PAB Phase I Final Report Final Report Planetary-scale Astronomical Bench NASA Institute for Advanced Concepts Phase I Study submitted by: SVS Inc. Timothy L. Howard (Principal Investigator) Lee M. Gutheinz Jack Sanders-Reed Carl Tuttle Bill Witt 9 Jan 2000 1 PAB Phase I Final Report Table of Contents 1. Executive Summary 3 2. Introduction 5 3. Science Objectives 7 a. Astrometry 7 b. Aperture Synthesis 12 c. Microlensing 17 d. Conventional Astronomy 18 e. Asteroid Studies 19 f. Other Applications 19 4. System Concepts 20 a. Station Concepts and Basing 22 b. Environments 29 5. Technology 31 a. Large Optics 31 b. Separated-spacecraft interferometry and metrology 32 d. Electronics 34 e. Robotics 35 f. Other Technology Areas 37 g. Technology Expectations 38 6. Exosolar Planet Detection 40 7. Summary and Conclusions 41 Appendices I. Limits on Separated-aperture interferometry 42 II. Libration orbit modeling 48 III. Environmental disturbances at Jupiter’s orbit 50 IV. Microlensing timescales 53 V. Astrometric depth sensing 54 VI. Summary data on the Trojan asteroids 55 References 57 2 PAB Phase I Final Report Executive Summary This report documents a concept study for a large-scale astronomical observatory that we refer to as the Planetary-scale Astronomical Bench, or PAB. The basic concept behind PAB is to place space telescopes at each of Jupiter’s Lagrange points L4 and L5 for use in cooperative, long-baseline astronomical observations. In conducting the study we looked at: the types of missions and science objectives that would be enabled or enhanced by the PAB concept; system configuration, basing, and the number and types of instruments; environmental considerations resulting from a location in the Jovian orbit; and technology needed for development of PAB systems and instruments. Figure 1 below shows the basic geometry of the PAB system. JUPITER ORBIT 1 AU EARTH TRANSFER ORBIT ORBIT JL5 JL4 5.2 AU Figure 1. PAB basic geometry. 3 PAB Phase I Final Report Among the principal conclusions resulting from the Phase I study are the following. · The concept has advantages for several areas of astronomical research, including astrometry, synthesis imaging, solar system exploration, and exosolar planet studies o Aperture synthesis across baselines 1000 times longer than currently under consideration will be possible o Synthesis imaging of exosolar planets may be enabled to much greater distances than currently planned – potentially, all earthlike planets out to the center of our galaxy · Both near-term and far-term missions are possible based on current technology projections o Precursor missions to explore the Trojan asteroid environment or to deploy a subscale “pathfinder” demonstration system at one of the Lagrange points would be a desirable early goal, and feasible with near-term technology · Environmentally-induced disturbances are relatively minor, so that long- term maintenance of the system geometry is possible · Many current areas of NASA research, including several NIAC Phase I and Phase II studies, will provide technology needed for PAB systems o Much of the astronomical instrument and space systems technology needed will be developed as a result of currently planned or proposed programs The use of the Jovian Lagrange points will provide a measurement baseline across planetary-sized scales of space and time. The PAB study documented here provides a basis for further studies. We believe the concept has much potential, including much that we have not foreseen, and should be explored further. 4 PAB Phase I Final Report Introduction There is currently intense interest in the astronomical community in developing spaceborne interferometers for high-accuracy astrometry and aperture synthesis. During the next decade it is expected that NASA will develop and fly one or more astronomical missions dedicated to the use of long-baseline interferometry in order to obtain higher-resolution astrometric and imaging data than is foreseeable from other types of instruments. Although even current plans are technologically challenging, we believe that eventually ultra-long-baseline aperture synthesis will be possible, and propose that a natural location for such a system already exists: the two Jovian Lagrange points at 60 degrees ahead of and behind Jupiter in its orbit. In fact, there are a number of astronomical and space science applications that would benefit from this concept. Such a system would be comprised of one or more platforms placed at each of these points, robotically maintained and designed for extremely long life and extensibility. The two stations would work cooperatively and would be suitable for multiple types of astronomical and space exploration instruments. Because the Lagrange points are spaced 9AU apart, PAB will use baselines of a size comparable to that of our solar system. Hence the name, Planetary-scale Astronomical Bench (PAB). This report documents a preliminary study of the use of the Jovian Lagrange points as long-term sites for astronomical instruments. These Lagrange points are locations of natural orbital stability; they are located 60 degrees