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Meteoroid & Debris

A First Look at the Clementine Interstage Adapter Satellite Orbital Meteoroid and Debris Counter (OMDC) Results

John P. Oliver, Senior Research Scientist, Institute for Space Science and Technology, 1810 NW 6th Street, Gainesville FL 32609

This paper is based on inputs from the OMDC science team: Henry Garrett, BMDO, John Oliver, Charles Simon, S. F. Singer and Jerry Weinberg, ISST, Donald Kessler and Herbert Zook, JSC, William Kinard, Donald Humes and Phillip Kassell, LaRC, Phillip Anz-Meador and Mark Matney, LESC, Jimmie Wortman, North Carolina State University, John Thomas and David Medina, Phillips Labs, and Dale Atkinson, POD Associates.

Abstract
For 95 days, from early February to early May, 1994, the Clementine Interstage Adaptor Satellite (ISAS) carried the Orbital Meteoroid and Debris Counter experiment (OMDC) in an eccentric Earth orbit with an (initial) perigee of 6700 km and apogee of 133,000 km. The OMDC was comprised of 48 MOS microparticle impact detectors arranged in three groups distributed circumferentially around the periphery of the ISAS. Neglecting five turn-on events, a total of 75 impacts were recorded. Fifty-three of these events were time-tagged, allowing a determination of the spacecraft location at impact. The data divide cleanly into two periods; 22 impacts were recorded during the first 76, while 53 impacts were recorded in the last 19 days. Only one-third of the early impacts were time-tagged; seven of these occurred within ten degrees of apogee. Almost all of the remaining impacts were time tagged and were distributed relatively uniformly around the orbit. No impacts were observed at altitudes less than 2000 km (i.e. LEO).

Introduction
The Interplanetary Dust Experiment (IDE), active on the Long Duration Exposure Facility (LDEF) from April 1984 to March 1985, has shown that the small particle orbital debris environment is distributed in a highly non-random fashion in both location and time. A significant fraction of the impacts observed occurred in highly concentrated "clumps" or clouds of material which were encountered during only a small portion of the LDEF orbit (Figure 1). In one such cloud the impact rate reached 12 impacts/second/sq. meter, almost four orders of magnitude greater than the 6 year average impact rate (Ref 1). The LDEF IDE data show that (a) the microparticle component of the orbital debris environment is more complex than the standard models suggest and (b) it is difficult to separate the orbital debris component from the interplanetary dust component in low Earth orbit.

The Clementine mission to map the moon and then observe an asteroid (Ref 2) provided an opportunity to gather new data on the microparticle meteoroid and debris environment. The Clementine Interstage boosted the main spacecraft into an elliptical orbit around the Earth, and then separated to become the Interstage Adaptor Satellite (ISAS; Ref 3). The orbit of the ISAS had an inclination of 66 degrees, an eccentricity of 0.90, and a period of 52 hours. The initial apogee was 133,370 km and the initial perigee was 6736 km. This highly eccentric orbit meant that the ISAS spent less than one hour in LEO each orbit. Thus most of the exposure was to a relatively debris-free environment. The Orbital Meteoroid and Debris Counter‡ experiment was carried on the ISAS.

The OMDC Experiment
The OMDC hardware has been described in Ref 4 . Briefly, 48 MOS microparticle sensors (essentially the same as those flown on LDEF) were installed on the ISAS in three groups of 16. Each group occupied a 120 degree arc of the circumference of the ISAS, just below the top ring. The OMDC sensors were designed to detect hypervelocity impacts of microparticles of 0.5 microns diameter and larger (Ref 5).

Each group of 16 sensors was connected to an accumulating counter which registered a count upon an impact on any one of the sensors in the group. The total bias current drawn by all 48 sensors was monitored to allow an indication of the overall health of the sensors. A typical sensor was expected to have a leakage current of approximately one microampere. A series resistor limited the short circuit current drawn by a single sensor to about 20 microamperes.

Data Stream Format and Downlinks
The status of the OMDC experiment was indicated by two items in the ISAS data stream. The ISAS Data Acquisition System (DAS) sampled the value of the OMDC counters once every five seconds. If any counter value was found to differ from its previous value, the values of all three counters were stored in the DAS buffer memory. Twice per hour the DAS read the total OMDC sensor bias current into the buffer memory, along with other analog data. Housekeeping data and the data from several other experiments was also stored in the buffer memory.

