What is currently known about the orbital debris flux is from a combination of ground based and in-space measurements. These measurements have revealed an increasing population with decreasing size. A summary of measurements is presented for the following sources: the North American Aerospace Defense Command Catalog, the Perimeter Acquisition and Attack Characterization System Radar, ground based optical telescopes, the Explorer 46 Meteoroid Bumper Experiment, spacecraft windows, and Solar Max surfaces

Kessler, D. J.

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ORBITAL DEBRIS MEASUREMENTS: D.J.. Kessler, NASA, Johnson Space
Center, Houston, Texas 77058
An any one time, there are about 200 kgm of meteoroid mass moving through
altitudes below 2000 km at an average speed of about 20 km/sec. Most of the
mass is found in particles of about 0.1 mm diameter [1]. The meteoroid
environment has been a design consideration for spacecraft. The Apollo and
Skylab spacecraft were built to withstand catastrophic impacts on critical
systems from meteoroids having sizes up to 3 mm. in diameter. Larger sizes
were so few in number as to be of no practical significance for the duration
of the mission. Some small spacecraft systems required additional shielding
against meteoroids as small as 0.3 mm. diameter in order to maintain an
acceptable reliability. The trend in the design of future spacecraft (as for
example, the Space Station) is towards larger structures, lighter
construction, and longer.stay times in orbit. These factors increase the
range of concern from about 0.1 to 10 mm. in diameter.
However, it is no longer sufficient to consider only the natural
meteoroid environment in spacecraft design. Since the time of the Apollo and
Skylab programs, launch activity has continued and increased. As a result,
the population of orbital debris has also increased substantially. The total
mass of debris in orbit is now approximately 2,000,000 kgm at altitudes below
2000 km. Relative to one another, this mass is moving at an average speed of
10 km/sec; or only half the relative speed of meteoroids. The most
significant difference between the orbital debris population and the
meteoroid population is that most of the debris mass is found in objects
several meters in diameter, rather than 0.1 mm diameter as for meteoroids
[2]. This large reservoir of mass may be thought of as a potential source
for particles in the 0.1 to 10 mm. range. That is, if only one ten-
thousandth of this mass were in this size range, the amount of debris would
exceed the natural meteoroid environment. The potential sources for
particles in this size range are many: (1) Explosions - more than 80
spacecraft are known to have exploded in low Earth orbit. The fragment size
distribution is a sensitive function of the intensity of the explosion [3,4].
(2) Hypervelocity collision in space - One or two of the known satellite
breakups may have been from hypervelocity collisions. The fragment size
distribution of such a collision is known to include a large number of
particles in the 0.1 to 10 mm size range. (3) Deterioration of spacecraft
surfaces - oxygen erosion, UV radiation and thermal stress are known to cause
certain types of surfaces to deterioate, producing small particles. (4) Solid
rocket motor firings - A third of the exhaust products of a solid rocket
motor is aluminum oxide particles in the size range 0.0001 to 0.01 mm. (5)
Unknown sources - Other sources are likely to exist. Particulates are
commonly observed originating from the Shuttle, and other objects in space.
What is currently known about the orbital debris flux is from a
combination of ground based and in space measurements. These measurements
have revealed an increasing population with decreasing size. Beginning with
the largest sizes, a summary of these measurements follows.
NORAD Catalogue. The North American Aerospace Defense Command (NORAD) is
responsible for tracking and maintaining a catalogue of "all man-made
objects" in space. The catalogue as of September 30, 1985, contained 5712
objects in space [5], most in low Earth orbit, and nearly half resulting from
satellite breakups. The ability to catalogue small objects is limited by the
power and wave length of individual radar sites, as well as the limitations
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on data transmission within the network of radar sites. Consequently,
objects smaller than about 20 cm are not usually catalogued.
PARCS Radar. The Perimeter Acquisition and Attack Characterization System
(PARCS) at Concrete, N.D. is NORAD's most powerful radar. It can typically
detect objects as small as about 8 cm in low Earth orbit. Past [6] and
continuing tests with this radar have shown an uncatalogued population which
is between 7% and 18% greater than the catalogued population.
Ground Based Optical Telescopes. Lincoln Laboratory was contracted to
use their Experimental Test Site (ETS) to search for centimeter sized debris
in low Earth orbit. The ETS consists of two, 31 inch telescopes located in
White Sands, New Mexico. The telescopes tracked identical overhead star
fields for about an hour when the region of space between 500 km to 1000 km
was sunlit. The star-like images were detected using low-light level TV
cameras and recorded on video tape. Centimeter size objects were seen as
16th magnitude objects moving at about a degree per second through the field
of view. Two telescopes were required to obtain the altitude of the object,
