Douglas O. Revelle

Meteor Phenomena and Bodies

Meteoroids can be observed at collision with the Earths atmosphere as meteors. Different methods of observing meteors are presented: besides the traditional counts of individual events, exact methods yield also data on the geometry of the atmospheric trajectory; on the dynamics and ablation of the body in the atmosphere; on radiation; on the spectral distribution of radiation; on ionization; on accompanying sounds; and also data on orbits. Theoretical models of meteoroid interaction with the atmosphere are given and applied to observational data. Attention is paid to radar observations; to spectroscopic observations; to experiments with artificial meteors and to different types of meteor sounds. The proposed composition and structure of meteoroids as well as their orbits link them to meteorites, asteroids and comets. Meteor streams can be observed as meteor showers and storms. The rate of influx of meteoroids of different sizes onto Earth is presented and potential hazards discussed.

The flux of small near-Earth objects colliding with the Earth.

Asteroids with diameters smaller than ∼50–100 m that collide with the Earth usually do not hit the ground as a single body; rather, they detonate in the atmosphere. These small objects can still cause considerable damage, such as occurred near Tunguska, Siberia, in 1908. The flux of small bodies is poorly constrained, however, in part because ground-based observational searches pursue strategies that lead them preferentially to find larger objects. A Tunguska-class event—the energy of which we take to be equivalent to 10 megatons of TNT—was previously estimated to occur every 200–300 years, with the largest annual airburst calculated to be ∼20 kilotons (kton) TNT equivalent (ref. 4). Here we report satellite records of bolide detonations in the atmosphere over the past 8.5 years. We find that the flux of objects in the 1–10-m size range has the same power-law distribution as bodies with diameters >50 m. From this we estimate that the Earth is hit on average annually by an object with ∼5 kton equivalent energy, and that Tunguska-like events occur about once every 1,000 years.

Historical Detection of Atmospheric Impacts by Large Bolides Using Acoustic‐Gravity Wavesa

ABSTRACT: During the period from about 1960 to the early 1980s a number of large bolides (meteor‐fireballs) entered the atmosphere which were sufficiently large to generate blast waves during their drag interaction with the air. For example, the remnant of the blast wave from a single kiloton class event was subsequently detected by up to six ground arrays of microbarographs which were operated by the U.S. Air Force during this presatellite period. Data have also been obtained from other sources during this period as well and are also discussed in this summary of the historical data. The Air Force data have been analyzed in terms of their observable properties in order to infer the influx rate of NEOs (near‐Earth objects) in the energy range from 0.2 to 1100 kt. The determined influx is in reasonable agreement with that determined by other methods currently available such as Rabinowitz [ 21 ], Ceplecha[ 4 ], [ 5 ] and by Chapman and Morrison [ 8 ] despite the fact that due to sampling deficiencies only a portion of the “true” flux of large bodies has been obtained by this method, i.e., only sources at relatively low elevations have been detected. Thus the weak, fragile cometary bodies which do not penetrate the atmosphere as deeply are less likely to have been sampled by this type of detection system. Future work using the proposed CTBT (Comprehensive Test Ban Treaty) global scale infrasonic network will be likely to improve upon this early estimate of the global influx of NEOs considerably.

Meteoritic dust from the atmospheric disintegration of a large meteoroid

Much of the mass of most meteoroids entering the Earths atmosphere is consumed in the process of ablation. Larger meteoroids (> 10 cm), which in some cases reach the ground as meteorites, typically have survival fractions near 1–25 per cent of their initial mass. The fate of the remaining ablated material is unclear, but theory suggests that much of it should recondense through coagulation as nanometre-sized particles. No direct measurements of such meteoric ‘smoke’ have hitherto been made. Here we report the disintegration of one of the largest meteoroids to have entered the Earths atmosphere during the past decade, and show that the dominant contribution to the mass of the residual atmospheric aerosol was in the form of micrometre-sized particles. This result is contrary to the usual view that most of the material in large meteoroids is efficiently converted to particles of much smaller size through ablation. Assuming that our observations are of a typical event, we suggest that large meteoroids provide the dominant source of micrometre-sized meteoritic dust at the Earths surface over long timescales.

An estimate of the terrestrial influx of large meteoroids from infrasonic measurements

[1] The influx rate of meteoroids hitting the Earth is most uncertain at sizes of � 10 m. Here we make use of historical data of large bolides recorded infrasonically over a period of 13 years by the U.S. Air Force Technical Applications Center (AFTAC) to refine the terrestrial influx rate at these sizes. Several independent techniques were applied to these airwave data to calculate bolide kinetic energies. At low energies our flux results are within a factor of two in agreement with previous estimates. For 5–20-m diameter objects, however, our measurements of the cumulative number of Earth-impacting meteoroids are as much as an order of magnitude higher than estimates from telescopic surveys of near-Earth objects and satellite-detected bolides impacting the Earth. The precise cause of this disagreement is unclear, though we propose several possible explanations. From our infrasound study, our best estimate for the cumulative annual flux of impactors with energy equal to or greater than E (in kilotons of TNT equivalent) is N = 4.5 E � 0.6 .

