AAS 97-729
SMALL SATELLITES FOR AERONOMIC MISSIONS IN THE LOWER THERMOSPHERE
G. Laneve - Aerospace Department, Rome, Italy
Abstract
This paper is devoted to study the possibility of using a satellite on a very eccentric orbit (î ÷ 0.5) for exploring the atmosphere in the altitude range from 140 to 200 km. The region between 100 - 200 km is referred to as lower thermosphere and it is sometimes called ignorosphere to emphasize the difficulties involved in the data interpretation coming from that region, due to the sporadicity of the observations obtained by rocket flights and by local ground-based measurements. This region is crucial to the development of improved empirical and theoretical models of the outer atmosphere, since most of the energy deposition and other important physical processes, controlling the outer atmosphere, occur in this relatively confined region. In addition to the altitude, the Earth's Thermosphere characteristics depend mainly on the latitude and solar local time. Thus we intend to explore the possibility of starting from this statement, for determining the necessary rate of variations of right ascension of the ascending node (ê') and perigee argument ( '), to meet our necessity in terms of latitudinal and solar local time coverage and then determine the characteristics of a suitable orbit in terms of semi-major-axis and inclination. In particular this paper is devoted to study the possibility of determining analytically the orbital characteristics of a satellite oriented to aeronomic applications. In fact, the use of a series of small satellites seems to be more rewarding in terms of cost and reliability with respect to a space tether. A consequence of the small effect of the geopotential on the right ascension and perigee argument of the considered orbits is that sometimes a complete revolution in ê and cannot be accomplished due to the limited lifetime of the satellite caused by strong drag effect. Thus, the solutions of the above stated problem can be theoretically correct but with low practical efficience due to a satellite lifetime shorter than the two periods 2ã/ ' or 2ã/ê'. We can solve this problem establishing in addition to the conditions on ê' and ' the further condition that the larger period between 2ã/ ' and 2ã/ê' is lower or at least equal to the satellite lifetime. Orbits meeting the requirements, depending on their eccentricity, allow us to collect some days of data at altitudes from 130 to 200 km. Using powered satellites the necessity of high eccentricity orbit can be reduced. The application of a constellation of small satellites can be considered suitable for fulfilling the requirements of the aeronomists to sound the lower thermosphere with the aim of understanding his variability with solar local time, latitude and longitude.