What Did Galileo Find at Jupiter?


Galileo is a NASA spacecraft mission to Jupiter, launched October 18, 1989, and designed to study the planet's atmosphere, satellites and surrounding magnetosphere for 2 years starting in December 1995. It was named for the Italian Renaissance scientist who discovered Jupiter's major moons in 1610 with the first astronomical telescope.

This mission will be the first to make direct measurements from an instrumented probe within Jupiter's atmosphere, and the first to conduct long-term observations of the planet and its magnetosphere and satellites from orbit around Jupiter. It is already the first to encounter an asteroid, and to photograph an asteroid's moon.

The Jet Propulsion Laboratory designed and developed the Galileo Jupiter orbiter spacecraft and is operating the mission; NASA's Ames Research Center developed the atmospheric probe with Hughes Aircraft Company as prime contractor. The German government is a partner in the mission through its provision of the spacecraft propulsion subsystem and two science experiments.

Scientists from six nations are participating in the mission.

Like Voyager and some other previous interplanetary missions, Galileo used planetary gravitational fields as auxiliary propulsion stages. The spacecraft dipped into the gravitational fields of Venus and Earth to pick up enough velocity to get to Jupiter. This 38-month Venus-Earth-Earth Gravity Assist phase (see figure) ended with the second Earth flyby on December 8, 1992. It provided, in addition to the velocity increment, opportunities for useful scientific observations and an exercise of the spacecraft's scientific capabilities.

Galileo's two planned visits to the asteroid belt provided the first and second opportunities for close observation of these bodies: in October 1991 the spacecraft flew by asteroid Gaspra, obtaining the world's first close-up asteroid images; in August 1993 it flew by a second asteroid, Ida, and discovered the first confirmed asteroid moon.

In late July 1994 Galileo was the only observer in a position to obtain images of the far side of Jupiter when more than 20 fragments of Comet Shoemaker-Levy plunged into the night-side atmosphere over a 6-day period.

In December 1995 the Galileo atmospheric probe will conduct a direct examination of Jupiter's atmosphere,

while the larger part of the craft, the orbiter, begins a 23-month, 11-orbit tour of the magnetosphere and the Galilean moons, including ten close satellite encounters.

Galileo's orbital science results will be transmitted to Earth over the low-gain antenna at significantly lower data rates than originally planned, because of the in-flight failure of the high-gain antenna to deploy as commanded in April 1991. The Project team has developed means to transmit the key scientific data and to accomplish the Project's Jupiter science objectives, using on-board data processing and compression, and various enhancements to the communications link performance, including new encoding systems and advanced technology in ground equipment.

The 2,223-kilogram (2-1/2-ton) Galileo orbiter spacecraft carries 10 scientific instruments; there are another six on the 339-kilogram (746-pound) probe. The spacecraft radio link to Earth and the probe-to-orbiter radio link serve as instruments for additional scientific investigations.

Galileo communicates with its controllers and scientists through the Deep Space Network, using tracking stations in California, Spain and Australia.

The Galileo Mission

Launched in 1989, the Galileo spacecraft arrived at Jupiter on December 7, 1995, when it fired its main engine for a successful orbit capture around Jupiter. On that day, Galileo's atmospheric probe plunged into Jupiter's atmosphere and relayed information on the structure and composition of the solar system's largest planet. The spacecraft's orbiter will spend the next two years orbiting the giant planet, studying Jupiter and its moons (encountering one moon during each orbit), and returning a steady stream of images and scientific data. After completing its primary mission, Galileo will then begin its two year extended mission called the Galileo Europa Mission. The Galileo Europa Mission (GEM) is a highly focused follow-on to Galileo's Jupiter system exploration and a precursor for future missions to Europa and Io. GEM will conduct a detailed study of Europa over 14 months, then plunge repeatedly through the Io Plasma Torus to reach volcanic Io.


The most difficult atmospheric entry in the solar system was successfully accomplished !
After a six year journey through the solar system and after being inexorably accelerated to a speed of 170,700 km/hour (106,000 mph) by Jupiter's tremendous gravitational pull, the Galileo Probe successfully entered Jupiter's atmosphere at 22:04 UT ( 2:04 P.M. PST) on December 7, 1995. During the first two minutes of this most difficult atmospheric entry ever attempted, near-probe air temperatures twice as hot as the Sun's surface and deceleration forces as great as 230 g's (230 times the acceleration of gravity at Earth's surface) were produced as the spacecraft was slowed down by Jupiter's atmosphere. The Galileo Probe and Orbiter separated on July 13, 1995 and both arrived at Jupiter on slightly different trajectories. The Galileo Orbiter successfully became the first spacecraft to enter an orbit about Jupiter a few hours after the Probe's successful descent into the atmosphere.

The Galileo Probe's radio transmission lasted for 57.6 minutes.
At a speed of 3,000 km/hour (1,900 mph), the Probe's parachutes deployed and the heat shields fell away for the start of the direct measurements of the atmosphere and the transmission of data via a radio link to the Galileo Orbiter which was 215,000 km (134,000 miles) above. The descent sequence was successfully executed, but for an as yet unknown reason, the start of the scientific measurements occurred 53 seconds late. This delay means that the direct atmospheric measurements started deeper in the atmosphere than originally planned- starting at the 0.35 bar (0.35 times sea-level atmospheric pressure on Earth) pressure level (at or below the estimated cloud tops) rather than the 0.1 bar pressure level. The Probe's transmissions to the Orbiter, which stored the data in its computer memory and on its tape recorder for later playback to Earth, lasted for 57.6 minutes, failing only after the communication system on the Probe succumbed to the extreme environmental conditions about 600 km (373 miles) after entering the tenuous upper reaches of Jupiter's atmosphere.

