Pergamon Adv. SpaccRes. Vol. 17, No. 12, pp. (12)177-(12)182, 1996

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COSMIC DUST MEASUREMENTS IN LUNAR ORBIT

H. lglseder,* K. Uesugi** and H. Svedhem***

* ZARM, Center of Applied Space 7~chnology and Microgravity, Bremen, Germany ** ISAS, The Institute of Space and Astronauttcal Science, Sagamihara, Japan *** ESTEC, European Space Research and Technology Centre. Noordwijk, The Netherlands

~ S ~ A ~

On 15th February 1992, ISAS space engineering satellite HITEN was successfully inserted into an elliptical orbit around the moon with perilune between some 100 km and 8000 km and apolune of about 50.000 kin. On board was a small scientific experiment designed to detect cosmic dust particles, MDC - Munich Dust Counter. During a period of more than one year, until I-liten's hard landing on the moon surface at 10th of April 1993 (UTC), measurements of impact velocity, mass and crude flight direction of micrometeoroid particles have been performed. In total 150 cosmic dust impacts were detected and evaluated. From these measurements, the impact rate versus time and the dust flux versus distance from the moon are derived. The evidence of moon ejecta and some indications of particles which are orbiting the moon will be discussed. The spatial distribution of the measured particles is shown in lunarcentric as wen as in heliocentric coordinate systems. The directional distribution is also given, showing the different populations of cosmic dust particles. Finally, the gathered data will be compared with previous results from measurements in the the vicinity of the Earth and in the geomagnetic tail regiov_

INTRODUCTION

The Munich Dust Counter (MDC) is a scientific space experiment on board the HITEN Satellite of the MUSES-A mission of the Institute of Space and Astronautical Science (ISAS) of Japan. It has been developed by the Chair for Astronautics of the Technical University Munich (TUM) of Germany and the Space Science Department of ESA with support by the German Federal Ministry for Research and Technology and the German industry. The MDC has been designed to determine mass and velocity of cosmic dust particles by measuring the impact charges generated by high velocity impacts of dust particles on a gold target. The mass of the MDC flight unit is 605 g, the power consumption is 1.8 W. The actual sensor area is 10 c m x 10 cm, the field of view is 148 deg corresponding to a solid angle of 3 steradian. Demil~ of the instrument are given by Igenbergs et al./1/.

The MUSES-A spacecraft HITEN is the 13th satellite developed by ISAS. HITEN is a spin stabilized spacecraft, with a diameter of 1.4 m, a height of 0.8 m and a mass of 197 kg. The MDC is installed on the main instrument platform behind an aperture of 12 cm x 12 cm in the solar cell panel. As the MDC is mounted on the perimeter of the spinning spacecraft, it scans the ecliptic plane within one revolution of the satellite. The spin period is 3 seconds. The spin axis of the spacecraft is perpendicular to the ecliptic plane.

The Satellite HITEN was launched from the Kagoshima Space Center, Japan, on January 24, 1990, into a highly-elliptical orbit around the Earth and about two years later on February 15, 1992 it has been inserted into an orbit around the moon, see figure 1. A description of the MUSES-A mission is given by Uesugi/2/. The insertion was carried out successfully and the investigation of moon's dust environment started and continued until 10th of April 1993.

JAs~ 1~-12~M (12)177

(12) 178 H. lg lseder et al.

The apolune altitude of the first orbit was about 49.400 kin, see fig 2. During this 14 month lunar orbiting period, the dust environment near moon has been investigated from perilune distances of some 100 kin to 8000 km and apolune of about 50.000 kin, see figure 2.

Y distance from moon [* I 0 0 0 km]

5O . . . . . . . . . . . : ... . . / >~:- . . . . . . . . . . . . . . . . . . . . .

: . --~~.~,~o;'~"~ 5o X

2 5 , 0 0 0 i ron - ] ~ "

I ~ IOO0 fo5o ftO0 f15 5o fiso ~00 days from launch I mr t l t l Coord lm, - (MSO-EC)

Fig. 1. HITEN's trajectory during the lunar orbiting Fig. 2. Distance from Moon versus time in days phase, from insertion 15 Feb. 1992 to 30 April 1992. (until mission end 10 April 1993, UTC).

MEASUREMENT AND EVALUATION PROCEDURE

The MDC experiment measures the electrical charges generated by high velocity impacts of small masses on a gold surface. Using the procedure described by Iglseder/3,4/, mass and velocity of the impacted dust par~cle can be derived. Particle velocities from 1.8 km/s up to over 70 km/s and particle masses between 10 -7 g and 10 -16 g were determined.

tog m/gl log m [gl

'° I ) ~,//L ' - i . ) ~ z { . ~ : .,o . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . ~, ...........

l i " - ~ I i _..Li i i ! .......... . . . . ........... .......... ............... - 1 , . . . . . . . . . . ~ . . . . . . . . . . . . . . . ! . . . . ~- .............. ; ........... ' .......... ~ ............... - I , . . . . . . . . . . . . . . . . . . . . . . . . . .

-13 ........................................ -13"

; / -16 . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . .

