CHAPTER 17: THE SPACE PROBES
© John F. Graham, 1995
Photos courtesy NASA
In the saga of space exploration, space probes have been the heroes. These
probes, both lunar and interplanetary, have acted as extraterrestrial
laboratories for Earthbound scientists, American or Russian, who have made
thousands of discoveries about our Solar System, including the Sun, the major
planets, the moons, asteroids, and comets.
These probes carried sophisticated equipment such as infrared sensors to
measure a planet's temperature, radars to see through clouds to the planetary
surface, ultraviolet sensors to analyze atmospheric compositions, and
magnetometers to measure the planet's magnetic field. The rest of an
extraterrestrial spacecraft's equipment powers it, protects its from radiation
or collision, and transmits the data back to scientists on Earth.
After a spacecraft has been launched from the Earth's surface on a trajectory
beyond the influence of Earth's gravitational field, the spacecraft's navigation
system keeps it on course. This job is performed by tracking sensors and an
on-board computer. One sensor fixes on the Sun and one sensor fixes on a star
such as Arcturus to determine the spacecraft's position in space. On-board
thrusters make the necessary corrections in both position and attitude.
More advanced computer systems such as on the Voyager spacecraft were
programmed to make major course corrections on their own. The ground controllers
can send new information to these on-board computers as the need arises.
Computationally, Voyager 2 w as a much stronger spacecraft 12 years after it was
launched due to the many sophisticated software changes made as technology
improved over the time it was in space.
A very critical part of the spacecraft is the communications system, which
receives and transmits signals across the vast reaches of space. All of the
space probes have low and medium-gain antennas for this purpose. Probes to the
outer planets such as the Voyager and the Pioneer carry large parabola antennas
to amplify the signals sent back to Earth.
A very reliable power source must be aboard a spacecraft for it to continue
to function in space. Craft exploring the inner solar system from the Sun out
through the Martian orbits can use solar panels for power. Solar panels employ
photovoltaic cells which convert light into electricity. Probes headed toward
the Sun only require two large solar panels while those going toward Mars
require four. If a robot explorer travels beyond the orbit of Mars, the craft
cannot depend on the Sun for electrical power because of the reduced intensity
of the photons; therefore, the spacecraft must use small nuclear generators
called radioisotopic thermal generators (RTG) on outriggers. The outriggers are
to insure that the RTG power source is kept far enough away from the sensors to
insure that excess radioactivity does not affect the accuracy of the probe's
data collecting capabilities. Erroneous radioactivity readings could change a
scientist's determination of the strength of a planet's magnetic field or the
strength of the solar wind near the investigated planet.
Temperature control blankets, ventilation louvers, and heat shields are used
to protect probes orbiting toward the Sun, while those flying outbound require
heaters to prevent the craft from freezing as it accomplishes its mission. These
are the probes which have started the exploration of our Solar System and four
of these are on their way to the stars. Because of these little robots we have
begun to understand how the Sun and the planets were formed and how our planet
is unique in the solar system.
TO THE MOON - RANGER
After the initial attempts to reach the Moon, the US finally formed a
University/Government team from the California Institute of Technology and the
Jet Propulsion Laboratory (JPL) which sought to explore the Moon via
photographs. The result of this affiliation was the Ranger program.
NASA approved the Atlas Agena combination to launch the Ranger spacecraft;
its mission was to take photographs until it impacted the lunar surface. The
spacecraft had solar panels for power, three axis stabilization and
The entire program was to be a series of nine launches; the first six failed.
In the first two launches the Agena failed; on the third the spacecraft failed,
on the fourth the computer failed, on the fifth the power system failed, and on
the sixth as the spacecraft neared the Moon's surface, its cameras failed to
operate. Ranger 7 was launched July 28,1964 on a near-perfect mission;
everything worked. It transmitted 4316 pictures of the Lunar surface above the
Sea of Storms. The last picture was taken 1400 feet above the surface with a
resolution of three feet in an area 100 feet by 160 feet.
By this time the Apollo Program was underway and the mission for Ranger was
to scout for possible landing sites. Ranger 8 was sent to the Sea of Tranquility
where it impacted on February 17, 1965 after transmitting 7100 pictures.
