By Marty McDowell/NASA
On October 18, 1989, the United States launched Galileo. The Galileo
spacecraft was designed to study Jupiter's atmosphere, satellites, and
surrounding magnetosphere. The spacecraft was named in honor of Galileo
Galilei, the Italian Renaissance scientist who discovered Jupiter's major
moons in 1610. The spacecraft is currently orbiting Jupiter and performing
an extended study of the planet's moons.
The Galileo mission consists of two spacecraft: an orbiter and an
atmospheric probe. Launched during the STS 34 flight of the Atlantis
orbiter, the two spacecraft were kicked out of Earth orbit by an inertial
upper stage (IUS) rocket, sending them careening through the inner solar
system. The trajectory which the spacecraft followed was called a VEEGA
(Venus-Earth-Earth Gravity Assist), traveling first in toward the Sun for
a gravity assist from Venus before encountering the Earth two times
(spaced two years apart). These encounters with Venus and the Earth
allowed Galileo to gain enough velocity to get it out to Jupiter.
False color image of the
Minos Linea region on icy and fractured surface of
Image courtesy of NASA.
During the flybys of Venus and the Earth, Galileo scientists took the
opportunity to study these two planets as well as the Moon, making some
unprecedented observations as a result. In addition, following each Earth
flyby, Galileo made excursions as far out in the solar system as the
asteroid belt, enabling scientists to make the first close-up studies of
two asteroids, Gaspra and Ida. As is this were not sufficient, Galileo
scientists were fortunate to be the only ones with a direct view of the
Comet Shoemaker-Levy 9 fragment impacts on Jupiter. All of this was prior
to the primary missions of sending an atmospheric probe into Jupiter's
atmosphere and studying Jupiter, its satellites, and its magnetosphere for
two years with the orbiter.
Interplanetary studies were also made sporadically by some of the other
Galileo instruments, including the dust detector, magnetometer, and
various plasma and particles detectors, during its six year journey to
The probe was released from the orbiter 147 days prior to its entry
into the Jovian atmosphere on 7 December 1995.
The science goals of the Galileo Probe were to:
- determine the chemical composition of the Jovian atmosphere;
- characterize the structure of the atmosphere to a depth of at least
- investigate the nature of cloud particles and the location and
structure of cloud layers;
- examine the Jovian radiative heat balance;
- study the nature of Jovian lightning activity; and,
- measure the flux of energetic charged particles down to the top of
The objectives of the Galileo Orbiter are to:
- investigate the circulation and dynamics of the Jovian atmosphere;
- investigate the upper Jovian atmosphere and ionosphere;
- characterize the morphology, geology, and physical state of the
- investigate the composition and distribution of surface minerals on
the Galilean satellites;
- determine the gravitational and magnetic fields and dynamic
properties of the Galilean satellites;
- study the atmospheres, ionospheres, and extended gas clouds of the
- study the interaction of the Jovian magnetosphere with the Galilean
- characterize the vector magnetic field and the energy spectra,
composition, and angular distribution of energetic particles and
plasma to a distance of 150 Rj.
Although Galileo was not the first mission to explore Jupiter
(actually, it is the sixth), it has established a number of
"firsts" during its journey.
- First mission to make a close flyby of an asteroid (Gaspra).
- First mission to discover a satellite of an asteroid (Ida's
- First multispectral study of the Moon.
- First atmospheric probe to enter Jupiter's atmosphere.
- First spacecraft to go into orbit around Jupiter.
- First direct observations of a comet impacting a planet
A comprehensive list of the science results of Galileo would be longer
than space permits. Here, then, is a short list of some important
discoveries (in no particular order).
- The discovery of a satellite (Dactyl) of an asteroid (Ida).
- Confirmation of the existence of a huge ancient impact basin in the
southern part of the Moon's far side (inferred from Apollo data but
never before mapped).
- Evidence of more extensive lunar volcanism than previously thought.
- Discovery of an intense interplanetary dust storm (the most intense
- Discovery of an intense new radiation belt approximately 50,000 km
(31,000 miles) above Jupiter's cloud tops.
- Jovian wind speeds in excess of 600 kilometers per hour (> 400
mph) were detected.
- Far less water was detected in Jupiter's atmosphere than estimated
from earlier Voyager observations and from models of the Comet
Shoemaker-Levy 9 impact.
- Far less lightning activity (about 10% of that found in an equal
area on Earth) than anticipated. The individual lightning events,
however, are about ten times stronger on Jupiter than the Earth.
- Helium abundance in Jupiter is very nearly the same as its abundance
in the Sun (24% compared to 25%).
- Extensive resurfacing of Io's surface due to continuing volcanic
activity since the Voyagers
flew by in 1979.
- Preliminary data support the tentative identification of intrinsic
magnetic fields for both Io and Ganymede.
- Evidence for liquid water ocean under Europa's surface.
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Galileo being deployed during the Shuttle STS 34 flight.
Courtesy of NASA
Launched: October 18, 1989
Arrival: December 7, 1995
Mission: Orbit and atmosphere studies