LDEF Interplanetary Dust Experiment (IDE) impact detector results

John P. Oliver, C. G. Simon, W. J. Cooke, S. F. Singer, J. L. Weinberg,

Institute for Space Science and Technology 1810 NW 6th Street, Gainesville, Florida 32609

P. C. Kassel, W. H. Kinard

NASA Langley Research Center Hampton, Virginia 23665-5225

J. J. Wortman

North Carolina State University Dept. Elec. & Comp. Eng. Box 791 Raleigh, North Carolina 27695-7911

ABSTRACT

The Interplanetary Dust Experiment (IDE) provided high time resolution detection of microparticle impacts on the Long Duration Exposure Facility satellite. Particles, in the diameter range from 0.2 microns to several hundred microns, were detected impacting on six orthogonal surfaces of the gravity-gradient stabilized LDEF spacecraft. The 11 1/2 month tape-recorded data set represents the most extensive record gathered of the number, orbital location, and incidence direction for microparticle impacts in low Earth orbit. Among the results to be discussed is the discovery of orbital debris rings and clouds. In some cases, impacts occurred in a localized region of the orbit for dozens or even hundreds of orbits in Multiple Orbit Event Sequences (MOES). In addition, more than a dozen intense and short-lived "spikes" were seen in which impact fluxes exceeded the background by several orders of magnitude. An analysis is given of the orbital parameters and of possible progenitor events for a selection of MOES.

1. LONG DURATION EXPOSURE FACILITY

The Long Duration Exposure Facility (LDEF) spacecraft was deployed by the Challenger space shuttle orbiter on April 7, 1984. Placed in a nearly circular orbit at an altitude of nearly 500 km and with an orbital inclination of 28.5°, its mission was to sample the near-Earth environment for 9 months. The entire spacecraft was to be returned to Earth at the end of the mission. The LDEF vehicle is a re-usable 12-sided cylinder about 9.1 m long and 4.3 m in diameter designed and constructed at the NASA Langley Research Center in Hampton, Virginia. Fifty-six experiments were mounted on 84 modular trays that covered the entire surface.

The LDEF was gravity-gradient stabilized, so that the long axis was always directed at the sub-satellite point on the Earth's surface. Rotation about the long axis was inhibited by a magnetic damping system. Thus, one of the long faces was always the "leading", or "ram" edge, facing the velocity direction. The opposite edge was then the "trailing" or "wake" edge. One end of the cylinder was always facing space, the other end always towards the Earth. The faces perpendicular to both the long axis and the velocity vector were nominally "north" and "south", continuously facing +61.5 ° and -61.5 ° declination respectively. Post-flight analysis of LDEF surfaces indicated that the spacecraft was rotated (in "yaw") about 8.2 ° from the intended orientation about the long axis. Analysis of the IDE solar sensor data confirms this result independently for the first 11½ months of the mission. The normal to the leading edge was thus rotated about 8.2 ° away from the velocity vector, with the normal to the "north" edge rotated 8.2 ° towards the velocity vector. The scheduled return of LDEF to Earth was delayed, first by scheduling problems, and then by the loss of the Challenger. Exceptionally intense solar maximum activity accelerated the decay of its orbit and it became clear that if LDEF was not recovered, it would re-enter the atmosphere in early 1990, with the loss of all of its data. It was retrieved at an altitude of 330 km by the crew of the Columbia on 12 January 1990, and returned to Earth on 20 January, the same day that NORAD had predicted as its most probable atmospheric re-entry date.

2. INTERPLANETARY DUST EXPERIMENT

The Interplanetary Dust Experiment (IDE), one of the fifty-six experiments mounted on LDEF, was conceived to permit a discrimination between cosmic dust and orbital debris, and to characterize the dust in terms of mass, velocity, time, and trajectory . The IDE experiment occupied portions of six trays, one each on the leading and trailing edges, the Earth and space ends, and the "north" and "south" edges. Five of the trays carried 80 active detectors, while the spaceward tray bore only 59. The total detector area was slightly less than one square meter. Each detector was a 50-micrometers diameter metal-oxide-silicon (MOS) semiconductor capacitor (Figure 1). Each detector was charged by a bias voltage supply through a current limiting resistor. An impacting microparticle with sufficient energy could vaporize the dielectric layer and cause a transient discharge of the capacitor. The associated electronics counted and time-tagged each discharge. The thickness of the oxide determines the energy required to trigger a discharge of the capacitor. The thinner dielectric requires less impact energy, providing higher sensitivity. The thickness of the oxide dielectric of 60 % of the detectors on each panel was 0.4 micrometers; it was 1.0 micrometers for the remaining 40%. Pre-flight calibration indicated that the sensors lower limits of detection, for hypervelocity particles, were roughly 0.2 micrometers and 0.5 micrometers diameter, respectively . The upper detection limit for both types (representing the particle size expected to physically break the detector substrate) was 100 mm in diameter. Identical detectors were flown on Explorer 46 (the Meteoroid Technology Satellite - MTS) in 1972.

