Detection of a high density meteoroid on cometary orbit

We report a detection of a unique fireball photographed at three Czech stations of the European Fireball Network on June 1, 1997. The main exceptionality of this fireball is given by its retrograde heliocentric orbit (i= 138°) typical for comets and, at the same time, a behavior in the Earth’s atmosphere typical for the hardest component of the interplanetary matter. The spectrum of the fireball is unique by the absence of the sodium line. With the entry velocity of 65 km s−1, the meteoroid of an estimated initial mass of 0.2 kg (diameter of several centimeters) penetrated down the altitude 65 km, i.e. significantly deeper (about 25 km) than cometary meteoroids of a comparable velocity and mass. A comparison with a typical cometary meteoroid is demonstrated on anηAquarid fireball photographed one month before.The event reported is unique in decades-long records of the fireball networks. The origin of the meteoroid in the asteroid belt is highly improbable owing to the orbit. The possibility that comets contain centimeter sized compact stones is therefore suggested.

Spatial Distribution and Orbital Properties of Zodiacal Dust

Within the recent years the spatial distribution of Zodiacal dust has been subject to a variety of modelling approaches. Whereas models derived from observations in the visual range tend to demand for an increase of interplanetary matter above the solar poles (bulges), models based on infrared measurements and extended to small r seem to favor a decrease there (holes). The models are reviewed, and the dynamical structure implicated in the models is outlined.

The characterization of lunar surface impact erosion and solar wind sputter processes on the lunar surface

A wide variety of processes have been instrumental in the evolutionary progression of the lunar surface towards its present state. During the early history of the Moon, chemical differentiation, volcanism and, particularly for the mare areas, infilling by magma flow have been major mass transport factors but, since further cooling some 3 x 10 to the power of 9 years ago, the development of surface features has been primarily determined by impact erosion from remnants of the primeval stellar condensation and matter associated with asteroids and comets. As a result of the evolutionary decrement of interplanetary matter impact erosion is time dependent but even so at a particular epoch the relative importance of impact erosion to solar wind sputter erosion in the determination of the topology of a feature will be dependent on the scale of the feature examined. An impact crater formed on the lunar surface is normally exposed to the 400 km s-1 solar wind spectrum which will result in sputter erosion of surface atoms. Under varying angles of incidence during the lunar cycle, features exposed to the solar wind will be gradually smoothed and will eventually disappear. A similar erosion effect is the abrasion of large craters due to the formation within them of much smaller craters, i.e. a process similar to sand blasting. We also have the possibility during these quasi-continuous processes of single and complete obliteration by an impacting particle at least as large as that which caused the original crater. Thus we may characterize any particular crater with three erosion lifetimes: (1) the sputter lifetime, (2) the (small particle) abrasion lifetime, and (3) the (large particle) erasure lifetime.