The final impacting spacecraft, Ranger 9, was launched on March 21, 1965. The
spacecraft was rotated so that it looked precisely along the flight path. This
permitted a resolution of one foot. Ranger 9 impacted into Alphonsus crater
after transmitting 5814 pictures showing craters within craters and some rocks.
TO THE MOON - SURVEYOR
Surveyor's mission was to answer the question, "What was the lunar
surface really like?" Many scientific hypotheses suggested that the Moon
was covered with deep fields of dust into which a spacecraft might sink.
Crevasses might be treacherously covered with dust very much like snow covering
some of Earth's crevasses in a glacier and an astronaut could fall into one of
them. Others theorized that the Moon's surface might resemble a World War I
battlefield, a wasteland of craters and debris of a uniform blanket of ejecta
over the entire Moon.
The Surveyor mission had been conceived in 1959 and it was modified to meet
the requirements dictated by Apollo. The mission remained much the same: to
soft-land scientific instruments on the Moon's surface. This mission required a
new launch vehicle and the new technology of a closed-loop, radar-controlled
automated landing. The new launch vehicle was the Atlas-Centaur which used the
propellant combination of Oxygen and Hydrogen for the first time. The first
stage was an Atlas D with enlarged tanks and increased thrust and the Centaur
upper stage had two engines with 15,000 pounds of thrust each.
The Surveyor spacecraft consisted of a tubular framework perched on three
shock-absorbing footpads. The 625 pound lander contained batteries and solar
panels for its 90 watts of electrical power, two 10 watt transmitters, and a TV
camera with two focal lengths of 25 and 100 mm having a resolution of 1mm at 4
meters. Seven of these spacecraft were launched; the first, Surveyor I, was
launched on May 31, 1966 and landed on the Moon on June 2, 1966. Surveyor I
performed a perfect landing in the Ocean of Storms and proceeded to transmit
The Russians once again beat the Americans in space exploration when Luna 9
landed on February 3, 1966 sixty miles northeast of the crater Calaverius and
sent back to Earth the first lunar surface pictures, but the Surveyors quickly
built a very impressive collection of photographs from various locations on the
Surveyor I showed the lunar surface to be cohesive and able to support a
spacecraft. The small spacecraft found a barren plain pitted with numerous
craters and strewn with rocks of all shapes and sizes.
Surveyor II was lost during a midcourse correction maneuver and Surveyor III
made a rough landing inside a 650 foot crater as it slid down the side of the
crater before stopping. Surveyor III had a remote digger on board which dug a
little pile of lunar dirt which was dumped on the footpad for closer
examination. In 1970 Apollo 12 astronauts visited Surveyor III and took home
parts of its structure. Surveyor IV failed minutes before touchdown and Surveyor
V landed in a crater on the Sea of Tranquillity on September 10, 1967. After
landing the Spacecraft's engines were reignited to see what effect they would
have on the lunar surface's dust. The small amounts of dust scattering indicated
that there would be no problem for the Apollo landings.
Surveyor VI landed in Sinus Medii and was flown to a new location eight feet
away from the original landing site. The fine dust covered a photometer plate
during this maneuver.
The last Surveyor, number VII, landed near the crater Tycho on January 9,
1968. This last spacecraft produced over 20,000 photographs and ended this
program with a resounding triumph as five of the seven missions had been
The Lunar Orbiter
The missions of the Lunar Orbiter were to photograph all possible Apollo
landing sites, to measure meteoroid flux around the Moon, and to determine the
lunar gravity field precisely. Lunar Orbiter I was launched on August 10, 1966
and photographed nine primary and two secondary Apollo landing sites.
The next two orbiter missions were launched on November 6, 1966 and February
4, 1967. They provided excellent coverage of all twenty Apollo landing sites,
far side photographs, and oblique photos as might be seen by orbiting
astronauts. The final two orbiters were launched on May 4 and August 1, 1967.
These were placed in high lunar polar orbits and mapped the Moon's entire
The Apollo site survey showed surprises. Some areas originally thought to be
perfect landing sites turned out to be unacceptable because of rough terrain or
boulder concentrations. All sites had craters so the astronauts received
training on crater avoidance. The other orbiter experiments showed no unexpected
high levels of radiation or meteoroids; the astronauts' safety was almost
assured. There were large gravitational variations noted in the Moon due to
great mass concentrations known as mascons which would perturb an Apollo orbit.