ISAS data downlinks were possible near the time of perigee passage (approximately every 52 hours) when the spacecraft was within 20,000 kms of a ground station. The planned procedure was to transmit a "TX-on" signal when the spacecraft came within range. The spacecraft was programmed to transmit its entire data buffer upon that ground command, and then to erase the buffer. Unfortunately, ground-based RF interference with the spacecraft telemetry receiver caused the spacecraft systems to transmit the data buffer and then erase it at every perigee, whether the ground station was in a position to receive the data or not. As a result, buffered data were successfully received and stored at the ground station for only about forty percent of the ISAS active data taking period.§

Observations
The OMDC sensor leakage current data are shown in Figure 2. This figure graphically illustrates the result of the spurious downlink triggers on the data collection process. The vertical rules mark perigee passage for each orbit. As can be seen, data were received on only 18.5 orbits out of 45.

The sensor leakage current started at about 75 microamps (µa), slowly declining to just less than 60 ua over the first three weeks.** This decrease is likely due to the outgassing and curing of insulation and potting compounds used on the sensor wiring. The plateau value of about 58 µa is consistent with the expected leakage for an array of 48 sensors.

These data indicate that it is unlikely that more than one sensor was short circuited and non-functional at OMDC turn on time although is possible that as many as two sensors were short circuited at that time. The small rise in sensor leakage current at the very end of the flight may be due to the relatively high number of impacts recorded at this time, or to a change in the sensor temperature. There are no direct sensor temperature measurements available. As may be seen in Figure 6, the solar array temperature reached relatively high values at this time.

The readings of the three OMDC counters are given in Table 1 for each data record. These are the "time tagged" readings for which the ISAS position could be determined. The ISAS radial distance, angle from perigee, and hours after perigee tabulated were calculated from orbital elements provided by the Naval Research Lab (Refs 6, 7). The column labeled "Accumulated" lists the contents of the three counters at the time of the record. The column labeled "New Counts" gives the change in counter value. Recall that no record was stored unless at least one counter recorded a count.

The first record was recorded less than five seconds after the ISAS data acquisition system startup. These five counts represent counter turn-on transients and (possibly) impacts that occurred during the week in low Earth orbit before the bias was applied to the sensors. These counts should be disregarded in the analysis of OMDC data. There are five other records in Table 1 that show more than one new count. Each of these occurs after spurious downloads had cleared the data buffer. There are no cases where more than one count can be assumed to have occurred in a single five second interval.

The orbit of the ISAS is shown in Figure 3 with the location of most of the time tagged records marked by orbit number. The counter readings are plotted in Figure 4 as a function of time. All of the time tagged records during the first 60 days occurred within ten degrees following apogee, however only one-third of the early impacts were time tagged. The time tagged records during the last 20 days were distributed over much of the orbit although no time tagged counts were recorded in LEO (altitude less than 2000 km).

The data appear to be divided into two distinct segments which have significantly different counting rates. During the first 76 days a total of 22 impacts were recorded giving an average counting rate of 0.28 impacts per day. During the last 19 days a total of 53 impacts were recorded giving an average counting rate of 2.8 impacts per day. The data are summarized in Table 2.

Table 2   OMDC Counting Statistics
                Total     Tagged     Rate
First 5 seconds   5          -          -
First 76 days    22          8        .28
Last 19 days     53         45        2.8
Total            80         53        .79

During the early period the counts increased linearly with no discrete events. This may represent the random background. The later period appears to be comprised of two discrete events with an intervening quiescent period. The random background component may well have continued through this period.

ISAS Dynamical Behavior
In order to interpret the OMDC counter data in terms of fluxes, it is necessary to know the orientation of the spacecraft while in orbit. Unfortunately, the ISAS was not instrumented to determine its attitude or rotation during flight. It appears that the ISAS began to rotate and/or tumble upon separation from Clementine. Optical images obtained by the Clementine imaging systems at separation have not proved helpful (Ref 8). Attempts at the Naval Research Lab to interpret the polarization of the incoming radio signals in terms of spacecraft attitude have not been successful (Ref 9). Attempts at ground based photography from sites in Hawaii and Florida were unsuccessful.

One approach to inferring the ISAS dynamical behavior that appears to yield some useful information is the analysis of the Solar Array temperature. Once each hour data from a variety of temperature sensors was stored in the DAS data buffer for transmission at the next downlink. One of these sensors was installed in close contact with the ISAS external skin, on the exterior of the spacecraft about mid-way between top and bottom of the cylindrical body. It was covered by approximately 15 mils of blanketing. This sensor was intended to monitor the temperatures encountered by the solar cells.

The data from Solar Array (SA) temperature sensor are plotted for the entire 95 day ISAS flight in Figure 5. Selected samples are plotted at an expanded scale in the upper portion of this figure while the entire data set is plotted at the bottom. Several points can be made about these data.

• The temperature appears to have been constant at about 10 degrees C with little scatter for the first 27 orbits (55 days) of the mission.
• Between orbits 166 and 174 the data appear scattered uniformly over a band of temperatures.
• Finally, during the last two orbits a large amplitude oscillation appeared.

