Richard E. Spalding

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.

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.

Detection of a meteoroid entry into the Earth's atmosphere on February 1, 1994

Infrared and visible wavelength sensors on board platforms operated by the U.S. Department of Defense detected an energy release over the central Pacific Ocean on February 1, 1994, estimated to be of the order of at least tens of kilotons of TNT. The event has been assessed by the Defense and Intelligence Community to be a meteoroid entry. The object broke up into several fragments and created debris clouds which were tracked for over an hour. The meteoroid entered at about 24 km/s and an angle of approximately 45° on a heading of approximately 300°. From this, the objects heliocentric orbit just prior to entry was calculated to have a semimajor axis of about 1.6 AU, an eccentricity of about 0.65, and inclination of 2.1°. The radiant energy released is modeled to be between 1.4×1013 J and 2.6×1014 J or equivalent to 3.4 to 63 kilotons of TNT, and the total kinetic energy of the meteoroid is estimated to be in the range 1.4×1014 J to 2.6×1015 J or equivalent to 34 to 630 kilotons of TNT. From the kinetic energy and if we model the object as composed of silicates with a density of 3.5 g/cm3, we derive a mass range of 5×105 to 9×106 kg and a diameter range of 6 to 17 m.

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.

Gamma-Ray Burst Observations by Pioneer Venus Orbiter

The Pioneer Venus orbiter gamma burst detector is an astrophysics experiment for monitoring cosmic gamma-ray bursts. It is included in this planetary mission to provide a long baseline for accurately locating the sources ofthese bursts in order to identify them with specific astronomical objects. Responses to 14 gammaray burst events were examined; these events were verified from data acquired by other systems. Preliminary locations are proposed for three events, based on data from the Pioneer Venus orbiter, ISEE C, and Vela spacecraft. These locations will be improved, and additional locations will be determined by including in the analyses data from Helios B and the Russian Venera 11, Venera 12, and Prognoz 7 spacecraft.

The Pioneer Venus Orbiter Gamma Burst Detector

The Orbiter Gamma Burst Detector was designed to record the temporal and spectral characteristics of cosmic gamma-ray bursts. The primary mission of the experiment is the accurate determination of the directions to the sources of such bursts through a technique of triangulation, as a member of a widely spaced array of similar instruments. The system consists of a pair of scintillation spectrometers sensitive in the range 100 < E < 2000 keV, together with logics and data storage to provide a capability for recording these events. Nineteen events which have been verified as cosmic gamma-ray bursts were recorded within the first years operation.