With the completion of Ranger, Surveyor, and Lunar Orbiter the robotic scouts
for Apollo had done their jobs. The scientists had high confidence that no
surprises awaited the first astronauts to land on the Moon.
THE MARINER PROGRAM
On the 27th of August 1962 the US entered the interplanetary race by
launching Mariner 2 to Venus. The Soviets had successfully passed within 100,000
kilometers of the planet in 1961 following a launch of an SL-3 from Tyuratam,
but the probe was only half of a triumph because contact with the probe was lost
less than a month into the mission. The US had tried to send Mariner 1 to Venus,
but a programming error resulted in the craft's destruction less than five
minutes after its launch.
On the 14th of December 1962, Mariner 2 flew within 34,000 kilometers of
Venus' surface; the probe did not lose contact and reported that the surface
temperature of Venus was extremely high. The probe's radiometers measured
surface temperatures in the 9 00º F range; this was about four times hotter
than scientists had anticipated. Mariner 2 continued into heliocentric orbit
where it remains after a job well done.
The Soviets launched Mars 1 in November 1962. On March 21, 1963 the Mars
bound vehicle lost all communications capability and was thought to have flown
by the red planet at 197,000 km on June 19, 1963. Mars 1 became the first
spacecraft from Earth to flyby Mars.
US scientists took their next interplanetary aim at Mars. Mariner 3, launched
on November 7, 1964, failed when its payload shroud did not separate from the
payload. Three weeks later, on November 28, 1964, Mariner 4 launched and
performed a successful flyby within 9800 kilometers of the red planet eight
months later. Mariner 4 transmitted 21 digital pictures back to Earth which
showed no surface water, no Martian canals, and a Moonlike surface with a number
of craters. This flyby also revealed a very thin, carbon dioxide atmosphere
Venera Program Begins
On November 12 and 16, 1965 the Soviets launched Venera 2 and 3 whose purpose
was to orbit Venus and possibly land on its surface. Again, both probes had
communications failure enroute to Venus. On February 27, 1966 Venera 2 flew past
the Sun side of Venus at a distance of 24,000 km above the surface. On March 1,
1966 Venera 3 entered the Venusian atmosphere on the planet's dark side and
became the first probe ever to land on another planet. These probes returned no
useful Venusian data.
In June of 1967 both the USSR and the US launched probes to Venus on June 12
and June 14 respectively. This time the Soviet probe, Venera 4, did not suffer
radio failure and on October 18, 1967 it landed on Venus. For the first time
data concerning Venus' temperature, pressure, and chemical composition were sent
to Earth. At 20 km above Venus' surface the temperature rose to 520ºF and the
pressure reached the equivalent of 18 Earth atmospheres and the probe failed.
Mariner 5 performed another fly by within 3950 km of the Venusian surface; it
looked for a magnetic field and performed ultraviolet photometry of Venus'
atmosphere before continuing into heliocentric orbit. Both Venera 4 and Mariner
5 reconfirmed the Mariner 2 data, Venus is a literal hellish place.
In January 1969 the Soviets sent Veneras 5 and 6 to duplicate and surpass
Venera 4's accomplishments. Both of these probes failed at about the same
altitude in Venus' atmosphere in May 1969 and reconfirmed the planet's high
temperature and pressure.
While the Soviets occupied themselves with Venus the Americans concentrated
on Mars. On February 24 and on March 27, 1969 the US launched Mariners six and
seven to Mars. This was the first time a pair of probes was ever sent to a
single planet. Both flyby probes arrived within five days of each other on July
30 and August 5, 1969. Mariner 6 passed 3400 km over the Martian equator and
took 76 pictures while Mariner 7 passed over Mars' south pole and returned 126
pictures. These photographs were of higher resolution and excited scientists as
they now saw deserts, ridges, and valleys along with the craters. The probes
confirmed that Mars has an atmosphere rich in carbon dioxide and suggested that
the polar caps of Mars could be made of dry ice.
