Planetary Exploration Missions 883
was delicately aerobraked down into circular mapping orbit
over a period of months, the long period being needed due
to structural failure of the attachment of one solar panel.
The mission has yielded a continuing stream of imaging and
other data, revolutionizing scientists’ knowledge and mod-
eling of Martian geology and atmospheric processes. (See
marsweb.jpl.nasa.gov/missions/.)
Failures of the 1990s
The years 1992, 1998, and 1999 saw three US missions
fail during arrival at the planet:Mars Observer,Mars Cli-
mate Orbiter, andMars Polar Lander. An elaborate interna-
tional Russian mission’s launch,Mars-96, failed in 1996—
a series of events that led in the USA to a management
overhaul and in Russia to the end of Mars exploration
for the time being. (See marsweb.jpl.nasa.gov/missions/ and
http://www.iki.rssi.ru/mars96/mars96hp.html.
Mars Odyssey
Mars Odyssey, a NASA orbiter launched in 2001, is in-
strumented for measurements complementing those of the
Global Surveyor. With infrared/visible, gamma-ray, and
particle spectrometers, it produces thermal imaging en-
abling evaluation of surface physical properties, subsurface
elemental chemistry, and the planet’s radiation environ-
ment.Odyssey’s findings have greatly stimulated interpre-
tations of many of Mars’s landforms as resulting from the
action of subsurface briny water, ice and carbon dioxide.
(See marsprogram.jpl.nasa.gov/odyssey/.)
Spirit and Opportunity
In an intense three-year effort, two NASA Mars rover mis-
sions,SpiritandOpportunity, were prepared for the 2003
launch opportunity. Both succeeded, and at the time of writ-
ing the two rovers are continuing to make astonishing dis-
coveries in Meridiani Planum and Gusev Crater, on oppo-
site sides of the planet, reinforcing the orbiters’ findings of a
history dominated by the effects of water. (See marsrovers.
jpl.nasa.gov/home/index.html.)
Mars Express
E S A’sMars Expressorbiter, launched on a RussianSoyuz-
Fregatvehicle in 2003, delivered the small BritishBeagle-2
lander, which failed, and has then gone on at the time of
writing to yield excellent imaging, plus spectrometric mea-
surements indicating, among other findings, that there is a
correlation between regions of enhanced water vapor and
methane concentrations in the atmosphere. Mars Express
also carries a ground-penetrating radar for detetecting the
signatures of subsurface brines and ices. (See sci.esa.int/
science-e/www/area/index.cfm?fareaid=9.)
Mars Reconnaissance Orbiter
Launched in 2005 and delivered in 2006 into aerobraking
orbit at Mars,Mars Reconnaissance Orbiteris expected to
increase by orders of magnitude the quantity and quality of
remote-sensing data from Mars, because of its powerful ra-
dio system and advanced on-board instruments and system
software. Imaging already obtained, while excellent, gives
only a small sample of the harvest to come.Future Mars Missions
The exciting discoveries of the missions listed here and the
ongoing debate over the prospect of human missions to
Mars have continued to energize an active NASA program.
Mars launches are planned for the 2007 and 2009 opportu-
nities.Phoenixis a reflight of the failedMars Polar Lander,
andMars Science Laboratoryis intended to expand on the
findings of the Mars rovers,SpiritandOpportunity. (See
marsweb.jpl.nasa.gov/missions/.)9. Small Bodies
As scientists have come to realize that comets and asteroids
contain clues to the ancient history of the solar system—
clues largely obliterated by geologic processes in planets
and moons—missions to small bodies have increased in im-
portance. Also studies of cratering and meteorite records
show that near-Earth asteroids present both a threat and an
opportunity. The threat is that of devastating impacts and
the opportunity is that of useful resources not found in the
Moon. (See http://www.permanent.com.)Ice
After completing its solar mission asISEE-3 (see Sun
and Heliosphere sections) the spacecraft was retargeted
and renamedInternational Cometary Explorer. It flew
through the tail of Comet Giaccobini-Zinner in 1985, then
continued on in heliocentric orbit where it sent low-rate
data for the next several years. (See heasarc.gsfc.nasa.gov/
docs/heasarc/missions/isee3.html# instrumentation.)The Halley Armada
As Halley’s comet arrived near the Sun in 1986 on its 76-
year orbit, it was met by spacecraft from Japan, Europe, and
the USSR. Comet enthusiasts lamented the absence of the
USA from this once-in-a-lifetime opportunity. Japan’sSui-
seiandSakigakimade distant observations of the ultraviolet
coma. ESA’sGiottopassed within 600 km of the nucleus
collecting imaging, spectra, and detailed chemical data.
The SovietVEGA 1and 2 flew by at intermediate distances
after their productive en route encounters with Venus (see