Figure 1. Cross-section of typical MOS impact detector. The IDE sensors used 0.4 and 1.0 micrometer thick dielectric.

An on-board tape recorder was included to record the time of each impact, identified by panel and by wafer thickness, but not by specific detector. The time resolution of the IDE clock was about 13.1 seconds. About every 2.4 hours, there was also a dump to the tape of the status (illuminated or dark) of six sun sensors, the status (active or shorted) of each detector, and other "housekeeping" information. Sunrise and sunset information from the sun sensors allowed calibration of the IDE clock. IDE activation occurred at 1984 April 07d 17h 23m 43.8s ± 0.3s UTC.

Tape was only supplied for the nominal 9-month mission, and it ran out on day 348. Post-flight verification shows that there was only one recording anomaly during this time and no significant data were lost. About 15,000 impacts were recorded on the 459 detectors during the active phase of the mission. For the remaining 4.7 years of flight, the detectors continued to receive impacts which left physical craters, but no time-resolved information was recorded. Since the LDEF IDE detectors were recovered, integrated fluxes for this time period have been determined by visual examination of the detector surfaces.

3. TIME-RESOLVED DATA

In general, information on orbital debris and cosmic dust in the near-Earth orbital environment has come from the examination of surfaces recovered after exposure in orbit. While such information has provided valuable information on mean fluxes, it has been deficient in detailing the near-Earth micrometeoroid and space debris environment in two crucial ways. First, most spacecraft have not maintained their orientation in an Earth-centered reference frame. Thus the measured fluxes have been averaged over a range of directions relative to the orbital velocity vector and to the celestial sphere. Second, since the time of occurrence of each impact cannot be determined, it is not possible to investigate variations in particle flux with position and time; that is, spatio-temporal information cannot be obtained from such data analysis.

In contrast with these earlier studies, the controlled orientation and high time-resolution of the IDE data provide, for the first time, a detailed, extensive data set well adapted to analysis of the spatio-temporal characteristics of orbital debris in near-Earth orbit. An examination of the IDE data (Figure 2) shows immediately that the detected particle fluxes were neither uniform in time nor in space. All impacts on the 0.4 micrometers IDE detectors are displayed in this "seismograph" plot. The entire 346-day active data recording phase of the mission is represented along the horizontal axis. The impact rate on each of the six orthogonal surfaces is indicated by the vertical amplitude of each trace. Note that the impact rates represented in this figure are raw rates, uncorrected for effective area. This does not significantly change the appearance of the plot.

Figure 2. All impacts recorded on the 0.4 micrometer dielectric thickness (high sensitivity) IDE detectors during the 346-day time-resolved phase of the mission.

Examination of Figure 2 suggests a number of interesting points:

The activity shown on the trailing edge and the Earth end is almost certainly due the shuttle orbiter. It is well established that the shuttle will be surrounded by a "Spacecraft Induced Atmosphere" . The orbiter moved away from LDEF almost immediately after deployment, partly to avoid contaminating LDEF. The (approximately) one week duration of the impact activity seen on the shielded surfaces of LDEF suggests that a significant amount of material was distributed by the shuttle in orbits which allowed "catching up" with the LDEF from behind and from beneath.

4. MULTIPLE ORBIT EVENT SEQUENCES AND SPIKES

Closer examination of the data shows that in addition to being non-uniformly distributed in time, the IDE impact data are non-randomly distributed in both time and space. In Figure, a portion (taken from the leading edge high sensitivity detector data) of the IDE data set is examined at increasing time resolution. The upper trace in this figure represents the same information as the leading edge trace of Figure 2. The data of 4 June 1984 are selected out and displayed in the middle trace.