The success of the Americans on Mars was quickly followed by a Soviet success
on Venus. Venera 7, launched from Tyuratam on August 17, 1970, softlanded on
Venus' equator on December 15, 1970 and transmitted data for 23 minutes before
succumbing to the high temperatures and pressure.
On July 22, 1972 Venera 8 became the first Soviet probe to land on the sunlit
side of Venus at 10ºS and 330ºE at about 600 km from Venus' terminator. This
lander operated on the surface for 50 minutes before failing. During this time
it took light-level readings, made temperature and pressure measurements, and
performed soil and atmosphere composition studies.
The Americans continued the two probe technique of exploring Mars and Mariner
8 was destroyed by a second stage malfunction shortly after liftoff. The
Soviets, using this technique, launched Mars 2 and Mars 3 in May of 1971. Both
of these craft orbited Mars in November 1971 and landed on the planet. Mars 2
probably crashed on the planet while Mars 3 landed and transmitted for 23
seconds before failure. The Americans' second probe, Mariner 9, launched on July
17, 1971 and entered Martian orbit on November 13, 1971. It was the first
artificial moon of any planet as it beat both of the Russian Mars probes to the
planet. Mariner 9 orbited Mars for more than a year taking 7,329 photographs.
Among these were startling pictures of huge volcanoes, deep canyons, and
dried-up rivers. Spectrometer readings discovered traces of water in the Martian
atmosphere and with it the possibility of life on another planet. On October 27,
1972 Mariner 9 ran out of control gas thus ending the most successful planetary
mission to that date.
Pioneer 10 and 11
Until March 2, 1972 all US and USSR interplanetary probes had been directed
at Earth's two closest neighbors, Venus and Mars. On this date an Atlas-Centaur
launched Pioneer 10 toward Jupiter. On December 3, 1973 this probe flew within
130,000 km of Jupiter's surface; it returned 300 photographs of the planet and
its Galilean moons while performing an extensive study of the Jovian
magnetosphere. During the flyby of Jupiter, Pioneer 10 acquired enough gravity
assist velocity to hurl the craft out of the solar system. On its surface was
placed a plaque, a cosmic greeting card with a message for extraterrestrial
intelligences. On this plaque stand a man and a woman before a facsimile of the
spacecraft to show the size of average humans. A radial pattern shows the
location of the Sun within the galaxy while depicted at the bottom are the Sun
and the Planets showing the path of Pioneer 10 during its voyage out of the
Sticking with their policy of launching dual spacecraft in the event of
failure, the Americans launched Pioneer 11 on May 5, 1973 to probe the outer
solar system aboard another Atlas-Centaur vehicle. On December 3, 1974 the probe
passed within 42,800 km of Jupiter's surface; in the Jovian system Pioneer 11,
exactly like its forbearer, performed extensive magnetospheric studies and
photographed Jupiter and the Galilean Moons. Unlike Pioneer 10, Pioneer 11
performed a gravity assist maneuver toward Saturn. On September 1, 1979 the
probe flew within 21,400 km of Saturn's surface. Passing outside of Saturn's
ring plane, Pioneer 11 photographed the planet, its ring system and the
Saturnian moons. While conducting an intensive study of Saturn's magnetosphere,
Pioneer 11 discovered another moon and determined that the ring system was much
more extensive than previously determined by telescope. After the Saturn
encounter, Pioneer 11 entered a trajectory which eventually took it out of the
solar system. Like its partner the probe also carried a plaque on its surface
locating our Sun and planet in the galaxy.
Soviet Mars Flights
In July of 1973 a launch opportunity to Mars presented itself and the Soviets
took full advantage of these launch windows by sending a small armada to the red
planet. On the 21st and 25th of July they launched Mars 4 and 5 aboard a pair of
powerful Proton Rockets. Mars 4 was intended as a radio relay for the other
craft which was to land on the surface, but the retro rockets failed and the
craft performed a flyby passing 2200 km above Mars' surface. Mars 4 did manage
to transmit several TV pictures of Mars during its unplanned passage. Mars 5 was
also intended as an orbital relay and it successfully achieved Mars orbit on
February 12, 1974.