.17 ;

-14 .......... ~ .............................. : - ~ ' . " ' . " - . , ! . . - -"

. ,5 .. . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . i I

- ' 6 " +~ I : :

: i

-,7 io i~o v_ imp [kmlsec]

Fig. 3a. Cosmic dust particles as measured by MDC during its 14 months lunar orbiting phase. The bars attached to the dots indicate the sensor pointing at the lime of impact. Impact velocity is relative to the sensor.

-17 I0 100

v_hel io [kmlsec]

Fig. 3b. Cosmic dust particles as in fig. 3a shown for heliocentric velocities.

C o s n u c Dust M e a s u r e m e n t s in Lunar Orb i t ( 1 2 ) 1 7 9

During the calibration of the experiment using iron particles, the associated measurement errors have been determined. Assuming a similar dependence for real dust particles, the measured velocity is accurate within 0.65 to 1.47 of the actual velocity, and the measured mass is accurate within 0.33 to 2.70 of the actual particle mass. The uncertainty in mass Includes the propagating effect of the velocity uncertainty.

OVERVIEW OF COSMIC DUST PARTICLE OBSERVATIONS

The nominal operation of the MDC experiment around moon started on February 15, 1992. During the 14 month of operation, until the 10th of April 1993, a total of 150 impacts have been recorded and evaluated. In figure 3a and 3b all data are shown in mass/velocity diagrams together with associated MDC impact direction (which is indicated with a bar) for particle impact velocity and calculated heliocentric velocity.

COSMIC DUST PARTICLE FLUX IN LUNAR ORBIT

Figure 4 gives the impact rates of cosmic dust particles as recorded by the MDC in lunar orbit. The average of slightly more than 0.5 impacts per day, is indicated in the graph by a dotted line. It corresponds to an overall flux of 1.3 to 4.1"10-4 m-2s -1, taking into account the sensor area of 100era 2, a solid angle of 3 sr for particles in a mass range 10 -16 g to 10 -7 g. The data of figure 4 indicates variations of the impact rates by about three orders of magnitudes. This is about more than one order of magnitude less than previous measurements in the vicinity of the Earth and geomagnetic tail region/5/. Clear indications for swarms and/or moon ejecta could not be detected. This observed behaviour confimas, that the dust flux near planets or moons is non-homogeneous and composed by random particles, groups (and swarms), as found with previous experiments, Hoffman et al./6/and Fechtig et al./7/.

1o-:2

1o-3

1o4

1o-5

le,-6

1 / time between two ira ~ t s [I/s]

r • • o

• • • • w

I I

1.-7~ 00 850

• " • • • " . • • • 0 * g l • e p • $8 • • • - . _ ___~_ j_o___ J , t _ ~ .

• "L : o ~,-° • . o . o - * • • •

, " " • I" "" ""l • •-I i - -7

~ 0 #50 i000 i050 l'100 1150 days.from launch

Fig. 4. Elapsed time between two consecutive impacts versus mission time in days.

Fig. 5. The particle flux for masses greater than 10-16g versus the distance from moon is shown. A variation of the particle flux rate by about a factor ten was measured.

SPATIAL DISTRIBUTION IN LUNAR- AND HELIOCENTRIC COORDINATES

Figure 6a indicates the locations of the impact events of cosmic dust particles in a lunar centric coordinate system with a fixed moon-sun line and figure 6b in a heliocentric coordinate system, with the vernal equinox line to the left. The lines marking every particle impact event indicating the direction in which the MDC was pointed at the time of measurement. The length of the line is proportional to the particle velocity in a logarithmic scale.

(12)180 H. Iglseder et al.

i APEX

: i / 20.oo0

. . . . . . . .

........ ............. ! . . . . . . . . . ~ @ - : ~ I " ~ - 7 - f - ~ - ~ : < ~ ~ . . . : ...........

H t r g N [ lmCsl

i : 1o t o o

. . . . . . _ . . . . . . . . . . . . . _ . . . . . .

Fig. 6a. Lunar-centric impact locations of cosmic dust particles

Fig. 6b. Heliocentric impact locations of cosmic dust impacts

DIRECTIONAL DISTRIBUTION OF COSMIC DUST PARTICLES

As the field of view of the MDC is 148 deg. in ecliptic and about ±74 deg in north/south direction, the information about the particle flight direction for a single measurement is only accurate to these numbers. But by adding many measurements, which have been obtained by scanning the ecliptic plane on a spinning spacecraft, some information about the directional distribution of the cosmic dust particles can be obtained, see fig. 7a.

APEX

APEX

SUN

SUN

ANTI-APEX Radius Axis : Mass [ g ] ANTI-APEX "9 Vectors Show Velocity in Rel. Units S P ~ "~"

Fig. 7a. Directional dependance of the impacts. The figure was obtained by assigning a cosine probability function with 148 deg between zeros to each particle. The maximum was set to 1 in the direction of the sensor pointing at the lime of impact. Finally all impacts were added.

Fig. 7b. Particle mass, relative velocity and direction for all particles encountered by the MDC during the lunar orbiting phase.

