878 Encyclopedia of the Solar System
Galileo Earth Flybys
En route to Jupiter (see Outer Planets section below) the
Galileospacecraft made gravity assist passes at Earth in
1990 and 1992. Spectrometric observations were made to
simulate a search for evidence of life on an unknown planet,
and the data did show an out-of-equilibrium, oxygen-rich
atmosphere. (See galileo.jpl.nasa.gov/facts.cfm.)
Terra
Launched in 1999, NASA’sTerraspacecraft carries five
advanced radiometric and spectrometric instruments ob-
serving global phenomena of land, oceans and atmosphere.
Measuring Earth’s radiation budget, its carbon cycle and
evolution of its climate and biosphere are main mission
goals. (See terra.nasa.gov/.)
Topex/Poseidon and Jason-1
Launched in 1992 and 2001 respectively as parts of a
collaboration between NASA and the French national
space agency CNES, Topex/Poseidon and Jason-1 use
radar altimetry and very precise orbit determination to
determine ocean topography, aiding studies of currents,
winds and climate effects including El Ni ̃no. (See topex-
http://www.jpl.nasa.gov.))
Grace
In a collaboration among NASA, the German space agency
DLR and other partners, two small satellites, Grace,
launched in 2002 use very precise measurements of the
distance between them to gain knowledge of the bumps
and hollows in Earth’s gravity field, leading to information
on the exchanges of mass, momentum and energy between
oceans and atmosphere. (See http://www.csr.utexas.edu/grace/.))
Envisat
ESA’s 8200-kg Earth observing satellite,Envisat, launched
in 2002, carries ten large instruments including a synthetic
aperture radar, a radar altimeter and a suite of radiometers
and spectrometers recording atmospheric, ocean, ice, land
and biosphere data, spanning the spectrum from ultravi-
olet to microwave frequencies. Its polar orbit gives global
coverage. (See http://www.esa.int/export/esaEO/.))
Aqua
NASA’sAquasatellite, launched in 2002, carries six ra-
diometric and spectrometric instruments surveying Earth’s
water cycle, sea and land ice, atmospheric temperature,
aerosols and trace gases, and soil moisture, so as to in-
crease understanding of climate and Earth’s radiation
balance, with both physical and biological influences. (See
science.hq.nasa.gov/missions/satelliteaqua/.)Aura
Launched in 2004, theAurasatellite’s four instruments
complement those ofTerraandAquaby measuring atmo-
spheric chemistry, including the formation and dissipation
of polar ozone holes and the distribution of greenhouse
gases. (See science.hq.nasa.gov/missions/satelliteaura/.)Other Recent Earth Observing Missions
In addition to the major efforts noted here, a host of other
orbital remote-sensing missions investigating Earth as a
planet with its evolving hydrosphere, cryosphere, atmo-
sphere, biosphere, and magnetosphere have been launched
in recent years. Summaries are given at the following Web
site with links to pages describing each mission in more
detail: science.hq.nasa.gov/missions/earthsun/.7. Moon
After centuries of careful naked-eye and telescopic obser-
vation from Earth, the Moon has at last become a body to
be investigated by robots, visited by human explorers, and
perhaps ultimately inhabited by the people of a first out-
ward wave of civilization. At its beginning, scientific lunar
exploration was caught up in the great 20th century struggle
between the USA and the USSR. With the end of the USSR,
the program fell victim to low priority and languished for
decades, but now a lively international revival is in progress.
Here we list the most important robotic missions of the past,
then briefly mention the grandApolloventure and its failed
Soviet competitor, and finally remark on the new missions
now established in a widening group of countries.Luna 1, 2, and 3
TheLunaSoviet missions in 1959 yielded the first es-
cape from Earth’s gravity, the first lunar impact, and
the first far-side images. (See nssdc.gsfc.nasa.gov/database
/MasterCatalog?sc=1959-008A.)Ranger 7, 8, and 9
After two nonlunar tests and three failed attempts to deliver
seismometers to the lunar surface, the NASARangermis-
sions, launched byAtlas-Agenasin 1964 and 1965, yielded
thousands of high-resolution television images of the lunar
surface showing that all features are mantled by the impact-
generated regolith. (See nssdc.gsfc.nasa.gov/planetary/
lunar/ranger.html.)