Two Proton rockets launched the Soviet landing craft, Mars 6 and Mars 7, on
the 5th and 9th of August respectively. Both craft contained a landing bus and a
flyby craft. On March 2, 1974 the Mars 6 landing craft entered the Martian
atmosphere at 25 6;W and 24ºS, but shortly before reaching the planet's
surface the capsule ceased transmitting. Mars 7 ran into more bad luck as the
separation maneuver was performed improperly and the Mars 7 lander missed the
planet by 1200 km. Both of the craft continue to orbit the Sun.
On November 3, 1973 another Atlas Centaur launched Mariner 10 to probe the
inner planets of the solar system. On February 5, 1974 the probe flew within
5770 km of Venus and took several pictures of that planet in the ultraviolet
spectra and determined t he true cloud patterns of the planet. Mariner 10
continued on to Mercury where it performed three flybys and returned 3700 images
of the inner planet. Mariner 10 determined that Mercury has a magnetic field and
a surface which is heavily cratered. The planet also appears to be shrinking in
on its core and as a result is leaving a very torturous landscape of deep rills.
Also discovered was the largest impact basin in the solar system - the Caloris
Basin. Mercury is a very unfriendly place, but could p resent the answers to
many questions about the solar system's formation and early life.
On December 10, 1974 a Titan III-E Centaur launched a joint US-West German
probe, Helios 1, to study the Sun. This craft entered a solar orbit with a
perihelion of 0.31 AU, the closest approach to the Sun at that time. Helios 1
performed measurements o f the magnetic fields, the solar wind, radio waves,
cosmic rays, dust and X-rays.
Helios 2 was launched on June 15, 1976 and flew to within 0.29 AU of the Sun.
Like its forbearer it measured the Sun's magnetic field, solar wind, cosmic
rays, radio waves, plasmas, dust, and X rays.
The Soviets continued their assault on Venus with two launches on June 8th
and June 14, 1975. The two craft were designated Veneras 8 and 9, but they were
different from their predecessors. Due to the desire for increased atmospheric
investigations, surface studies, and TV pictures the Soviets designed an
entirely new craft. Drawing on the technology developed for the Mars missions,
the new Venera station were to be a heat-shielded sphere with a landing module
On October 22 and 25 each landing craft separated from its respective orbiter
which would serve as a radio relay from the planet's surface. After aerodynamic
braking the landers jettisoned the upper half of their protective shell and
deployed a parachute; data transmission began at this time. Three main
parachutes deployed to slow the craft's descent. After passing through the main
cloud layers, the parachutes were discarded and the final descent was
accomplished using a braking disc at the top of the landers. This disc was very
effective as the Venusian atmosphere became denser near the planet's surface. As
the lander touched down at a velocity of about 7 m/sec a titanium ring shock
absorber cushioned the landing and supported the module on the surface. Two
minutes after touchdown, the TV system was operated and picture of a very
foreboding surface was transmitted. Venera 9 operated for 53 minutes while
Venera 10 survived 65 minutes.
The US continued the exploration of Mars with two landers Viking 1 and 2. Two
Titan III E's launched the craft on August 20 and September 9, 1975. Viking
consisted of two major parts: a Mariner-like orbiter to photograph the entire
planet and a lander to photograph the Martian surface with a laboratory to
search for Martian microbes. The two landers were the size of a jeep and each
contained three scientific laboratories. Both landers contained two cameras and
a ten foot arm for collecting soil samples. Small nuclear generators powered
these machines and their control was accomplished by on-board computers
In June 1976 the craft began orbiting Mars and began to inspect potential
landing sites. When the intended landing sites were determined to be unsuitable,
the Vikings searched for another location. Upon finding a suitable landing site
the Viking lander separated from the orbiter, fired a retro rocket to leave
orbit and deployed a parachute to slow its descent to the surface. About a mile
above the Martian surface the craft fired three retro rockets to slow the
spacecraft to 5 mph and the craft settle d gently on Mars at a place designated
as the Plains of Chryse. On July 20, 1976 the first pictures of the Martian
surface appeared at the Jet Propulsion Laboratory in Pasadena, California. The
surface appeared to be like an Arizona desert with large rocks and red soil into
the far distance and the sky was a light pink.
