. The approximately 90,000 of these elements (Figure (ITA) is currently underway, with release smallest analysis feature on the FEM is the 1). Each element has an associated Property planned for early 1999. Ana- Identification Number, or lysts at the NASA/Johnson PID, and a Ballistic Limit Space Center’s Hypervelocity Equation, or BLE. Impact Technology Facility Groups of elements are (HIT-F) have been producing used to make up module ITAs since the early 1990s in surface features; these in support of the International turn are used to make up Space Station effort. Frequent ISS modules. The ISS assembly plan changes modules are then have facilitated the need to re- assembled further into a peat the assessments every 9 to complete ISS FEM. This 12 months. Although the as- FEM information is used sessments typically require 6 by the analysis computer months to produce, each is the program, BUMPER-II, to result of many years of testing perform the fundamental and research. The overall as- M/OD risk calculations. sessment process is complex but can be simplified into four The meteoroid and orbital general areas of concentration: debris environments im- (1) Finite Element Model defi- plemented in BUMPER-II nition, (2) M/OD environment are described in NASA model definition, (3) ballistic documents SSP 30425 limit equation definition, and and TM 104825. Each of (4) overall probability Figure 1. ISS Finite Element Model ITA-6: Assembly Complete Configuration. these documents contains analysis. Approximately 90,000 elements total. Large PV arrays and orbiter not included. (Continued on page 2) Inside... Update from NASA’s Orbital Debris Observatory .................................................................... 3 1998 Ends with Eighth Satellite Breakup .................................................................................... 5 EVOLVE 4.0 Preliminary Results ............................................................................................... 6 Post-Flight Inspection STS-89 ..................................................................................................... 7 1 The Orbital Debris Quarterly News Project Reviews, Continued International Space Station Debris Risk Assessment Process, Continued (Continued from page 1) PNI. But what if we want to know how many angle, and particle diameter (Figure 3). The information about M/OD particle of these impacting particles will penetrate the element property identification numbers, or concentrations, sizes, orbits, and altitudes. surface? To take this extra analysis step with PIDs mentioned earlier, are used to link a par- Figure 2 illustrates the M/OD threat directions BUMPER-II, it is necessary to describe the ticular BLE to a region of the FEM. A penetra- assessed in BUMPER-II. threshold size of an impacting particle that just tion occurs above a particular BLE curve for a penetrates each particular surface of the FEM, given impact angle and impact velocity. The FEM and M/OD environment provide suf- and requires use of the BLE. BUMPER-II uses the discrete velocity and ficient information for BUMPER-II to deter- angle distributions with the BLEs for each PID mine how many M/OD particles of a particular The BLEs have been derived from analysis to assess the Probability of Non-Penetration, or diameter will impact the FEM over a specific based on thousands of hypervelocity impact PNP (Figure 4). period of time including the effects of tests. A BLE defines the maximum, or critical, shadowing. An M/OD analysis of this type is particle diameter that a particular surface can called a Probability of Non-Impact analysis, or withstand as a function of velocity, impact Figure 2. M/OD Threat Directions Figure 3. Typical PID Ballistic Limit Equation Curves Meteoroid - 149 threat directions. Debris - 90 threat directions. Impact angles of 0o and 60o (for illustration only) (Each threat direction for EACH element assessed.) Figure 4. Examples of BUMPER PNI & PNP Results. Upper Table provides the PNI and PNP for each element of the FEM. Graphic in lower left-hand corner describes the assessed orbital debris penetration risk on a particular surface PID as a function of impact angle and velocity. Graphic in lower right-hand corner provides a color-coded illustration of predicted high (red) versus low (blue) impact zones on the FEM. (for illustration only) 2 The Orbital Debris Quarterly News Project Reviews Update from NASA’s Orbital Debris Observatory John L. Africano and John V. Lambert components: a dual-processor Pentium-II PC with 54 Gbytes of disk space, an IEEE-1394 Observational Results: Liquid Mirror Telescope SCSI-PCI card, and a Sony DHR-1000 digital video tape player. The software consists of We have completed manual