Planetary Exploration Missions 875
FIGURE 3 The ESA/NASA Solar and Heliospheric Observatory
(SOHO) orbits about the L1 Lagrangian libration point
1.5 million km sunward from the Earth, where its 14 instruments
continuously observe phenomena relevant to understanding the
solar interior, the solar atmosphere, and the solar wind.
sunward from the Earth, where its 14 instruments con-
tinuously observe phenomena relevant to understanding
the solar interior, the solar atmosphere, and the solar wind
(Fig. 3). SOHO’s observations are immediately fed to users
via the Internet at umbra.nascom.nasa.gov. The mission has
already made observations through most of an 11-year so-
lar cycle, and it is expected to continue for several more
years. It too survived a massive onboard failure with a dra-
matic rescue—this time by remote control from Earth. (See:
sohowww.estec.esa.nl.)
ACE
TheAdvanced Composition Explorer, a NASA mission with
nine instruments and an international team of 20 investi-
gators, was launched by aDelta IIvehicle in 1997. Like
SOHO, it orbits in the L1 region where it continuously
surveys the isotopic and elemental composition of parti-
cles from the solar corona, the interplanetary medium and
interstellar space. In 1998, the ACE data system began pro-
viding public, real-time observations that can give warning
of solar events that cause geomagnetic storms. (See http://www.srl
.caltech.edu/ACE/,)
TRACE
A smallExplorersatellite launched in 1998 by the innova-
tive air-launchedPegasusrocket system, TRACE provides
nearly continuous solar coronal observations with high spa-
tial and temporal resolution, complementing the data from
SOHO. (See umbra.nascom.nasa.gov/trace/.)
Genesis
In an audacious venture using gravity assist at Earth and li-
bration orbiting for two years near L1, theGenesismission,
launched by aDelta IIin 2001, in 2004 returned a capsule
to Earth bearing actual samples of the solar wind and inter-
planetary medium embedded in ultraclean collector plates.
Due to a failure to signal its parachute to open, the capsule
crashed in the Utah desert, but not all was lost: A number
of the collector units survived in condition good enough for
the recovery of isotopic information and other science data.
(See: genesis.lanl.gov.)RHESSI
Launched byPegasusin 2002, the Reuven Ramaty High
Energy Solar Spectroscopic Imager is a smallExplorer
spacecraft dedicated to x-ray and gamma-ray observations
for exploring the basic physics of particle acceleration
and energy release in solar flares. (See hesperia.gsfc.nasa.
gov/hessi/index.html.)4. Mercury
Mariner 10
Flight to the innermost planet began withMariner 10,
launched on 3 November 1973 by an Atlas-Centaur
(Fig. 4). It was the first mission to use gravity assist, fly-
ing by Venus on 5 February 1974 enroute to Mercury,
where it arrived on 29 March. Then using Mercury grav-
ity assist, it flew by again on 21 September 1974 and 16
March 1975, each time passing over the same side of the
planet.Mariner 10’s images showed a scorched, Moon-like
cratered surface, while its infrared and ultraviolet spec-
trometers recorded mineral composition and its magnetic
and plasma instruments surveyed Mercury’s surroundings,
revealing a weak magnetic field. Precise trajectory analysisFIGURE 4 Flight to the innermost planet, Mercury, began with
Mariner 10, launched on 3 November 1973 by an Atlas-Centaur.