L-1 and L-2 Observatories for Earth Science in the Post-2010 Era Warren Wiscombe and Jay Herman NASA Goddard Space Flight Center, Greenbelt, MD 20771 Francisco Valero Scripps Institute of Oceanography, La Jolla, CA 91109 Abstract–Twin observatories 1.5 million km from Earth along the Earth-Sun line offer revolutionary possibilities for Earth observation and scientific progress. INTRODUCTION The L1 and L2 Lagrange points are 1.5 million km from Earth along the Earth-Sun line (Figs. 1, 2). Objects at L1 and L2 experience the same gravitational force as the Earth itself, and thus become new “planets” with the same orbital period as Earth. The points have saddle-point gravitational instabilities, but even current spacecraft have enough fuel and thruster authority to stationkeep for five years or more. From L1 or L2, the Earth occupies almost the same 0.5° FOV as the Sun does from Earth—although slightly smaller since the rim of the Sun is visible from L2 (an advantage for limb scanning!). The full-disk view of the Earth from L1 and L2 holds tremendous potential for Earth science. Both L1 and L2 have been occupied by space science missions, but not by any Earth science mission. NASA’s $100M Triana (http://triana.gsfc.nasa.gov ) was to be the first Earth observing platform at L–1, but became mired in political wrangling and now sits in storage with no scheduled launch date. The L1 point is towards the Sun and provides a full-disk view of the sunlit half of the Earth; the L2 point is away from the Sun and provides a complementary view of the night side of the Earth. Twin observatories at L1 and L2 can observe every point on Earth once a minute—true synoptic coverage, the weather forecaster’s dream! (Compare this with the five geostationary weather satellites which view the Earth to 60° latitude once every 15 min, or the 24 polar orbiting satellites required to obtain half-hourly coverage of the whole Earth.) This combination of global synoptic coverage and rapid temporal sampling allows the study of diurnal cycles and short-lived events. Precise long-term calibration using the Moon allows the measurement of delicate global changes unobservable with any current satellites. Not everything can be done from L1 and L2, of course. The antennas required to do microwave or radar remote sensing, for example, would be immense. But ultraviolet, visible, near-IR and thermal-IR remote sensing from L1/L2, while extremely challenging, are all within current technological capabilities. (Indeed, the challenge of measuring from L1/L2 creates technical spinoffs that benefit all spaceborne Earth observation.) Thus, the view to take is that each vantage point will do what it does best: some remote sensing now done from low Earth (LEO) and Fig. 1 The injection path for Triana showing the distances and time to reach the L1 halo orbit. Moon’s orbit in blue. Sun distance not to scale. Fig. 2 Summary of major L1 orbital parameters and comparisons with the distances of conventional low Earth orbit (LEO) and geostationary (GEO) satellites. The Moon is approximately where it would be viewed for calibration purposes, with the Earth in a solstice configuration allowing a complete view of a polar region. geostationary (GEO) orbits can better be done from L1/L2, while LEO and GEO are more appropriately used for complex instruments that may not work well so far from Earth. Moreover, some observing tasks are best done by constellations of robot aircraft and Ultra-Long Duration Balloons (ULDB’s). L1 and L2 observatories are essential components of an emerging integrated multi-perspective approach to observing all elements of the Earth system. These multiple perspectives range from at and below the Earth’s surface, to robot aircraft, to ULDB’s, to satellite formations in LEO and GEO, out to L1 and L2. The National Academy report [1] on Triana states: “Perhaps Triana’s most important contribution ... is