876 Encyclopedia of the Solar System
confirmed that Mercury has a huge iron core reaching to two
thirds of its outer diameter. (See nssdc.gsfc.nasa.gov/nmc/
tmp/1973-085A.html.)
Messenger
TheMessengerspacecraft, launched by aDelta IIon 2 Au-
gust 2004, will enter obit about Mercury in 2011 after an
Earth gravity assist in 2005, Venus gravity assists in 2006
and 2007, then three Mercury assists in 2008 and 2009.
The spacecraft carries a suite of instruments to investigate
Mercury’s surface and interior composition, its gravity and
magnetic fields, its particles and radiation environment and
the polar regions where Earth-based radar observations
show the possible presence of ices in permanently shad-
owed craters. (See messenger.jhuapl.edu/.)
5. Venus
Mariner 2
The first mission to return data from another planet,
Mariner 2in 1962, had amazing escapes from disaster.
During ascent itsAtlaswent into uncontrolled rolling and
miraculously stopped in an orientation such that theAgena
upper stage could deliver the spacecraft onto a trajectory
toward Venus. En route, the spacecraft survived a series
of mortal threats, and shortly after flying by Venus it suc-
cumbed to overheating. But during the flyby, as described
in the Venus chapters of this encyclopedia, it produced
proof of the planet’s hellish greenhouse. (See nssdc.gsfc
.nasa.gov/nmc/tmp/1962-041A.html.)
Veneras 4 through 16 and Vega
First to enter another atmosphere,Venera 4in 1967 car-
ried the emblem of the USSR to Venus. It began the Sovi-
ets’ most successful interplanetary program. As shown in
the table,Veneramissions of increasing complexity and
scientific yield continued to be launched at nearly ev-
ery celestial mechanics opportunity until 1983, and then
in 1985 the two VEGA spacecraft, en route to Halley’s
comet, delivered balloons into the Venus atmosphere. Sci-
entific results of this decades long exploration are described
in the Venus chapter. (See http://www.russianspaceweb.com/
spacecraftplanetaryvenus/.)
Mariner 5
Launched two days afterVenera 4in 1967,Mariner 5
made flyby observations, including ultraviolet cloud imag-
ing, that revealed the rapid rotation and spiraling equator-
to-pole circulation of the Venusian atmosphere. (See nssdc
.gsfc.nasa.gov/nmc/tmp/1967-060A.html.)
Mariner 10
During its gravity assist flyby of Venus in 1974 en route to
Mercury,Mariner 10made observations of the Venusian
atmosphere and ionosphere, confirming the equator-to-
pole circulation and absence of a magnetosphere. (See
nssdc. gsfc.nasa.gov/nmc/tmp/1973-085A.html.)Pioneer Venus
The twoPioneer Venusspacecraft, launched in 1978, had
complementary objectives.Pioneer Venus 1went into or-
bit with a radar altimeter to survey the surface though the
planet’s permanent cloud cover.Pioneer Venus 2delivered
four probes into the atmosphere to measure its charac-
ter and composition down to the surface. (See nssdc.nasa
.gov/planetary/pioneervenus.html.)Vega 1 and 2
Two large Soviet spacecraftVega 1and 2 (Fig. 5) flew by
Venus in 1985 en route to close encounters with Halley’s
comet. Their spherical entry capsules released balloons
that were inflated and floated in the Venus atmosphere,
returning data for several days. (See http://www.iki.rssi.ru/IPL/
vega.html.)FIGURE 5 Vega 1, a large Soviet spacecraft, flew by Venus in
1985 en route to Halley’s comet.