using parallax. This permitted discrimination between orbiting debris and
meteors which are only found below 120 km. Part of the observing program was
coordinated with NORAD. The search detected eight times as many orbiting
objects as were predicted using the catalogued population [7], indicating
that a total population of approximately 45,000 objects larger than about 1
cm are in low Earth orbit. Additional tests of this type are planned to
determine probable sources, and to improve statistical uncertainties.
Explorer 46 Meteoroid Bumper Experiment. Explorer 46 was launched into
Earth orbit in August, 1972. One of the experiments, the Meteoroid Bumper
Experiment, consisted of 3 orthogonal surfaces with areas totaling 19.2 sq.
met. These areas were sensitive to penetrations by particles larger than
0.075 mm at 7 km/sec. The distribution of impacts on these orthogonal
surfaces illustrated that the surfaces experienced a highly directional flux,
one direction measuring a factor of 10 greater flux than another. Assuming
the experiment performed as expected, the only explanation is that the
satellite became gravity gradient stablized and mostly measured an Earth
orbiting population [8].
Spacecraft Windows. Since the beginning of manned space activities,
returned windows have been examined for meteoroid impacts [1]. Beginning
with the Apollo/Skylab windows, the windows were examined in the Scanning
Electron Microscope (SEM), and some chemistry of the impacting particles was
determined. About half of the Apollo/Skylab hypervelocity pits were aluminum
lined, and concluded to likely be man-made in origin [9]. However, since
Shuttle windows are reused, they cannot be as easily examined.
Three days after the launch of STS-7, the crew reported a pit, about 4 mm
in diameter, on the external surface of one of the windows. This damage
exceeded safety requirements for launch and the window was replaced.
Consequently, this window was examined to the same detail as were previous
Apollo windows for meteoroid impacts. Energy Dispersive X-ray Analysis (EDS)
was again used to determine the composition of partially fused material found
in the bottom of the pit. Titanium oxide and small amounts of aluminum,
carbon, and potassium were found added to the pit glass. Crater morphology
places the impacting particle diameter at 0.2 mm, and a velocity between 3
km/sec and 6 km/sec. From this data, it is concluded that the particle was
man-made and likely an orbiting paint fleck [10] . This is the first
conclusive case where orbital debris can be shown to have caused the
operational loss to a space vehicle subsystem.
ORBITAL DEBRIS MEASUREMENTS
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 63
Solar-Max Surfaces. Approximately 1.5 sq. met. of thermal insulation
surface, and 1.0 sq. met. of aluminum louvers were returned from the
Solar-Max satellite after 50 months of exposure to space at more than 500 km
altitude. The thermal insulation consisted of 17 layers of aluminized
Kapton, each separated by a dacron net. This type of surface has capture
properties similar to the capture cell experiment on LDEF and offers an
excellent opportunity to obtain chemistry of impacting particles. About 160
impacts which had penetrated the outer layer were found in 0.5 sq. met.
These penetrations deposited ejecta on the following layers. Over a thousand
craters were found which did not penetrate the 1st layer — more than
expected from meteoroid impacts alone. EDS analyses shows clear evidence
that most of the smaller craters were produced by particles with sufficient
velocity to produce melting. EDS analysis also shows that a large number of
these pits contain titanium, zinc, potassium, silicon and chlorine. Except
for chlorine, this chemistry corresponds to the chemistry of thermal paints
currently used by NASA for space applications. Meteoroid impacts have also
been identified. While analysis is far from complete, the preliminary
results are finding twice as many orbital debris impacts as meteoroids,
suggesting that billions of 0.1 mm debris particles are in Earth orbit [10,
11,12].
Nearly all of the orbital debris measurements to date show an orbital
debris flux which exceeds the meteoroid flux. These measurements are
summarized and compared with the meteoroid flux in Chapter 7 of this volume.
References:
1. Zook H.A., Flaherty R.E., and Kessler D.J. (1970) Planetary and Space
Sciences, 18, 953.
2. Kessler D.J. and Cour-Palais B.C. (1978) JGR, 83, 2637. •
3.- Kessler D.J. (1981) Jour, of Spacecraft and Rockets, 18, 357.
4. Johnson N.L., Gabbard J.R., DeVere G.T., and Johnson E.E. (1984) Teledyne
Brown Engineering Report CS84-BMDSC-0018.
5. Office of Public Affairs, Goddard Space Flight Center (1985), 25.
6. Kessler D.J. in Orbital Debris NASA CR2360 (1985), 39..
7. Taff L.G., Beatty D.E., Yakutis A.J.,. and Randall P.M.S. In Advances in
Space Research (1985) 5,.2, 35.
8. Kessler D.J., in Properties and Interactions of Interplanetary Dust
(1985), 97.
9. Clanton U.S., Zook H.A., and Schultz R.A., In Orbital Debris NASA CR2360
(1985), 177.
10. McKay D.S., (1985) personal communication.
11. Kessler D.J., Zook H.A., Potter A.E., McKay D.S., Warren J.L., Watts L.A.,
Schramm, L.S., Wentworth S.J., Robinson G.A., In Lunar and Planetary
Science XVI (1985), 1, 434.
12.. Schramm L.S., McKay D.S., Zook H.A., Robinson G.A., In Lunar and Planetary
Science XVI (1985), 2, 736.


Orbital debris measurements

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