ahead of and behind Jupiter in its orbit, and are a result of the combined gravitational attraction of the Sun and Jupiter. A large population of asteroids is trapped in clouds which are localized in the vicinity of the JL1 points. Lagrange points exist around other solar system bodies, but those at Jupiter are the most stable against perturbations. The PAB study examined potential science objectives, basic feasibility issues, system architecture and design concepts, and technology challenges. In conducting the study we used the notional NIAC timeframe of 10-40 years to set a time "horizon” for development. Our findings show that the JL points can be utilized for long-baseline measurements of various types, and offer significant advantages over other locations in the solar system as sites for future astronomical instruments. 1 In the remainder of this report we use the notation “JL” to refer to the Jovian Lagrange points, followed by the numerals 4 or 5 to denote the leading or trailing JL points which are coorbital with Jupiter, or generically, JLx where we do not wish to distinguish between them. We use ELx and MLx to refer to Earth and Mars Lagrange points. In all cases we refer to the Lagrange points formed by interaction between the principal planet and the sun. 5 PAB Phase I Final Report Table I gives a listing of basic information about PAB that will be useful as a reference in the remainder of this report. Table I. Basic PAB geometry. Stations: JL4 (leading) 60 degrees ahead of Jupiter JL5 (trailing) 60 degrees behind Jupiter Orbit: radius 5.2 AU, period = 11.86 year JL4-JL5 distance: 9.01 AU (nominal) JLx-E1 distance: 4.2 – 6.2 AU (periodic) Solar irradiance at 5.2 AU: 50 W/m2 Light-travel time: JLx-E1: 34.9 – 51.6 min. JL4-JL5: 74.9 min. Hohmann transfer orbit: transfer time 2.6 years (one way) Orbital velocities: 13.06 km/sec (nominal Jovian orbit) Synodic periods: Earth - Jupiter: 13.3 months Mars - Jupiter: 26.0 months 6 PAB Phase I Final Report Science Objectives In 1996 the “HST and Beyond” report of the Dressler committee [1] identified key goals for future space-based astronomical instruments which would be needed during the first decades of the 21st century. These goals included the development of space-based interferometry for use in both astrometry and synthesis imaging, and the development of an advanced near-infrared space telescope for studies of galaxies at high redshifts. At the beginning of our study we took these as preliminary instrumental objectives, and added to them, preparing a list of six broad astronomical and space science application areas that could benefit from the PAB architecture, i.e., from a location at the Jovian orbit and/or a cooperative set of stations at each JL point. Potential PAB applications include: astrometry, conventional astronomical imaging and spectroscopy, gravitational wave detection, microlensing, aperture synthesis and interferometry, and solar system science. We examined use of the PAB concept as a capability enabler for each of these application areas. There are also secondary objectives which we did not examine in detail, and some variations to the primary list which we uncovered as the study progressed. Each of these will benefit from a location in a Jovian orbit. Conventional imaging and spectroscopy will have increased sensitivity due to a reduced zodiacal background in the infrared. Astrometry, interferometry, microlensing, and gravitational-wave detection will benefit from long baselines, and will require spatially separated observations. Astrometry can be done with simultaneous observations across a long baseline, or from a single site over time using natural orbital motion; interferometry, microlensing, and gravitational-wave detection will require simultaneous observations from at least two sites. Asteroid investigations can be done using autonomous robot explorers as we initially envisioned, but also using conventional telescopes and radar imaging. The following sections discuss these applications in more detail. Astrometry Astrometry will be enabled by the simultaneous use of both stations for parallax- based measurements. The baseline established would be 9 AU station-to- station and from 4.2 to 6.2 AU from station to earth. Temporal motion of the stations in their orbits will allow full sky coverage at the full baseline in one-half of an orbital period (approximately six years). Use of long-baseline parallax-based astrometry in conjunction with high-angular resolution interferometric astrometry will allow unprecedented measurements of stellar positions throughout our galaxy, and potentially into neighboring galaxies as well. Figure 2 shows a calculation of astrometric parallax “range reach” available versus the achievable angular resolution. This calculation is based on a 7 PAB Phase I Final Report parallactic baseline between JL4 and JL5. Distances to the Large Magellanic Cloud and to the Andromeda Galaxy are shown for comparison, along with the approximate range reach which would be achieved using resolutions comparable to those expected for the planned SIM and GAIA missions [2,3].