1999 was an important year for Earth Science, with $2 billion worth of observation systems launched into space. These civilian earth-observing missions, coordinated under the auspices of NASA's Earth Science Enterprise (ESE), began major new initiatives of a complex program that includes science and applications research, observational systems and technology development, and commensurate information system management, in cooperation with domestic and international partners. Its stated long-range goal is to enable policy and decision makers at all levels of government, public, and private sector practitioners to establish sound, knowledge-based environmental decisions in the 21st Century.

Landsat 7. As a major new part of NASA's Mission to Planet Earth program, the 4,662-lbs. (2100 kg) Landsat 7 Earth-imaging satellite was successfully launched on 4/15 on a Delta 2 into a 438-mi. (750-km) sun-synchronous polar orbit. The $650-million spacecraft traces its heritage to the earlier Landsat 4/5 series, the Defense Meteorological Satellite Program (DMSP) and the Tiros satellites. Mostly distinguished by its Enhanced Thematic Mapper Plus (ETM Plus), it is a considerable improvement over Landsat 6 which suffered a failed launch on a Titan rocket in 1993. Expectations are that with today's substantial cost reduction and systematic acquisition strategies employed with Landsat 7, its Earth observations will once again, as with the Landsats in the past, become the foundation for both terrestrial research and application activities.

Terra. As the first in a new spacecraft series -- the Earth Observation System (EOS) satellites -- the 10,506-lbs. (4765 kg) Terra was launched successfully on 12/18 on an Atlas/Centaur, the largest payload ever flown on that launcher. Formerly called EOS-AM-1, Terra achieved a 437-mi. (705-km) circular, sun-synchronous orbit, similar to Landsat 7 (which will have daily equatorial crossings four hours after Terra). The new spacecraft has five closely coupled science instruments for integrated studies of the Earth, from the top of its atmosphere down through its cloud layers to the surface of its seas and lands. Two smaller satellites in the series, EOS-PM and EOS-Chemistry, are in advanced stages of development for launches over the next two years. Altogether, the EOS program will eventually comprise 25 spacecraft of various sizes to be launched through 2003.

ACRIMSAT. The Active Cavity Radiometer Irradiance Monitor Satellite (ACRIMSAT) was launched on December 20 on an Orbital Science (OSC) Taurus rocket. The 115-kilogram (253-pound) satellite is currently circling Earth in a polar orbit at an altitude of 425 mi. (680 km). The instrument is designed to measure small, sustained changes in the total solar irradiance (TSI) of as little as 0.5% per century which could be the primary causal factor for significant climate change on time scales of many decades. There is mounting evidence that this has occurred in the past. Resolution of a century of TSI variation will require the flight of many instruments with overlapping missions to maintain the high precision of the data necessary to "see" the solar variability.

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