Figure 3. Observed activity on the leading (ram) edge of LDEF as recorded by the high sensitivity (0.4 micrometer dielectric) detectors of IDE. Note that every impact is displayed. The lack of impacts between the obvious events is real.

Figure 4. Impacts on the high activity surfaces as a function of LDEF orbital position and time for the 346 day time-resolved data set. Note that the orbital longitude axis partially repeats at the top.

The regular spacing of the impacts matches the LDEF orbital period of approximately 94 minutes. The final trace of this figure displays a single 94-minute segment of the 4 June data. All the impacts during this segment occurred during a period of less than five minutes. As may be seen in these figures, the IDE data set contains many impacts which occurred in "bursts", during which numerous impacts were recorded in a short time. Such a burst we have designated an event. At the finest resolution, events may show structure. For example, the 4 June event illustrated here appears to be double. A number of these multi-event sequences appear in the IDE data set. As illustrated in Figure 3, events may be seen to reoccur each time the LDEF returned to the same point in its orbit. These we call multi-orbit event sequences (MOES).

A significant conclusion resulting from the high-time resolution of LDEF IDE data displayed in Figure 3 is that the instantaneous fluxes observed are much greater than the mean fluxes. As shown in the text imbedded in the figure, the mean flux calculated from the 346-day data set is 0.0017 impacts/second/sq. meter. The peak flux, observed with the IDE time resolution of 13 seconds, was 12 impacts/second/sq. meter, almost 4 orders of magnitude greater! While long-term fluxes may be useful for engineering structures and similar purposes, there are circumstances where peak fluxes may be more useful. The IDE results indicate that an optical surface such as a window (which could be degraded by small particle impacts) could need replacement far sooner than would be predicted by mean fluxes.

While the structure of an individual MOES is illustrated in Figure 3, the distribution of the observed MOES with orbital location and time is best illustrated in a plot such as Figure 4. In this figure, all impacts on the high sensitivity detectors mounted on the North, Ram, and South LDEF surfaces are plotted as a function of LDEF orbital longitude and time. A number of MOES are indicated. Also indicated are “spikes”, defined as sudden bursts of impacts which occurred on only a single orbit. These spikes were the most intense individual events observed by IDE. In addition to their lack of multiple-orbit repetition, spikes differ from the events of an MOES by frequently appearing as pairs of events, separated in orbital longitude by 10 to 30 degrees. This "bifurcated" structure of spikes is visible in Figure 4.

5. DETERMINATION OF MOES IMPACTOR ORBITS

Most of the 15,000 impacts recorded by IDE occurred in MOES. It is natural to assume that such events result from the intersection of the orbit of the LDEF with that of a concentration of orbital debris. An examination of a typical MOES (e.g. the June 4 event shown in Figure 3.) shows two important characteristics:

  1. the orbital debris particle orbits are eccentric; if they were circular, the IDE detectors would register the group twice each orbit since a circular orbit must intersect LDEF’s orbit (which is essentially circular) at two points, and
  2. the particles must be "smeared out" along the orbit in some ring-like or torus structure. If the particles were concentrated in a "clump", the encounters with LDEF would not occur at integer multiples of the LDEF orbital period, unless the period of the particle orbit was the same as that of LDEF, an unlikely circumstance in general.
In order to deduce as much as possible about the orbit of the impacting particles in an MOES, we have developed the Method of Differential Precession. The goal of this method is to obtain the orbital characteristics of the particles which struck the IDE detectors during a MOES by an analysis of the time variation of the LDEF position over the series of encounters. This analysis makes use of the fact that the non-sphericity of the Earth induces the pole of an object’s orbit to precess, resulting in a cyclic change in the position of the line of nodes of the orbit (in the case of LDEF, the period of this precession is approximately 53 days). The oblateness of the Earth also causes the line of apsides of the orbit to precess, the point of perigee advancing if the orbital inclination is low and regressing otherwise. In general, bodies in different orbits will have different rates of these precessions, and should two of these orbits intersect, the differences in the precession rates will cause the point(s) of intersection to vary with time. If the characteristics of one of the intersecting orbits are known, the migration of the point of intersection may be used to determine the precession rates and orientation of the unknown orbit, which then may be used to calculate a family of candidate orbits.

