ERS-1 and ERS-2 operational and precise orbit determination

This paper presents the European Space Operations Centre's orbit determination and prediction systems for the ERS-1 mission. The routine operational orbit determination and prediction subsystem is discussed briefly, and statistics of the accuracy compared to the requirements are given. The precise orbit determination subsystem is then described, and the accuracy of its results are compared to those of the operational orbit system and to the D-PAF preliminary orbit solutions. Some geophysical results from the altimeter data, processed in these orbit determinations, are also presented. The ESOC/OAD ‘ERS-1 Orbit Report’ is introduced as a document providing this information on a monthly basis. Finally, this paper describes how the experience gained with the precise orbit determination will be exploited to further improve the accuracy of the routine system that will be used for ERS-2, and provides an estimate of this accuracy.     

Mathematical description of the NaK model for MASTER-2005

The release of NaK droplets has been modeled for the new version of the European Meteoroid and Space Debris Terrestrial Environment Reference model MASTER-2005. Previously published versions of the model have been revised. The parameters of the model are introduced and discussed. NaK droplets consist of eutectic sodium–potassium alloy and have been released during RORSAT reactor core ejections. They contributed to the space debris environment in the centimeter and millimeter size regime. Sixteen nuclear powered RORSATs launched between 1980 and 1988 activated a reactor core ejection system in Sufficiently High Orbits (SHO), mostly between 900 and 950 km altitude. The core ejection caused an opening of the primary coolant circuit. The liquid coolant has been released into space during these core ejections. The outflow is considered as a discrete event for each of the sixteen core ejections in total. The NaK coolant has been forming droplets up to a diameter of 5.5 cm. NaK releases are restricted to a very narrow region near 65° inclination. This paper gives the parameters of the NaK release model as it is implemented in MASTER-2005. The quantitative values of all model parameters including characteristic diameter and uniformity parameter are presented. The ratio of the characteristic droplet size to the orifice diameter is discussed. It is estimated that altogether 128 kg of NaK-78 (8 kg per RORSAT) was released on orbit. Simulation runs show that there are still 45,000 droplets with a total mass of 97 kg in orbit at the reference epoch 1 May 2005, whereas the smallest droplet has a diameter of 5 mm. Results of orbit propagation simulation runs are presented in terms of spatial density.     

Metabolic changes in rats subjected to space flight for 18.5 days in the biosatellite Cosmos 936

From an investigation of the activity of six glucocorticoid dependent liver enzymes, the existence of chronic, transient, stress-induced hypercorticosteronaemia during flight is probable. This hypercorticosteronaemia arises from weightlessness and induces gluconeogenesis. Weightlessness also caused substantial increases in liver glycogen level. The increased lipolytic activity and that of lipoprotein lipase in several groups of animals could be interpreted as enhancement of fat mobilization and utilization under the influence of stress. As this latter enhancement was also found in ground-based controls, it may have been due to the stress of handling rather than to space flight per se.     

Antenna pattern control of planar arrays for long distance communications

As an alternative to standard antennas and due to the advent of low-cost digital processors and materials, adaptive antenna arrays are increasingly considered for applications in long distance communications. The pattern beam control potentialities provide the performance improvement of the communication systems. In this context, this work presents a technique that permits to synthesize the antenna pattern of a planar array. Considering that the relationship between the array factor and the array excitations for the far field region is a Fourier transform in the appropriate variables, the non-uniform sampling procedure is extended for planar arrays. These arrays permit a further control of the antenna pattern since two dimensions of space are used. The technique considers a number of array factor points equal to the array size, which is used to impose appropriated values in the antenna pattern. The produced array factor passes through those points with the desired values, which allows the synthesis of antenna patterns with appropriated characteristics. The theory is presented in order to develop the necessary direct expressions to calculate the array excitation. The array factor can be obtained using the inverse Fourier transform. As application examples, with this technique it is possible to define the level of each sidelobe of the antenna pattern, to control the main beam and to impose nulls in prescribed directions of the pattern.     

Results from the ESA SREM monitors and comparison with existing radiation belt models

The Standard Radiation Environment Monitor (SREM) is a simple particle detector developed for wide application on ESA satellites. It measures high-energy protons and electrons of the space environment with a 20° angular resolution and limited spectral information. Of the ten SREMs that have been manufactured, four have so far flown. The first model on STRV-1c functioned well until an early spacecraft failure. The other three are on-board, the ESA spacecraft INTEGRAL, ROSETTA and PROBA-1. Another model is flying on GIOVE-B, launched in April 2008 with three L-2 science missions to follow: both Herschel and Planck in 2008, and GAIA in 2011). The diverse orbits of these spacecraft and the common calibration of the monitors provides a unique dataset covering a wide range of B-L* space, providing a direct comparison of the radiation levels in the belts at different locations, and the effects of geomagnetic shielding. Data from the PROBA/SREM and INTEGRAL/IREM are compared with existing radiation belt models.     