Calibrating infrasonic to seismic coupling using the Stardust sample return capsule shockwave: Implications for seismic observations of meteors

[1] Shock waves produced by meteoroids are detectable by seismograph networks, but a lack of calibration has limited quantitative analysis of signal amplitudes. We report colocated seismic and infrasound observations from reentry of NASA’s Stardust sample return capsule (SSRC) on 15 January 2006. The velocity of the SSRC (initially 12.5 km/s) was the highest ever for an artificial object, lying near the low end of the 11.2–72 km/s range typical of meteoroids. Our infrasonic/seismic recordings contain an initial N wave produced by the hypersonic shock front, followed � 10 s later by an enigmatic series of weak, secondary pulses. The seismic signals also include an intervening dispersed wave train with the characteristics of an air-coupled Rayleigh wave. We determine an acoustic-seismic coupling coefficient of 7.3 ± 0.2 m ms � 1 /Pa. This represents an energy admittance of 2.13 ± 0.15%, several orders of magnitude larger than previous estimates derived from earthquake or explosive analogs. Theoretical seismic response was computed using in situ VP and VS measurements, together with laboratory density measurements from samples of the clay-rich playa. Treatment of the air-ground interface as an idealized air-solid contact correctly predicts the initial pulse shape but underestimates its seismic amplitude by a factor of � 2. Full-wave synthetic seismograms simulate the air-coupled Rayleigh wave and suggest that the secondary arrivals are higher-order Airy phases. Part of the infrasound signal appears to arise from coupling of ground motion into the air, much like earthquake-induced sounds.

Surface Heating from Remote Sensing of the Hypervelocity Entry of the NASA GENESIS Sample Return Capsule

An instrumented aircraft and ground-based observing campaign was mounted to measure the radiation from the hypervelocity (11.0 km/s) reentry of the Genesis Sample Return Capsule prior to landing on the Utah Test and Training Range on September 08, 2004. The goal was to validate predictions of surface heating, the physical conditions in the shock layer, and the amount and nature of gaseous and solid ablation products as a function of altitude. This was the first hypervelocity reentry of a NASA spacecraft since the Apollo era. Estimates of anticipated emissions were made. Erroneous pointing instructions prevented us from acquiring spectroscopic data, but staring instruments measured broadband photometric and acoustic information. A surface-averaged brightness temperature was derived as a function of altitude. From this, we conclude that the observed optical emissions were consistent with most of the emitted light originating from a gray body continuum, but with a surface averaged temperature of 570 K less than our estimate from the predicted heat flux. Also, the surface remained warm longer than expected. We surmise that this is on account of conduction into the heat shield material, ablative cooling, and finite-rate wall catalycity. Preparations are underway to observe a second hypervelocity reentry (12.8 km/s) when the Stardust Sample Return Capsule returns to land at U.T.T.R. on January 15, 2006.

NEO fireball diversity: energetics-based entry modeling and analysis techniques

We have examined the behavior of a number of bolides in Earths atmosphere from the standpoint of recent entry modeling techniques. The entry modeling has been carried out including a triggered progressive fragmentation model (TPFM) which maintains a maximum drag orientation for the fragments in either the collective or a non-collective wake limit during entry (ReVelle 2004). Specifically in this paper, we have proposed a new method of estimating the terminal bolide mass and have compared it against the corresponding single-body mass loss prediction. A new expression for the terminal mass is proposed that corrects the mass of the body for the changing mass to area ratio during the fragmentation process. As a result of this new work we have found two very interesting features that correspond very closely to those found from a direct analysis of the observational data. These include an instantaneous mass that closely resembles that directly observed and an ablation coefficient behavior that also strongly resembles meteor observations (such as those found recently by Ceplecha & ReVelle 2005). During fragmentation, the apparent ablation coefficient has now been shown to decrease dramatically approaching the intrinsic ablation coefficient proposed by Ceplecha & ReVelle (2005). In our modeling we have assumed a breakup into equal size fragments that are consistently and progressively multiples of two of the original unbroken leading piece. Had we assumed a multitude of many much smaller pieces that made up the totality of the original body, our predicted ablation coefficient would indeed have approached the very small intrinsic ablation parameter values predicted by Ceplecha and ReVelle. This is especially evident in the case of Sumava, but is also true in a number of other cases as well. The bolides whose properties have been modeled using our detailed entry code including a prediction of the panchromatic luminosity consist of the 1965 Revelstoke meteorite fall (Folinsbee 1967; Carr 1970; Shoemaker 1983), the 1974 Sumava fireball and the 1991 Benesov fireball as presented in Borovicka & Spurný (1996) and in Borovicka et al . (1998), the Tagish Lake meteorite fall of January 8, 2000 (Brown et al . 2002), the March 9, 2002 Park Forest meteorite fall (Brown et al . 2004), the June 6, 2002 Mediterranean (Crete) bolide as presented in Brown et al . (2002) and finally the September 4, 2004 Antarctic bolide respectively (Klekociuk et al . 2005). A self-consistent assessment of the detailed properties of each of the fireballs was made using all available information for each event. In the future, more reliable estimates of all of the necessary source parameters (including their overall degree of bulk porosity) will be made if all channels of information are reliably retrieved for bolide events (channels such as acoustic-gravity waves and specifically its infrasound emission, seismic waves, satellite optical and IR data, ground-based spectroscopy, ground-based photometry and radiometry, VLF radiation, meteorite fragment recovery, etc.).