As developed by us, the process is an iterative one. The orbital inclination of the debris orbit is estimated. We assume that the location on the LDEF orbit of the MOES impacts represents the point of closest approach between the two orbits. The differential precession between the resulting candidate orbit and that of LDEF is calculated. The result is compared with the observed change in the orbital intersection point and a new estimate of the candidate orbit inclination is obtained. In general, convergence is rapid.

One of the most prominent multiple orbit event sequences observed by IDE began on May 13, 1984, and so has become known as the "May swarm." This MOES can be characterized as being of low intensity (~3 impacts per orbit) and long duration, lasting for over 20 days (300 LDEF orbits), with several hundred impacts recorded on the IDE trays facing in the LDEF ram direction and towards the south pole of the orbit, the majority occurring on the south-facing tray. The long duration of this MOES made it an especially suitable choice for analysis by the differential precession.

The results of applying the Method of Differential Precession are illustrated in Figure 5 where apogee and perigee are plotted as a function of eccentricity for a set of candidate orbits. Deduced values of the inclination, Longitude of the Ascending Node, and Longitude of Perigee are indicated on the figure. Note that the semi-major axis varies little with the eccentricity; in this case, the variation in a is so small that we could confidently set a = 6746.5 km, regardless of the eccentricity. The dual requirement that the candidate orbits have perigees of greater than 200 km in altitude (i.e. that they lie above the atmosphere), and that they intersect the LDEF orbit, placed strict limits on the allowed values of the eccentricity which must be greater than 0.0165 and less than 0.025.

Figure 5. Perigee and Apogee of family of May swarm candidate orbits.

One of the candidate orbits (e = 0.017) was then chosen for a series of checks on the results of the method. The first check involved the computation of the particle velocity of impact over the duration of the May swarm. These velocities were then resolved into components along the LDEF body axes in order to determine the impact speeds on the IDE trays. For this particular orbit, only the south tray and the ram-facing tray were struck, with the south impact speed being larger than that for the other tray for most of the encounter (see Figure 6). This is in good agreement with the IDE observations of the May swarm, in which these same two trays recorded large numbers of impacts, with the south tray receiving the most hits.

Figure 6. Particle impact speeds in the direction IDE tray normals for the e=0.017 candidate orbit.

The second check consisted of a comparison of the sky track of the points of closest approach between the two orbits to the sky positions of the individual impacts comprising the May swarm. As can be seen from Figure 7, the agreement is excellent, with the sky track of close approach passing neatly through diffuse band of impact positions. The particles are moving in a northerly direction, whereas LDEF is moving along its orbit from left to right. At the onset of the May swarm, the impacts are located at the position labeled "Onset", with the impact positions gradually moving towards the lower right as time progresses.

Figure 7. Sky track of close approach between the May swarm candidate orbit and that of LDEF. Arrows on the orbital paths at time of onset indicate direction of motion. Both orbit tracks shift to the right with time due to precession.

It is important to note that the orbits determined are the instantaneous orbits of the microparticles at the time of impact. Because micrometer sized particles are subject to rapid decay due to non-gravitational forces, the particle orbits do not necessarily represent the original orbits of these particles. To find the progenitor orbits, it is necessary to "back-project" the impact orbits, taking into account all appropriate forces. Work is currently in progress to carry this process out on a sample of MOES.

Figure 8. Location of LDEF ground track (sub LDEF terrestrial latitude and longitude) during spikes.

6. DISTRIBUTION OF SPIKES IN TIME AND LOCATION

Spikes are not directly subject to analysis by the Method of Differential Precession since, by definition, they appear to be single events (albeit bifurcated). Never-the-less, the observed spikes show interesting patterns in their times of incidence that may yield useful clues as to their origin. Most spikes occurred on the North and Ram LDEF surfaces. Three spikes occurred primarily on the Space surface. Virtually no spike activity was visible on the South surface.