Viking 2 landed two weeks later on the Plains of Utopia. The lander's
instruments determined that the highest temperature at the landing area was
about -24ºF in the Martian summer, its atmosphere contained 3% nitrogen and
water was found as ice under the dry ice caps of the poles. This led scientists
to be optimistic about the search for Mars microbes.
The search for Martian life was accomplished on its soil samples. The arm
took the sample and conveyed the soil into the Viking Lander laboratory. There
the sample were subjected to three separate tests to incubate the microbes and
to detect waste products such as carbon dioxide. Initially the soil samples
showed water and a very active chemical reaction to the tests. The biologists
disappointedly stated that this activity could be explained in terms of chemical
reaction rather than biological activity. Another experiment detected no forms
of the organic compounds of which life is composed on Earth. The search for life
on Mars was deemed to be inconclusive.
The Viking missions were giant successes. The landers and the orbiters
produced more than 50,000 images and operated much longer than the original
design specifications. The Viking 1 orbiter was designed for 90 days of
operation; it lasted four years until it ran out of gas. The Viking 1 lander
lasted until 1982 when a mistaken computer command erased part of the lander's
Voyager 1 and 2
The most successful space voyages of any robot explorers were launched on
August 20, 1977 and September 5, 1977 by two Titan III E Centaurs. Voyager 1 and
Voyager 2 opened up the outer solar system for human view for the first time.
Both craft flew by Jupiter in March of 1979 sending back 68,000 images of the
planet and its moons. The most striking discovery was of the volcanoes on the
Galilean Moon, Io; this was the first discovery of volcanic activity on a planet
other than Earth.
Following the Jovian encounter, both craft used a gravity assist trajectory
to proceed to Saturn arriving in 1980 and 1981. Saturn was a cold gaseous ball
with the most gorgeous ring system of any planet. This beautiful body was also
racked by the strongest winds in the solar system reaching over 1800 km/hr.
Saturn's rings were determined to be actually thousands of ringlets being formed
by a number of "shepherd moons." Six new moons were discovered around
Saturn bringing its total to 23 moons. Voyager 1 returned over 18,000 images of
Saturn and its moons. It flew within 4000 km of the moon Titan covered with a
thick orange atmosphere. After its Saturn encounter, Voyager 1 entered a
trajectory from which it will ultimately leave the solar system. Like the
Pioneer 10 and 11, the craft carries a recorded message for extraterrestrials
about our planet and a gold plaque similar to the Pioneer signs. Voyager 2 was
sent on a trajectory to Uranus.
Voyager 2 flew past Uranus on January 24, 1986 at a distance of 107,080 km.
It measured the magnetosphere of the planet and returned photographs of the
clouded planet and its moons. Voyager 2 passed within 29,000 km of the moon,
Miranda, which may have produced some ideas about why Uranus rotates on its
side. Strange chevrons stretched across Miranda's landscape considered to be the
strangest in the solar system. The craft also discovered additional ring
components and 10 new Uranian moons before using another gravity assist
trajectory to journey to Neptune.
On August 29, 1989, Voyager 2 flew past Neptune. Many great discoveries were
made at this planet including a large storm located in the planet's southern
hemisphere. Voyager also showed that the winds on Neptune are nearly as fast as
those on Saturn. Additionally, Voyager discovered that Neptune has an entire
ring system like the rest of the gas giants. Small layers of stratus clouds also
flew through Neptune's atmosphere. Other great discoveries were accomplished at
Neptune's moon, Triton. Here, some of the strangest terrain ever noted anywhere
in the solar system was seen. One side of the moon looked like the skin of a
cantaloupe, while the other side looked extremely rugged. Pancake-like terrain
was located over half of the moon and one image even showed some sort of
volcanic action on Triton's surface. A plume of either liquid nitrogen or water
was noted streaming from a vent on the moon. Following this grand finale,
Voyager 2 traveled to the edge of the solar system and into interstellar space
carrying with it a recorded message for any extraterrestrials which may retrieve
this probe along with a gold plaque telling from whence it came.
