David W. Hughes

The D-CIXS X-ray mapping spectrometer on SMART-1

The D-CIXS Compact X-ray Spectrometer will provide high quality spectroscopic mapping of the Moon, the primary science target of the ESA SMART-1 mission. D-CIXS consists of a high throughput spectrometer, which will perform spatially localised X-ray fluorescence spectroscopy. It will also carry a solar monitor, to provide the direct calibration needed to produce a global map of absolute lunar elemental abundances, the first time this has been done. Thus it will achieve ground breaking science within a resource envelope far smaller than previously thought possible for this type of instrument, by exploiting two new technologies, swept charge devices and micro-structure collimators. The new technology does not require cold running, with its associated overheads to the spacecraft. At the same time it will demonstrate a radically novel approach to building a type of instrument essential for the BepiColombo mission and potential future planetary science targets.

Cometary magnitude distribution and the ratio between the numbers of long- and short-period comets

Abstract The magnitude distribution of comets reveals information as to the way in which cometary numbers vary as a function of the radius and mass of the cometary nucleus. In this paper comets are divided into three groups according to their orbital period. P : long ( P > 200 years), intermediate (200 > P > 15 years), and short ( P N , varies as a function of their absolute magnitudes, H 10 ; these absolute magnitudes are from the Vsekhsvyatskii Catalog and its supplements. Typical relationships are, for long-period comets, log N=0.549 + 0.30 H 10 ( for H 10 m ) , for intermediate-period comets, log N=−0.270 + 0.163 H 10 ( for H 10 m ) , and for short-period comets, log N=− + 0.30 H 10 ( for H 10 m ) . Features of these curves are then used to quantize certain aspects of the physical evolution of comets. The Vsekhsvyatskii list contains 522 long-period and 84 short-period comets. Marsdens Catalogue of Cometary Orbits (5th ed., 1986) contains 613 long-period and 108 short-period comets. The ratio of about 6:1 between these two groups is of limited significance. This paper shows that what is of more importance is that the number of bright short-period comets that at present can be seen from Earth (there are 42 with H 10 m and P H 10 m ; P > 200 years) to the Earths vicinity equal to the observed flux of 0.83 ± 0.11 year −1 . The relationship between absolute magnitude and the size, shape, and surface structure of the nucleus is discussed in detail, and special reference is made to the recent observations of P/Halley carried out using the Giotto spacecraft.

The meteoroid influx and the maintenance of the solar system dust cloud

Abstract The total mass influx onto the Earths surface has been calculated over the mass range 10−13–1010 g by using cumulative influx rate data. Interpolation formulae have been fitted to this data and the resulting influx is found to vary between 3 × 108 and 5 × 1013 g. (Earth surface)−1 yr−1 depending on which data are used to obtain the formulae. By considering the latest measurements of comet masses and estimates for the rate of cometary decay, the mass influx to the solar system dust cloud has been calculated. Considering the processes responsible for removing particles from the cloud it is concluded that, if the cloud is to remain in equilibrium, the influx to the Earths surface must be about 108.7 ± 0.3 g (Earth surface)−1 yr−1.

The history of meteors and meteor showers

Abstract The history of meteors and meteor showers can effectively start with the work of Edmond Halley who overcome the Aristotelean view of meteors as being an upper atmospheric phenomenon and introduced their extraterrestrial nature. Halley also estimated their height and velocity. The observations of the Leonids in 1799, 1833 and 1866 established meteoroids as cometary debris. Two red herrings were caught — fixed radiants and hyperbolic velocities. But the 1890 to 1950 period with two-station meteor photography, meteor spectroscopy and the radar detection of meteors saw the subject well established.

The meteorite flux

Meteorites are being retrieved from the surface of our planet all the time, the number of located falls being at maximum in the range 0.3 to 1.0 yr-1 (106 km2)-1. Using the known flux of meteorite parent bodies to the top of the atmosphere this paper reviews suggested ablation processes and predicts the actual flux of meteorites to the Earth. It is found that this is some 70 to 300 times what is observed.

Scientific rationale for the D-CIXS X-ray spectrometer on board ESA's SMART-1 mission to the Moon

The D-CIXS X-ray spectrometer on ESAs SMART-1 mission will provide the first global coverage of the lunar surface in X-rays, providing absolute measurements of elemental abundances. The instrument will be able to detect elemental Fe, Mg, Al and Si under normal solar conditions and several other elements during solar flare events. These data will allow for advances in several areas of lunar science, including an improved estimate of the bulk composition of the Moon, detailed observations of the lateral and vertical nature of the crust, chemical observations of the maria, investigations into the lunar regolith, and mapping of potential lunar resources. In combination with information to be obtained by the other instruments on SMART-1 and the data already provided by the Clementine and Lunar Prospector missions, this information will allow for a more detailed look at some of the fundamental questions that remain regarding the origin and evolution of the Moon.

The Perseid meteor shower

The Perseids usually appear, with no remarkable variation in numbers, every August, maximising about the 12th. They have a parent, Comet Swift-Tuttle (1862 III), and, with the exception of planet Earth, the causative meteoroids keep away from perturbing planets. Of all the known streams the origin and evolution of the Perseids should be the easiest to understand. The meteoroids are fast and are prone to fragmentation.

The distribution of asteroid sizes and its significance

Abstract Eleven years ago, Hughes (Mon. Not. R. astr. Soc.199, 1149, 1982) gave the size distribution of 740 asteroids. This has now been updated using 1778 known asteroidal diameters. The mass distribution index of the large main-belt asteroids is now found to be 2.09 ± 0.10. A power law relationship has been obtained between the perihelion distance and the diameter of the smallest known asteroid in any group of asteroids that have a specific range of perihelion distance. This has been used, in conjunction with the size distribution function, to produce a “corrected” histogram of asteroidal perihelion distances in which the relative significance (in number) of the Trojan asteroids has been increased dramatically. It is also concluded that the mass of the asteroid belt, MA (g), is given by log MA = 26.21 (+1.4, −1.0).

The secular variation of cometary magnitude

Abstract This paper calculates the mean variation in absolute magnitude per perihelion passage, ΔH 10 , for short-period comets from the data of Vsekhsvyatskii and finds a value of 0.30 ± 0.06. Other mechanisms used for estimating cometary decay are reviewed and it is concluded that a more probable value for ΔH 10 is about 0.002. Reasons for the discrepancy between these two values are given.

THE INTERIOR OF A COMETARY NUCLEUS

Abstract The interior of a cometary nucleus is probably full of holes, these having a size distribution that echoes that of the major snowball cometesimal components. Considering the compositional similarities between comets and the satellites of the outer planets, coupled with the “fractal” nature of cometary structure it is concluded that comets probably have mean densities in the 0.03–0.1 gcm−3 range. Comets are thus extremely fragile objects with very low surface strengths.