A plot of all spikes observed on the North LDEF surface as a function of the sub-LDEF terrestrial latitude and longitude is shown in Figure 8. These events were almost all concentrated above the northern hemisphere, and between longitudes 80 and 200 degrees east. An examination of launch activities has shown no correlation between Soviet or other launches and the occurrence of spikes. In the process of studying the event times of spikes, it was realized that many, especially those that exhibited bifurcation, appeared to have a 15.5 day periodicity (see Figure 9). This could be explained if the source of the spikes was a highly concentrated clump of material in an orbit whose beat frequency with LDEF was 15.5 days. Unfortunately, the short lifetimes of micrometer sized orbital debris particles does not allow such a clump to have a lifetime measurable in days, much less months. It appears more likely that the spikes result from material leaving some long-lifetime orbiting object. Again, this hypothetical source object must have an orbit which has a 15.5 day beat frequency with LDEF. One possibility that we have examined is the Solar Maximum Mission satellite (SMM). SMM was in virtually the same orbit as LDEF, differing only in semi-major axis. After deploying LDEF in April, 1984, the shuttle Challenge then increased its altitude by about 20 km. and undertook the repair of the SMM. The beat frequency between the LDEF orbit and that of an object 19.3 km. above (or below) is 15.5 days. The spikes do not, however, coincide with the times of closest approach between LDEF and SMM, as calculated from the appropriate orbital elements. It seems likely, however, that material from SMM is involved in the IDE spikes. The terrestrial latitude and longitude concentration shown in Figure 8 would then presumably be a coincidence.

Figure 9. Spikes on LDEF North surface as a function on time. Note that many spikes occurred very near to multiples of 15.5 days (as indicated by the vertical plot divisions).

7. SUMMARY

The LDEF IDE experiment detected many discrete events which can be associated with orbital debris. Indeed, the majority of micrometer sized particles detected by IDE were contained in the MOES debris cloud events. Many of these events were long-lived enough that they could be analyzed in terms of the impactor orbital elements. This longevity, in itself, suggests that much of the microparticle orbital debris environment results from material being released from longer-lived larger objects. Discrete events termed spikes were observed that may be the result of material released from the Solar Maximum Mission satellite.

8. ACKNOWLEDGMENTS

We have had a number of useful discussions of these data with many of our colleagues. We wish to particularly acknowledge discussions with Don Kessler. This research was supported, in part, by NASA grant NAG 1-1218.

9. REFERENCES

  1. Peters, P. N. and Gregory, J. C., "Pinhole Cameras as Sensors for Atomic Oxygen in Orbit; Application to Attitude Determination of the LDEF", in First LDEF Post-Retrieval Symposium, NASA Conference Publication 3134, 61-68 (1991).
  2. Singer, S. F., Stanley, J. E., and Kassel, P. C., "The LDEF Interplanetary Dust Experiment", in: Properties and Interactions of Interplanetary Dust, eds. Giese, R. H. and Lamy, P., Reidel, Dordecht, 117-120 (1985).
  3. Kassel, P. C., "Characteristics of capacitor-type micrometeoroid flux detectors when impacted with simulated micrometeoroids", Technical Note D-7359, NASA, Washington D.C. (1973).
  4. C.G. Simon, J.D. Mulholland, W.J. Cooke, J.P. Oliver, P.C. Kassel, "Long-term Particle Flux Variability Indicated by Comparison of Interplanetary Dust Experiment (IDE) Timed Impacts for LDEF's First Year in Orbit with Impact Data for the Entire 5.77 Year Orbital Lifetime", in LDEF - 69 Months in Space: Second Post-Retrieval Symposium, (NASA CP-3194), April, 1993, pp. 693-704
  5. Zook, H. A., McKay, D. S., and Bernhard, R. P., "Results From Returned Spacecraft Surfaces", in AIAA/NASA/DOD Orbital Debris Conference: Technical Issues & Future Directions, AIAA-90-1349 (1990)
  6. Laurance, M. R. and Brownlee, D. E., "The flux of meteoroids and orbital space debris striking satellites in low Earth orbit", Nature, Vol. 323, 136-138 (1986)
  7. Warren, J. L. et al., "The Detection and Observation of Meteoroid and Space Debris Impact Features on the Solar Max Satellite", in Proceedings of the 19th Lunar and Planetary Conference, Lunar and Planetary Institute, Houston, 641-657 (1989)
  8. Weinberg, J. L., "Optical Observations From the Space Shuttle", in Advances in Space Research, vol. 7, no. 5, 203-205 (1987)
  9. Cooke, W. J., Oliver, J. P., and Simon, C. G., "The Orbital Characteristics of Debris Particle Rings as Derived from IDE Observations of Multiple Orbit Intersections with LDEF", presented at the Third LDEF Post-Retrieval Conference, Williamsburg, VA, November 1993.
This document produced by
Dr. Bill Cooke; bjc@ufl.edu