processing of the 1998 Summer Shutdown: special-purpose digital video C-code and IDL first forty-seven video tapes, about seventy-five processing routines developed by Lockheed- hours of data, collected by the LMT. In these Each year the LMT is closed for maintenance Martin at JSC. The program controls the digital observations, we detected 62 correlated objects during the summer months because of poor video tape player; decodes the compressed and 231 uncorrelated targets (UCTs). weather. As part of the general housekeeping video images; automatically detects and Assuming an albedo of 0.1 for the debris this year, the dome and prime focus areas were measures the positions and brightnesses of objects we are detecting objects down to about thoroughly cleaned, and all electrical moving objects; and compiles a report on all 2 to 3 cm in diameter. About 30 of these connections were checked and reseated. The detected debris. The human operator can uncorrelated objects may be associated with the mirror support system was re-aligned so that review and verify the video record for each RORSAT sodium potassium (NaK) droplets. there is less than a one arc-second error detection event. The debris detection software Assuming a 0.8 albedo for these objects leads between the main bearing rotational axis and is now in the final stages of development and to detections as small as 1.5 cm. the gravity vector. A new optics package, an testing. aspherical doublet corrector, was also installed The LMT and the Haystack radar detection replacing the rear single element of the old We have tested the automated debris detection rates as a function of object size are compared corrector. This new corrector removes field system using about thirty-two hours of digital in Figure 1. The observed Haystack detection distortion, provides better image quality over a video data, some 3.6 million images, previously rate was increased by a factor of 5.2 to account larger field-of-view, and covers a broader processed by human screeners. The automated for the difference between the LMT (0.24 spectral range. Preliminary results confirm processing is somewhat slower (sixteen times degrees) and Haystack (0.05 degrees) beam improved image quality from the new corrector real time) than the manual processing, but well sizes. While there are still several additional which should permit the LMT to detect fainter worth it. Comparisons between the results of observational biases to be considered in objects. Regular operations resumed on the automated system and the human screeners processing the LMT data, the optical results are October 12, 1998 and, by mid-November, fifty indicate that, while the automated detection very consistent with the radar results. The main hours of debris observations had been obtained. system may occasionally miss a debris event difference occurs in the 5 to 20 cm size regime. located very close to the edge of the field-of- At this time we are exploring possible Automated Debris Detection System: view, it almost always finds one or two explanations for these differences. additional events per tape not found by either We have developed a prototype hardware/ screener. Once operational, the automated The total number of detectable objects in orbit software system to automate the debris debris detection system will allow us to keep up was estimated from the LMT observations detection process. The hardware portion of the with the large volume of data being collected using a variation of the technique developed by system is composed entirely of off-the-shelf each clear night by the LMT. (Continued on page 4) 100 Haystack 6000 10 5000 LMT ects j 4000 1 3000 Detections/Hour 2000 0.1 Number of Ob 1000 0 0.01 0 500 1000 1500 2000 0.001 0.01 0.1 1 10 Estimated Size (m) Altitude (km) Figure 1. Comparison of Optical and Radar Size Distributions. Figure 2. LMT Optical Population Estimates. 3 The Orbital Debris Quarterly News Project Reviews Update from NASA’s Orbital Debris Observatory, Continued (Continued from page 3) size sensitivity of the LMT with altitude in the Figure 3 presents a typical CCD exposure. The Seniw (1992). This approach assumes that the above estimate. Examination of the cataloged telescope is moved to a particular azimuth and arrival rate of orbiting objects into the LMT and derived altitude distributions in Figure 2 elevation. The telescope is allowed to settle field-of-view obeys Poisson statistics and that suggests a possible undersampling of objects in with the drives turned off, then a 20 second the objects are randomly distributed in circular sun-synchronous orbits which can not be exposure is taken. Since the telescope is not orbits. A basic limitation in deriving the orbital observed by the LMT.