The US entered Venus exploration seriously when Pioneer-Venus 1 and 2 were
launched on May 20 and August 8, 1978. Pioneer-Venus 1 entered a polar orbit
around Venus and measured the planet's ionosphere, its clouds and radiation. An
on-board radar altimeter made a radar map of the entire planet with a resolution
of 75 km. These images revealed canyons, continent-sized plateaus, craters, and
possibly dormant volcanoes. Pioneer-Venus 1 also photographed Halley's Comet as
it passed the planet in 1986. T he craft was still functional until it deorbited
into Venus in 1993.
Pioneer-Venus 2 was built for one purpose - to send several probes to the
surface of Venus. This spacecraft carried four probes - one large sounder probe
and three smaller probes. These probes studied the temperature, the pressure,
the atmospheric composition, and cloud structure and composition. One of the
probes survived 67 minutes on Venus' surface. Twelve hours after depositing the
probes, the spacecraft bus entered the Venusian atmosphere. One important
finding from these probes confirmed earlier suggestions as to why Venus is so
hot. The carbon dioxide in the atmosphere acts like glass in a greenhouse,
trapping solar radiation and overheating Venus' surface.
The International Sun-Earth Explorer was launched from Cape Canaveral by a
Delta rocket on August 12, 1978. Placed in a "halo" orbit where the
gravitational forces of the Sun equal those of the Earth, the spacecraft
performed a long-term study of solar phenomena. In 1982 the craft was renamed to
International Comet Explorer (ICE) and sent to investigate Comet
Giacobini-Zinner and performed the first spacecraft encounter with a comet by
passing through the comet's tail. Even though no cameras were carried aboard the
craft it was able to study the comet's magnetic field and its plasma within the
comet's coma. In March of 1986 ICE encountered Halley's Comet and relayed
important data about the comet's coma. ICE is scheduled to return to the
vicinity of the Earth in 2005 where it may be recovered and studied.
The Soviets relentlessly continued their Venus exploration program by
launching probes to the veiled planet during every launch window. In September
1978 they launched Veneras 11 and 12 which landed in the Beta-Phoebe region of
Venus. Venera 11 lasted 95 minutes and Venera 12 operated 110 minutes on Venus'
surface before failing. Both craft may have attempted to transmit photographs
from the surface, but failed.
Another Venus launch window opened in October and November of 1981 and the
Soviets promptly launched Veneras 13 and 14. The Proton vehicle placed both
spacecraft into the correct trajectories for landing on March 5, 1982. Venera 13
operated for 127 minutes before failing. It took color pictures of the surface
and performed chemical analysis of a soil sample. Venera 14 only operated for 53
minutes, but it, too, took color pictures of the surface and performed chemical
analysis of the soil.
VEGA 1 AND 2
The next Venus window opened in June of 1983 and the Soviets sent two radar
mappers, Veneras 15 and 16 to the planet in October 1983. Both spacecraft were
equipped with side-looking synthetic-aperture radar and mapped the northern
hemisphere of Venus at a 1 to 2 kilometer resolution. On December 15 and 21 1984
a new series of two landers was launched to Venus. These were the Vega1 and 2
spacecraft. Both spacecraft had a double mission: 1. Dispatch a probe to the
Venusian surface and 2. Continue to Halley's comet to perform studies of the
comet's dust, gas, plasma, and magnetic fields within the coma of Halley. The
probe entered the Venusian atmosphere on the night side and dispatched an
instrumented balloon that drifted with the wind at an altitude of 55 km for 2
days before the batteries failed. The balloon instruments measured wind speeds,
pressures, and temperatures in the clouds. The Venus 1 lander set down near
Aphrodite Terra and operated 56 minutes before failing. A surface drilling
experiment failed and there were no provisions for surface photos on either
mission. Vega 2 had a successful balloon deployment which lasted two days and
its lander set down in the Alta Regio region. This lander operated for 57
minutes before it failed. It man aged to measure the atmospheric composition and
soil before the failure.
Both Vega 1 and 2 rendezvoused with Halley's Comet on March 3 and 9, 1986
respectively. Vega 1 passed within 8890 km of Halley's nucleus while Vega 2
passed within 8030 km. Both craft studied the composition of the comet's gas and
dust and both craft obtained images of Halley's nucleus.