It seems probable that the ISAS was rotating rapidly relative to the one-per-hour temperature sampling rate during the early part of the mission since it appears unlikely that the Solar Array temperature sensor remained at a constant angle to the sun during this entire period. The oscillations during the last two orbits may well represent the actual rotation rate during this period. The apparent rotation period varied between 12 and 24 hours. Less probably, the oscillation rate could represent a "beat period" against the temperature sampling rate. The data scattered in a band between orbits 166 and 174 is characteristic of "under-sampled" data. The spacecraft appears to have been rotating slowly enough that the skin temperature was varying with time, but enough faster than the temperature sampling rate to yield under-sampling.

The data of Table 1 indicate that the counts were relatively evenly distributed among the three counter channels during the early part of the mission. During the last part, however, counts were clearly not uniformly distributed. There were almost no counts recorded by counter number 1. There is some apparent tendency of counters 2 and 3 to alternate active periods. The Solar Array temperature and the OMDC new count activity during the last five orbits are shown in Figure 6. The counts on counters 2 and 3 appear "clumped" to some extent but there does not seem to be a clear correlation with the Solar Array temperature (and thus the inferred rotation).

Fluxes
Plots of ISAS impact rates versus time can be misleading in terms of fluxes because of the large eccentricity, and the resultant wide range in spacecraft velocity as a function of orbital position. It is necessary to take this varying velocity into account in order to calculate fluxes. During the early portion of the mission, most of the impacts were not time tagged and thus orbital positions are unknown. However, most of the impacts during the later part of the mission were time tagged. In Figure 7 and Figure 8 fluxes are plotted for two orbits, corresponding to the two rapid counting periods previously mentioned (Ref 10). During orbit 178, counts were registered only on counter 3. Counters 2 and 3 seemed to alternate during orbit 181. The concentration of counts near geo-synchronous altitudes in Figure 6 is striking. No data were received from orbit 179 but at most one count occurred since only two counts were registered in the first record of orbit 180, one on counter 1 and one on counter 3. During orbit 181, the counts were widely distributed around the orbit. Additional discussion of the OMDC impact fluxes may be found in Ref 11 .

Acknowledgments
We want especially to thank Jerry Golba, NRL, for his assistance in accessing the data and understanding the downlink process, and Robert Dasenbrock, NRL, for providing the orbital elements of the ISAS. The author would like to thank Jack McKisson of Radiation Technologies for a number of useful discussions on the support electronics for MOS detectors, as well as his assistance in understanding the nature of the ISAS data stream. This work was performed under NASA grant NAG 1-1218.

References
‡ Also known as "Oh My Darling Clementine."

§ Note: no counts were lost, only time tags.

** Note that the "banding" of values visible in the figure is due to problems with the analog-to-digital converter in the ISAS data acquisition system but the interpretation of the data is not significantly affected.

1. Oliver J. P., Simon C. G., Cooke W. J., Singer S. F., Weinberg J. L., Kassel P. C., Kinard W. H., Wortman J. J., "LDEF Interplanetary Dust Experiment (IDE) Impact Detector Results", presented at the SPIE International Symposium on Optical Engineering in Aerospace Sensing, April, 1994.
2. Regeon, P. A., Chapman, R. J., and Baugh, R., "Clementine; The Deep Space Program Science Experiment", presented at the Clementine Engineering and Technology Workshop, Lake Tahoe, July 18-19, 1994, to be published in Journal of Spacecraft and Rockets, 1994.
3. J. Golba, "Interstage Adapter", presented at the Clementine Engineering and Technology Workshop, Lake Tahoe, July 18-19, 1994, to be published in Journal of Spacecraft and Rockets, 1994.
4. Kinard, W. H., "Orbiting Meteoroids and Debris Counting Experiment", presented at the Clementine Engineering and Technology Workshop, Lake Tahoe, July 18-19, 1994, to be published in Journal of Spacecraft and Rockets, 1994.
5. Wortman, Kassel, and Oliver, on the IDE Archive CD-ROM, LaRC.
6. Robert Dasenbrock, private communication.
7. Dasenbrock, R., "Interstage Adapter Satellite Orbit Characteristics", presented at the Clementine Engineering and Technology Workshop, Lake Tahoe, July 18-19, 1994, to be published in Journal of Spacecraft and Rockets, 1994.
8. Henry Garrett, personal communication.
9. Jerry Golba, personal communication.
10. Mark Matney and Phillip Anz-Meador, Lockheed Engineering and Sciences Company; private communication.
11. Thomas, J. N., Medina, D. F., and Garrett, H. B., "Measurements of Interplanetary Dust and Orbital Debris on the Clementine Interstage Adapter Satellite, to be published in Journal of Spacecraft and Rockets, 1994.

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