Tethered Formation Configurations: Meeting the Scientific Objectives of Large Aperture and Interferometric Science

With the success of the Hubble Space Telescope, it has become apparent that new frontiers of science and discovery are made every time an improvement in imaging resolution is made. For the HST working primarily in the visible and near-visible spectrum, this meant designing, building and launching a primary mirror approximately three meters in diameter. Conventional thinking tells us that accomplishing a comparable improvement in resolution at longer wavelengths for Earth and Space Science applications requires a corresponding increase in the size of the primary mirror. For wavelengths in the sub-millimeter range, a very large telescope with an effective aperture in excess of one kilometer in diameter would be needed to obtain high quality angular resolution. Realistically a single aperture this large is practically impossible. Fortunately such large apertures can be constructed synthetically. Possibly as few as three 34 meter diameter mirrors flying in precision formation could be used to collect light at these longer wavelengths permitting not only very large virtual aperture science to be carried out, but high-resolution interferometry as well. To ensure the longest possible mission duration, a system of tethered spacecraft will be needed to mitigate the need for a great deal of propellant. A spin-stabilized, tethered formation will likely meet these requirements. Several configurations have been proposed which possibly meet the needs of the Space Science community. This paper discusses two of them, weighing the relative pros and cons of each concept. The ultimate goal being to settle on a configuration which combines the best features of structure, tethers and formation flying to meet the ambitious requirements necessary to make future large synthetic aperture and interferometric science missions successful. 1 Aerospace Engineer, Mechanical Systems Center, Code 543, NASA Goddard Space Flight Center, Greenbelt, MD 20771. 2 Aerospace Engineer, Guidance-Navigation and Control Center, Code 572, NASA Goddard Space Flight Center, Greenbelt, MD. Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner. Introduction It follows intuition as well as optical theory that the larger the diameter of a telescope’s mirror, the greater the amount of light that can be captured. That satisfies the quantity side of the equation, but as far as the quality side of the equation goes, a larger diameter mirror also implies a sharper image, which we would also intuitively expect. Here, quantity and quality have been used as euphemisms for sensitivity and resolving power. When faced with the desire for fine resolution and sufficient sensitivity to detect small dim objects (especially at long wavelengths), we find ourselves wanting impossibly large space observatories, and so we ask the question; is it possible to achieve both the quantitative and qualitative science objectives with realistically sized hardware? Fortunately, methods have been pioneered in the field of synthetic apertures, where small subapertures achieve resolutions comparable to a large mirror having a diameter equal to the spacing between the subapertures. The sensitivity is achieved by ‘staring’ at the target for a longer period of time, sampling different parts of the synthetic aperture. An exciting prospect is to merge the methods of a synthetic aperture with an imaging spectrometer optical system. The space science mission SPECS (Submillimeter Probe of the Evolution of Cosmic Structures; Mather et al. 2001) proposes to combine far-infrared interferometry in a synthetic aperture 1000 meters in diameter to produce a space platform imaging spectroscope. The heart of the instrument is a cryogenic Michelson interferometer with a stroking optical delay line. This type of optical set-up can detect spatial structures having time-invarient (during the observation), incoherent sources and has the ability to measure two basic properties: 1) Spatial brightness distribution