Co,~ n u c D u s t M e a s u r e me nt~ I n Lu nar ( ) rb i t (12) 181

The graph in Fig. 7a gives the number of particles measurements observed from the different directions from apex, sun, anti-apex and anti-sun. It was generated by calculating the particle impact probability according to the projection of the sensor area to the incident angles spread over 148 (leg and adding up all measurements obtained so far. The measurements shown in Pig. 7b have been arranged according to the particle velocity. Here it can be seen quite clearly that the fast dust particles, having velocities of more than 40 kin/s, are eneotmtered most often from the inner solar system. On the other hand rather slow dust particles, with velocities of less than 9 kin/s, are coming most often from the apex direction. It seems that particles having masses greater than 10 -10 g are coming more frequently from approximately the apex direction, while particles with masses less than 10 -14 g are more frequently coming from the sun direction.

Figure 7b and 3b give an indication for two populations of dust particles: The apex-particles, as described by Griin and Zook/8/, coming roughly from the apex direction, having greater masses and low velocities (relative to the earth system). These might be "overtaken" by the sateUite while spiralling down towards the sun. On the other hand the beta-meteoroids, Zook and Berg 19/, come from the inner solar system, having small masses and hyperbolic orbits. These may be accelerated outwards by interaction with the radiation pressure.

SUMMARY OF THE ACHIEVED RESULTS

Determination of the mass-, velocity- and angular distribution of cosmic dust particles and beta-meteoroids in lunar orbit.

Measurements of the impact rate versus time.

Accurate measurements of the cumulative particle flux and variation in the moon environment.

Determination of the directional- and spatial distributiorL

No significant indications for dust clouds generated by moon ejectao

Good correspondence with previous measurements and results.

CONCLUSIONS

During the 14 month of operation of the MDC in lunar orbit the cosmic dust experiment has observed 150 cosmic dust particles, which masses between 10 -16 g and 10 -7 g and velocities from 1.8 km/s to over 100 kin/s (beta-meteoroids).

On the basis of these measurements, flux rates have been calculated in dependence of mass and velocities of the dust particles. The overall flux rate for particle masses between 10-16g and 10-7g has been measured from 1.3 to 4.1"10 -4 m-2s -1 not much different to previous measurements/10/.

An overview about the directional distribution of the observed dust particles with respect to their masses and velocities was presented. This gives evidence of the populations of apex-particles and beta-meteoroids. On the basis of the data gathered so far further investigations of the detailed properties will be possible.

REFERENCES

1. Igenbergs E., Hfldepohl A., Uesugi K., Hayashi T., Svedhem H. Igiseder H., Koller G., G1asmachers A., Griin E., Schwehm G., Mizutani H., Yamamoto T., Fujimura A., Ishii N., Araki H., Yamakoshi K., Nogarni K., The Munich Dust Counter - A Cosmic Dust Experiment on Board of the MUSES-A Mission of Japan, in Origin and Evolution of Interplanetary Dust, (eds. A.C. Levasseur-Regourd and H. Hasegawa), Kluwer Tokyo, 45-48 (1991).

2. Uesugi K., Space Odyssey of an Angel - Summary of the HITEN's Three Year Mission, AAS 93-292, (1993).

3. Iglseder H., Ladungsemission beim Hochgeschwindigkeitseinschlag, Dissertation, TU-Mtinchen, (1986).

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4.

1t, Iglseder et al.

Iglseder H., Igenbergs E., Measured Charge Generation by Small Mass Impact at Velocities Between 1 and 45 kin/s, Int. J. Impact Engineering, Vol 5, San Antonio, 381-388 (1987).

5. Iglseder H., Mtlnzenmayer R., Svedhem H., Grfln E., Cosmic Dust and Space Debris Measurements with the Munich Dust Counter on Board the Satellites HITEN and BREMSAT, Advances in Space Research, Vol. 13/8, Space Debris, ed. W. Flury, Pergamon Press, Oxford, N.Y., Soul, Tokyo, August, 129-132 (1993).

6. Hoffmann H.-J., Fechtig H., Griin E., and Kissel J., Temporal Fluctuations and Anisotropy of the Micrometeoroid Flux in the Earth-Moon System Measured by HEOS-2, Planet. Space Sci., 23,981 (1975).

7. Fechtig H., Grtin E. and Morrill G., Micrometeoroids within Ten Earth Radii, Planet. Space Sci., Vol. 27; 511-531 (1979).

8. Griln E. and Zook H. A., Dynamics of Micrometeoroids in the Inner Solar System, in Solid Particles in the Solar System, (I. Halliday and B. A. Mclntosh, Eds.), Reidel, Dordrecht' 293-298 (1980).

9. Zook H. A. and Berg O. E., A Source for Hyperbolic Cosmic Dust Particles, Planet. Space Sci. Vol. 23, 183-203 (1975).

10. Iglseder H., Cosmic Dust and Space Debris Measurements on Board of the Small Satellites HITEN and BREM-SAT, Editors., in Small Satellites Systems and Services, Centre National d'Etudes Spatiales, CNES, C6padu~s-l~litions, Toulouse, France, (1993).