Small satellites for studies in the lower thermosphere

Among the solutions of the problem of filling the gap in aeronomic data at altitudes ranging from 130 to 200 km an experiment based on the exploitatio n of a tethered satellite deployed for 100 km or more from the Shuttle, has been proposed. However, this solution seems to be still far away from impl ementation; whereas other methods based on sounding rocket flights and meas urements from the ground present problem like the sporadicness and/or scarc e spatial coverage of the observations. As an alternative to these previous approaches this paper studies the possibility of using a satellite on a ve ry eccentric orbit (e approximate to 0.5) as a possible alternative, more r ewarding in terms of cost and reliability. In addition to the altitude, the Earth's Thermosphere characteristics depend mainly on the latitude and sol ar local time. Thus we intend to study the rate of variation of these terms as function of the rate of variations of right ascension of the ascending node and perigee argument, in order to evaluate the time necessary for meet ing our requirements in terms of latitudinal and solar local time coverage. Then, the characteristics of a suitable orbit in terms of semi-major-axis and inclination can be determined according to the type of study to be perf ormed. Furthermore we intend to analyze the possible difficulties that coul d arise when the full separation of any effect is afforded with the aim of selecting the orbit, for a single satellite or for a constellation of small satellites, able to minimize the limitations in the spatial and temporal e ffects separation. (C) 2001 Elsevier Science Ltd. All rights reserved.