Use of combined light flash and plasma measurements to study hypervelocity impact processes

We present new measurements concerning generation of light flash during hypervelocity impacts. We use iron particles (10⁻¹³ to 10⁻¹⁷ kg) with velocities over the range 1 to 42 km/s impacting semi-infinite targets (aluminium and molybdenum). The main results of previous work in the field are found to be reproduced with some slight deviations. For iron projectiles with given mass and velocity the energy of the flash (normalized to mass) is proportional to velocity to the power of 3.5 for aluminium targets and 3.9 for molybdenum targets. The duration of the flash is of order 1 microsecond. Simultaneous measurements of the generation of impact plasma do not change this. The onset of plasma generation of the bulk target material does not affect the total light flash energy. We discuss the duration of the flash compared to a simple calculation of temperature in the target and plasma vs time.     

Coronal fast wave trains of the decimetric type IV radio event observed during the decay phase of the June 6, 2000 flare

The 22 min long decimetric type IV radio event observed during the decay phase of the June 6, 2000 flare simultaneously by the Brazilian Solar Spectroscope (BSS) and the Ond?ejov radiospectrograph in frequency range 1200–4500 MHz has been analyzed. We have found that the characteristic periods of about 60 s belong to the long-period spectral component of the fast wave trains with a tadpole pattern in their wavelet power spectra. We have detected these trains in the whole frequency range 1200–4500 MHz. The behavior of individual wave trains at lower frequencies is different from that at higher frequencies. These individual wave trains have some common as well as different properties. In this paper, we focus on two examples of wave trains in a loop segment and the main statistical parameters in their wavelet power and global spectra are studied and discussed.     

Models of P/Borrelly: Activity and dust mantle formation

Comet 19P/Borrelly was observed by Deep Space One spacecraft on September 22, 2001 (Soderblom et al., 2002).The DS1 images show a very dark and elongate nucleus with a complex topography; the IR spectra show a strong red-ward slope consistent with a very hot and dry surface (345K to 300K). During DS1 encounter the comet coma was dominated by a prominent jet but most of the comet was inactive, confirming the Earth-based observations that <10% of the surface is actively sublimating. We have developed a thermal evolution model of comet PBorrelly, using a numerical code that is able to solve the heat conduction and gas diffusion equations at the same time across an idealized spherical nucleus ( De Sanctis et al., 1999, 2000; Capria et al., 2000; Coradini et al., 1997a,b). The comet nucleus is composed by water, volatiles ices and dust in different proportions. The refractory component is made by grains that are embedded in the icy matrix. The code is able to account for the dust release, contributing to the dust flux, and the formation of dust mantles on the comet surface. The model was applied to a cometary nucleus with the estimated physical and dynamical characteristics of P/Borrelly in order to infer the status and activity level of a body on such an orbit during the DS1 observation. The comet gas flux, differentiation and thermal behavior were simulated and reproduced. The model results are in good agreement with the DS1 flyby results and the ground based observations, in terms of activity, dust coverage and temperatures of the surface.     

Evolution of the sun''s near-surface asphericities over the activity cycle

Solar oscillations provide the most accurate measures of cycle dependent changes in the sun, and the Solar and Heliospheric Observatory/Michelson Doppler Imager (MDI) data are the most precise of all. They give us the opportunity to address the real challenge — connecting the MDI seismic measures to observed characteristics of the dynamic sun. From inversions of the evolving MDI data, one expects to determine the nature of the evolution, through the solar cycle, of the layers just beneath the sun's surface. Such inversions require one to guess the form of the causal perturbation — usually beginning with asking whether it is thermal or magnetic. Matters here are complicated because the inversion kernels for these two are quite similar, which means that we don't have much chance of disentangling them by inversion. However, since the perturbation lies very close to the solar surface, one can use synoptic data as an outer boundary condition to fix the choice. It turns out that magnetic and thermal synoptic signals are also quite similar. Thus, the most precise measure of the surface is required.

We argue that the most precise synoptic data come from the Big Bear Solar Observatory (BBSO) Solar Disk Photometer (SDP). A preliminary analysis of these data implies a magnetic origin of the cycle-dependent sub-surface perturbation. However, we still need to do a more careful removal of the facular signal to determine the true thermal signal.     

Thymine photoproduct formation and inactivation of intact spores of Bacillus subtilis irradiated with short wavelength UV (200-300nm) at atmospheric pressure and in vacuo.

Vacuum exposure renders the survival of spores of Bacillus subtilis approximately five times more sensitive to ultraviolet light irradiation than exposure under atmospheric conditions. The photoproduct formation in spores irradiated under ultrahigh vacuum (UHV) conditions is compared to the photoproduct formation in spores irradiated at atmospheric pressure. Compared to irradiation at atmospheric pressure, where only the "spore photoproduct" 5-thyminyl-5,6-dihydrothymine (TDHT) can be detected, two additional photoproducts, known as the c,s and t,s isomers of thymine dimer (T<>T) are produced in vacuo. The spectral efficiencies for photoproduct formation in spores under atmospheric and vacuum conditions are compared. Since there is no increased formation of TDHT after irradiation in vacuum, TDHT cannot be made responsible for the observed vacuum effect. "Vacuum specific" photoproducts may cause a synergistic response of spores to the simultaneous action of ultraviolet light (UV) and UHV. Three different mechanisms are discussed for the enhanced sensitivity of B. subtilis spores to UV radiation in vacuum. The experiments described contribute valuable research information on the chance for survival of microorganisms in outer space.