The most successful Halley Comet probe sent by the armada of space nations
was Giotto launched by the European Space Agency on July 2, 1985. On the 13th of
March 1986, Giotto pass within 610 km of the comet's nucleus and obtained some
fascinating images . The probe also returned data about the comet's gas, dust,
magnetic fields, radiation and plasma. Several of the spacecraft's instruments
were heavily damaged by the cometary encounter, but Giotto was revived in 1992
for another comet encounter which it performed excellently providing much
additional data about comets.
In 1988 the Soviets launched two probes to Mars called Phobos 1 and 2. Both
probes failed due to computer errors from initiated commands. Phobos 2 managed
to obtain pictures of Mars and of Phobos before its failure.
In 1989 the space shuttle launched a European spacecraft, Ulysses which was
sent into orbit to study the poles of the Sun. The spacecraft was sent to
Jupiter to obtain enough of a gravity assist to place the craft on a path around
the Sun's south pole in October 1994 and the north pole in March 1995. Ulysses
discovered that the solar wind at the south pole is actually faster than the
solar wind at the Sun's ecliptic.
Also in 1989 Galileo was launched on a winding path to Jupiter. After
launching from the space shuttle, Galileo flew by Venus, flew by Earth out to
the asteroid Gaspra, flew by Earth again past the asteroid Ida and has a current
arrival date of December of 1995 at Jupiter. The craft's primary antenna did not
deploy fully and it is unable to send high speed data which means a great amount
of data from the probe will take a long time to reach Earth or may be lost
entirely. Data about the impact of the Comet Shoemaker-Levy returned to Earth a
couple of months after the main event, but it showed that Galileo's data, even
if it is transmitted slower than normal will be extremely valuable. Another
major discovery of the probe was that of a minor moon orbiting the asteroid Ida.
Id has a diameter of about 22 km and its moon has a diameter of about 1 km. The
small moon was named Dactyl. This is the first evidence of a extremely small
body orbiting a very small primary body. As of July 1995 Galileo is still on a
direct course for Jupiter and will begin orbiting the planet in December 1995.
The atmospheric probe of the planet will be launched shortly. The probe's
mission is to penetrate as deeply into the Jovian atmosphere as possible and to
determine the gases and winds in this large planet.
In 1989 the spacecraft Magellan was launched to Venus to radar image the
entire planet. The job was very successfully accomplished and 98% of the
planet's surface has been imaged by radar. Many discoveries have been made which
show that Venus has volcanoes which may or may not be active. After performing
brilliantly for three years, Magellan performed an aerobraking maneuver and
crashed into Venus in 1994.
In 1992 the US launched the Mars Observer to obtain higher surface resolution
of the planet. After a flawless launch and a very good trajectory, Mars Observer
mysteriously quit working when a propulsion maneuver was performed. It probable
The next generation of space probes was launched in 1994 when Clementine was
dispatched to the Moon. Built to the specifications of the new NASA program of
"better, faster, cheaper", Clementine was actually built by the US
Department of Defense under the Strategic Defense Initiative Program. The small
spacecraft was designed, built and launched in less than two years for a total
cost of $80 million in a program consisting of 55 people total. The craft had an
ambitious mission plan of imaging the entire Moon and then rendezvousing with a
near Earth asteroid, Geographos, and imaging it as well.
The probe reached the Moon and obtained over 1.6 million images; on its way
to Geographos, Clementine rendered inoperative by a computer commanding error
which started spinning the entire spacecraft and depleting its fuel. As of
December 1994, Clementine has escaped Earth's gravitational influence and was
going into a heliocentric orbit. The command center in Virginia gained contact
with the probe as it flew farther from the Earth. There is some hope of
controlling it although most of the data will be concerning the particles in the
Solar Wind. Clementine will probably be the model for other future robotic
These robotic probes have led the way for humans to follow. Even though
humans have explored a very small part of the Moon for a very short time, most
of human explorations have been confined exclusively to LEO. Explorers are now
concerned with the very basics of space flight pursuing such questions as how
does the human body operate in space? In the early part of the next century an
international group of astronauts and cosmonauts will inhabit a permanent
International Space Station to determine not only the effects of space upon the
human body, but also to explore a number of areas that will change life on
Earth. In the next chapter we will learn about the evolution of the space
station from fiction to fact, and how it may be used as a springboard for
further human exploration of outer space.