Infrasound from the El Paso superbolide of October 9, 1997

During the noon hour on October 9, 1997 an extremely bright fireball (approximately -21.5 in stellar magnitude putting it into the class of a super-bolide) was observed over western Texas with visual sightings from as far away as Arizona to northern Mexico and even in northern New Mexico over 300 miles away. This event produced tremendously loud sonic boom reports in the El Paso area. It was also detected locally by 4 seismometers which are part of a network of 5 seismic stations operated by the University of Texas at El Paso (UTEP). Subsequent investigations of the data from the six infrasound arrays used by LANL (Los Alamos National Laboratory) and operated for the DOE (Department of Energy) as a part of the CTB (Comprehensive Test Ban) Research and Development program for the IMS (International Monitoring System) showed the presence of an infrasonic signal from the proper direction at the correct time for this super-bolide from two of our six arrays. Both the seismic and infrasound recordings indicated that an explosion occurred in the atmosphere at source heights from 28 - 30 km, having its epicenter slightly to the northeast of Horizon City, Texas. The signal characteristics, analyzed from approximately 0.1 to 5.0 Hz, include a total duration of approximately 4 min (at Los Alamos, LA) to greater than approximately 5 min at Lajitas, Texas, TXAR, another CTB IMS array operated by E. Herrin at Southern Methodist University (SMU) for a source directed from LA toward approximately 171 - 180 deg and from TXAR of approximately 321 - 4 deg respectively from true north. The observed signal trace velocities (for the part of the recording with the highest cross-correlation) at LA ranged from 300 - 360 m/sec with a signal velocity of 0.30 plus or minus 0.03 km/sec, implying a Stratospheric (S Type) ducted path. The dominant signal frequency at LA was from 0.20 to 0.80 Hz, with a peak near 0.3 Hz. These highly correlated signals at LA had a very large, peak to peak, maximum amplitude of 21.0 microbars (2.1 Pa). Our analysis, using several methods that incorporate various observed signal characteristics, total distance traveled, etc., indicates that the super-bolide probably had a source energy in the range between 10 - 100 tons (TNT equivlaent). This is somewhat smaller than the source energy estimate made using U.S. DoD satellite data (USAF news release, June 8, 1998).

Infrasonic observations of bolides on October 4, 1996

During the evening of October 3, 1996, at least six bright fireballs were observed over the western United States with reports from California to Louisiana. The event over California produced tremendous sonic boom reports in the Los Angeles area. This event was also detected locally by 31 seismometers which are part of a network of seismic stations operated by the California Institute of Technology. Subsequent investigations of the data from the four infrasound arrays used by LANL (Los Alamo National Laboratory) and operated for the DOE (Department of Energy) as part of the CTBT Program (Comprehensive Test Ban Treaty) Research and Development program showed the presence of an infrasonic signal from the proper direction at the correct time for this bolide from two of our four arrays (Nevada Test Site; NTS and Pinedale, WY; PDL). Both the seismic and infrasound recordings indicated that an explosion occurred in the atmosphere, having its epicenter near Little Lake, Calif. for possible sources heights from 40 - 60 km. The infrasonic arrays are each composed of four elements, i.e., low frequency pressure sensors that are in near-continuous operation. The nominal spacing between elements is 150 - 200 m depending on the specific site. The basic sensor is a Globe Universal Sciences Model 100C microphone whose amplitude response is flat from 0.1 to 300 Hz. Each sensor is connected to 12 porous hoses which act to reduce wind noise. The signal characteristics, analyzed from 0.1 to 5.0 Hz, includes a total duration of 5 (NTS) to 20 minutes (PDL) for a source directed toward 230 - 240 degrees from true North. The signal trace velocities ranged from 300 - 360 m/sec with a signal velocity of 0.30 plus or minus 0.03 km/sec, implying a stratospheric (S type) ducted path (with a reflection altitude of from 40 - 60 km). The dominant signal frequency is from 0.20 to 0.80 Hz, with a peak near 0.2 to 0.25 Hz. These highly correlated signals had a maximum amplitude of 1.0 microbars (0.1 Pa) at PDL and 4.0 microbars (0.4 Pa) at NTS. Our analysis indicates that the bolide had a probable, maximum source energy in the range from 150 - 390 tons (TNT equivalent).