874 Encyclopedia of the Solar System
or the Sun itself. Some were errant vehicles from plane-
tary misses. The notablePioneerseries began in 1959 and
continued in the late 1960s. The first international solar
mission,Helios, a U.S.–German cooperative mission, with
interplanetary spacecraft observing the solar wind and ra-
diation, was launched in the mid 1970s (see below).
Now the Sun is continuously observed from space. Cur-
rently operating missions includeUlysses,SOHO, andACE,
plus particles-and-fields instruments carried on some new
planetary missions. In the aggregate, as described in the
chapter on the heliosphere, these investigations have shown
a common portrait, with variations, of what happens as the
Sun’s streaming plasma, coronal mass ejections and elec-
tromagnetic radiations interact with the magnetic fields,
ionospheres, and atmospheres and surfaces of solar system
bodies. These effects are most dramatic when they result
in spectacular comet ion tails, but they are also important
in causing magnetic storms and driving the evolution of at-
mospheres due to dissociation of molecules and ionization
and sweeping away of atoms.
Study of these interactions as they are imagined to
have happened in the ancient past, for example when our
star is thought to have gone through a hugely energetic
T Tauri phase, enables not only analyses of early plane-
tary history here but also productive reasoning about what
may be observed in other star-planet systems as they are
found.
Over the history of spaceflight many space-borne inves-
tigations, for example surveys of Earth’s magnetosphere by
missions includingISEE-2and 3 ,Interbol,Geotail,Wind,
Polar, andClusterhave added to knowledge of the Sun
via its interactions with the rest of the solar system. Here
we do not dwell on those ventures; instead we focus on
missions dedicated to investigating the Sun itself as a star,
with improved planetary magnetospheric, ionospheric, and
atmospheric knowledge being extra benefits. It is appropri-
ate, however, to observe that the long tradition is vigorously
continuing with the worldwide International Heliosphere
Year (IHY) due to begin in 2007.
Pioneer 6, 7, 8, 9
These missions, making ingenious use of the technology of
their time, employed small spinning spacecraft to obtain
a rich harvest of data on the solar wind and other inter-
planetary phenomena over a period beginning in 1965 and
continuing for more than 30 years. (See http://samadhi.jpl.
nasa.gov/msl/QuickLooks/pioneer6QL.html)
Helios
Two German spacecraft, launched by NASA Titan-
Centaursin 1975 and 1976, explored solar phenomena
between Earth’s orbit and as close as 0.29 AU from the
Sun. An arrangement of mirrors and radiators enabled the
spinning spacecraft to survive the consequent extreme
heating. (See http://www.linmpi.mpg.de/english/projekte/
helios/Isee-3
Launched in 1978, theInternational Sun-Earth Explorer
was a small spacecraft maneuvered into a halo orbit around
the L1 libration point, 1.5 million km sunward from
Earth, where its x-ray and gamma-ray spectrometers en-
abled the study of both solar flares and cosmic gamma-
ray bursts. In 1982 it was maneuvered onto a trajectory
toward Comet Giaccobini-Zinner and renamed theInter-
national Cometary Explorer.(SeeSmall Bodiessection
below.) (See heasarc.gsfc.nasa.gov/docs/heasarc/missions/
isee3.html# instrumentation.)Solar Maximum Mission
Launched in 1980 by Space Shuttle,SMMcarried a suite
of instruments investigating the Sun at the height of the
sunspot cycle. Ultraviolet, x-ray, gamma ray, and visible light
observations combined to give a picture of the Sun’s total ra-
diation and its variations due to flares. The spacecraft failed
and was dramatically rescued by a shuttle crew in 1984,
whence it continued until atmospheric reentry in 1989. (See
umbra.nascom.nasa.gov/smm/.)Ulysses
Launched in 1990 by the space shuttle with propulsion be-
yond LEO to send it to Jupiter, ESA’sUlyssesused the
giant planet’s gravity to kick its orbit out of the plane of
the ecliptic and send the spacecraft back inward, passing
over the Sun’s poles to survey a region never before ex-
plored. Now the craft goes out to the distance of Jupiter’s
orbit and back to the Sun every five years. Its mission is
expected to continue until at least 2007. In addition to its
huge yield of information about the Sun, solar magnetism,
and the solar wind, Ulysses has observed interstellar dust
and interstellar helium atoms in interplanetary space. (See
helio.estec.esa.nl/ulysses/.)Yohkoh
Launched in 1991 from Kagoshima, this mission of ISAS,
with contributions from the US and UK, was an x-ray and
gamma-ray observatory that gave 10 years of nearly contin-
uous imaging of the solar atmosphere. (See solar.physics.
montana.edu/sxt/.)SOHO
The ESA/NASA Solar and Heliospheric Observatory,
launched by an AmericanAtlas-Centaurin 1995, orbits
about the L1 Lagrangian libration point 1.5 million km