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MICROMETEORITE STUDIES FROM EARTH SATELLITES W. M. Alexander National Aeronautics and Space Administration Goddard Space Flight Center At the present time, solid particles of less than 1 x 10-1 millimeters or so in radius cannot be detected by means of radar or photographic images, and thus can only be recorded through the dynamical effects of an impact. Both the mass spectrum and velocity spectrum of the impacting particles are initially of major interest, and these quantities may be resolved by means of a pair of complementary detectors, one of which measures momentum and the other energy. Of the sensors now in use, the momentum detector is in the highest state of development and has been flown in a number of space vehicles. It consists of a crystal microphone mounted against a sounding board. The output of the crystal-microphone-amplifier system has been calibrated for particles of various masses at velocities from 20 cm/sec to 2 km/sec and has been found to be a linear function of momentum (Kells 1959). Variation of velocity over six orders of magnitude with momentum held constant produced a change in output of less than a factor of two. The calibration will be extended to velocities above 10 km/sec to place the microphone sensor on a perfectly sound basis, and this may be achieved this year. The energy detector consists of a phototube made opaque by means of a 1000 R coating of aluminum over the surface. When particles hit the surface some of their energy is converted into a light flash. The inte- grated emission in the flash appears to be a single-valued function of energy, but the light-flash calibration is not as advanced as that of the crystal microphone detector. The mechanisms of energy conversion and conversion efficiencies are still inadequately understood. In any event the device is limited to high velocity particles; no energy detector has yet been fully developed for low velocity particles. Actually, it has not been possible to use the maximum capacity of the microphone sensors on space vehicle flights up to this time. The mo- mentum detector has recently flown in three such vehicles, a '58, 1 '59, and Pioneer I, and has obtained information principally on impact fre- quencies of particles with momenta larger than a given threshold: 69
Impact Frequency Momentum Thres- Space Craft No. of Events (m-2 sec-1) hold (dyne sec) a '58 153 1 x 10-2 2. 5 x 10-3 (Dubin 1960) >/'59 3725 ± 25 2xl<r3 1xl0-2 (LaGow 1960) (Alexander 1960) Pioneer I 17 4 x 10-3 2 x 10-4 (Dubin 1960) The data for »»'59 has been corrected for spurious events caused by inter- ference with the magnetometer interrogation system, and the correction is believed to be reliable within the error quoted. Rocket flights of the energy detector in 1955 (Berg 1956), and 1960 (Berg 1960), have shown that impacts on the leeward side of the Earth, with respect to the Earth's orbital motion are ~50 to 100 times less frequent than on the windward side. This is about the magnitude of the effect which would be expected under the assumption that the micromete- orites converge toward the Earth's orbit from regions farther away from the Sun, and possess no less than Earth's escape velocity (11 km/sec). On the basis of the flights of the energy and momentum detectors a crude mass spectrum can be derived from the micrometeorites. The spectrum is, at this time, indefinite and warrants only a few conclusions: 1. The line relating the logarithm of the number of impacts above threshold to the mass of the impacting particles has a slope near -1. 2. The line relating the logarithm of the number of impacts to the logarithm of the momentum of the impacting particles has a slope near -1. 3. On the basis of extrapolations of the size distributions of cm- and mm-sized meteors detected by means of photography and radar (Watson 1952; Manning 1959, Whipple 1958), it would appear that 1 - 10 \i radius micrometeorites are between one and three orders of magnitude less abundant than the satellites indicate. Whipple (1959) however points out that the luminous efficiencies of meteors depends highly on particle mass and density, and are probably not known to better than within two orders of magnitude. Whipple 's own estimate of the luminous efficiency of meteors, based on an assumed density of p = 0. 05 gm/cm3, leaves less than one order of magnitude between the frequency value extrapolated from meteors down to micron size and the observed frequency of 70
micrometeorites. The discrepancy is probably not to be regarded as a serious problem in view of the large extrapolations involved--at least until the composition of meteors can be established much more definitely. 4. According to satellite measurements to date, the amount of material falling to Earth in the micrometeorite range is of the order of 5 x 10-12 g/m2/sec, which is equivalent to about 104 tons per day. Pure impact detectors have been flown on several occasions, but are thus far capable of giving only semi-quantitative data. Wire grids may have been broken on Explorer HI but not on Explorer I (Manning 1959). A 6(i film, made opaque with 1000 R of Al, has apparently received an impact opening 15fi in diameter, as measured by a CdS photocell on Explorer VII (LaGow 1960). A package ejection system is being developed which will be tried this year. The package will carry cosmic ray emul- sions, and attempts will also be made to recover micrometeorite material by stopping them in a lucite trap. Van Allen: How sure are you that micrometeorites exist at all? Alexander: We have tested the momentum detector for several possible sources of spurious signals: thermal shock, vibration, interference from interrogation of other instruments. On TJ '59, magnetometer interrogations have produced spurious signals, and corrections have been made to this date--in fact, the lowest false count rate was in a month of high magnetometer interrogation. An in-flight calibration system which produces a diaphragm mechanical shock, is being placed in operation to check the functioning of the microphone sensor. Anders: From his studies of deep-sea sediments Pettersson (1950 and 1960) has concluded that the volume of nickel-rich spherules falling to Earth each day must amount to a few thousand tons. This material is in a limited size range and constitutes only iron which has been melted. Naturally there is some unmelted iron falling too, and much silaceous and cometary material. It is interesting that the total micrometeoritic material that has been found in the satellite studies is only about ten times the amount of material in the form of spherules. REFERENCES Kells, M. C., et al. (1959) in Proceedings of the Third Symposium on Hypervelocity, Vol. 1, 361-384. Dubin, M. (1960) Planetary and Space Science, 2, 121-129. 71
LaGow, H. E., and Alexander, W. M. (1960) in Proceedings of First International Space Science Symposium, Vol. 1. Alexander, W. M., LaGow, H. E., and Secretan L. (1960) NASA Tech- nical Notes (in press). Dubin, M. (1960) in Proceedings of the First International Space Science Symposium. Vol. 1 Berg, O. E., and Meredith, L. H. (1956) J. Geophys. Res. 61, 751. Berg, O. E., and Alexander, W. M. (1960) NASA Technical Notes (in press). Watson, F. G. (1952) Between the Planets. Whipple, F. L. (1958) Am. Rocket Soc., Paper No. 499-57. Manning, L. A. and Eshleman, U. R. (1959) Proc. IRE 47 (2), 186. Whipple, F. L. (1959) SNAP sponsored conference on Meteoroid Hazard to Space Power Plants. Manring, E. R. (1959) Planet Space Sci. 1, 27-31. LaGow, H. E., and Secretan, L. (1960) NASA-MSFC Report on Scientific Results on Explorer VII. Pettersson, H., and Rotschi, H. (1950) Nature 166. 302. Pettersson, H. (1950) Sci. Ann. (August) Ibid. (1960), Sci. Ann. 202 (2). 123.
