The Solar Sail as Planetary Aerobrake

摘要

Certain planetary-exploration missions (such as comet-sample returns and Neptune orbiters) are simplified if a solar sail is used for both primary propulsion and planetary aerobraking. In the comet sample-return, a solar sail is used after Earth escape for inner-solarsystem cometrendezvous. Sailaerobraking places the payload (with comet samples) into Earth orbit. The Neptune mission accelerates by solar sailing to solar-system escape velocity after Earth escape. Solar-sail aerobraking places the spacecraft into Neptune orbit. Various aspects of sail aerobraking are considered: planetary atmosphere-density profiles, atmospheric-drag deceleration, sail heating by interaction with atmospheric molecules, and mechanical effects of atmosphere-sail interaction. We propose experiments to further investigate the interaction between upper-atmospheric molecules and thin sail films. INTRODUCTION : A NEW APPLICATION FOR THE SOLAR SAIL The solar sail, a spacecraft accelerated by the pressure of sunlight, is under consideration for a wide variety of missions both within the solar system and beyond it. Primitive sails have been unfurled experimentally from space station Mir. In the near future, The Planetary Society plans to conduct an orbital test of Cosmos 1, the first spacecraft in which the prime propulsion system will be the solar sail. Unless laser or maser beams are used to transmit collected and converted sunlight deep into space, it is generally assumed that the sail can only be utilized close to the Sun or a Sun-like star. Thus, in a planetary exploration mission more demanding than a simple flyby, the solar sail has not been considered as the sole propulsion system. Here, we investigate sail utilization as a planetary aerobrake. If feasible, application of this technique will greatly simplify the design of certain types of planetary exploration missions. One mission that could utilize sailaerobraking is a comet sample return 54th International Astronautical Congress of the International Astronautical Federation, the International Academy of Astronautics, and the International Institute of Space Law 29 September 3 October 2003, Bremen, Germany IAC-03-S.6.02 Copyright © 2003 by the International Astronautical Federation. All rights reserved. 2 mission. Here, the spacecraft unfurls its sail after Earth-escape. The sail is used to alter both the energy and inclination of the spacecraft so that a rendezvous with the target comet can be accomplished. After the robotic payload collects samples from the comet’s nucleus, coma, or tail, the sail is utilized to alter the trajectory for Earthintercept. The sail is finally utilized in parachute fashion to interact with the Earth’s upper atmosphere and decelerate so that the spacecraft is captured into Earth orbit. Another mission class utilizing this technique is rendezvous with outer solarsystem gas-giant planets (Jupiter, Saturn, Uranus, and Neptune). We discuss here a Neptune rendezvous example. The sail accelerates the spacecraft from an Earthescape to a solar-escape trajectory. Upon encountering the giant planet some years after launch, the sail is used once again as a parachute. Here, the goal is to decelerate the spacecraft so that a closed orbit around Neptune is achieved. SAIL-AEROBRAKING DECELERATION If a spacecraft moving at velocity Vs/c relative to a planet’s atmosphere with density ρatm has a cross-sectional area As/c and travels through the planet’s atmosphere, the atmospheric-drag deceleration of the spacecraft is : ACCdrag = -0.5 Cd ρatm As/c Vs/c 2 / Ms/c, (1) where Cd , the drag coefficient has a numerical value between 2 and 2.3 and Ms/c is the spacecraft mass. 5 If the spacecraft is a solar-sail with cross-sectional area Asail projected normal to the direction of travel and areal mass thickness σs/c, Eq. (1) can be rewritten using the approximation; ACCdrag ≈ -ρatm Vs/c 2 / σs/c. (2) For the atmospheric density of the Earth, a curve match to the Standard Atmosphere was deve