All the scientific instruments operated successfully.
The Probe data stored in the Orbiter's computer memory has been successfully received on Earth (transmission of the more complete data on the tape recorder begins the week of January 22, 1996). Preliminary analysis of the received data indicates all scientific instruments operated properly and returned valuable measurements of the complex atmosphere of Jupiter and the innermost regions of Jupiter's intense radiation belts. Six scientific instruments and an experiment utilizing the Probe's radio transmissions to determine wind speeds collected information on the environmental conditions down to about 200 km (125 miles) below the visible cloud tops of Jupiter. The atmospheric probe did not include a camera.

Why send a Probe into Jupiter's atmosphere?
Prior to Galileo's arrival, many fundamental questions about Jupiter's nature remained unanswered due to the obscuring veil of its uppermost clouds, which is what is seen when looking at Jupiter from Earth or from a passing spacecraft such as Voyager or Pioneer. Jupiter, the largest of the planets, is one of the four so-called Giant Planets (Jupiter, Saturn, Uranus, and Neptune), which do not have solid surfaces like the icy and terrestrial planets - of which Earth is the largest. Jupiter is principally composed of hydrogen and helium in the form of gas, fluid, and fluid metal. In addition, because of Jupiter's strong gravitational pull, the original materials which made up the cloud of gas and dust that formed the planets are believed to remain trapped on Jupiter, unlike Earth where they have largely escaped. Thus, by accurately determining the composition of Jupiter, scientists believe they can obtain a better understanding of the formation of the planets, which occurred 4.5 billion years ago, as well as their subsequent evolution.

A new intense radiation belt was discovered.
Six hours before atmospheric entry, the Galileo Probe, which had been in a dormant state for the 155 days since its separation from the Galileo Orbiter, came to life to begin preparing for the atmospheric entry. Three hours before entry the only scientific experiment not designed for studies of the atmosphere started to take measurements. The Energetic Particle Instrument (EPI) measured the radiation (high energy charged particles) in the previously unexplored inner regions of Jupiter's magnetosphere -- the gigantic region about the planet in which the magnetic field of Jupiter dominates the interplanetary magnetic field produced by the Sun. Jupiter's magnetosphere is by far the largest in the solar system and its magnetic field and radiation belts are by far the strongest. The Earth has its own radiation belts known as the Van Allen belts. The radiation belts on Jupiter are so strong that the Galileo Orbiter is limited to maintaining an orbit quite high above Jupiter's cloud tops to avoid exposing its electronics to this damaging radiation. The EPI discovered a new intense radiation belt between Jupiter's ring and the uppermost atmospheric layers. This belt is approximately 10 times as strong as Earth's Van Allen radiation belts. A surprise discovery in this new radiation belt occurred with the finding of high energy Helium ions of unknown origin. With further analysis, these discoveries will increase our understanding of Jupiter's magnetosphere and of its high frequency radio emissions. Many bodies in the universe (stars, galaxies, pulsars, etc.) have extensive magnetic fields and trapped radiation so studying the particularly strong magnetosphere of Jupiter can provide us with new understanding of the nature of these objects as well.

Measurements of temperature, pressure, and vertical winds reveal several surprises.
As the plunge into Jupiter's atmosphere began, the Atmosphere Structure Instrument (ASI) started to probe the uppermost regions of the atmosphere through its influence on the probe's motion. The objective of this investigation was to measure the temperature, pressure, and density structure of Jupiter's atmosphere throughout the Probe's descent into the atmosphere. Such information is essential for understanding Jupiter's atmosphere and for interpreting the results of the other experiments. Temperature and pressure were directly measured during the parachute descent phase of the mission. Initial results include the detection of upper atmospheric densities and temperatures that are significantly higher than expected. An additional source of heating beyond sunlight appears necessary to account for this result. In the lower reaches of the atmosphere, temperatures were found to be close to the expected temperatures. The vertical variation of temperature in the 6-15 bar pressure range (about 100-150 km ( 62-93 miles) below the visible clouds) indicates the deep atmosphere is dryer than expected. This lapse rate of the temperature also shows the atmosphere is convective at these levels. The end of data transmission occurred at an atmospheric pressure of about 23 bars and a temperature of 305 degrees F (152 C). These initial results of the ASI experiment have various important implications. Our ideas about the abundance and distribution of water on Jupiter may need to be reconsidered. The ASI measurements will also increase our understanding of the escape of Jupiter's internal heat -- a power source for its dynamic atmosphere. In addition, becuase of the convective nature of the lower levels of the atmosphere, the deep atmosphere must be well mixed, and composition measurements obtained by other instruments must be representative of the deeper levels of Jupiter's atmosphere as well.

Visibility in the atmosphere is much greater than expected in the immediate vicinity of the Probe entry site.
Since we are seeing clouds when we look at Jupiter from afar, detecting and understanding the nature of its clouds can reveal a great deal about this cloud enshrouded world. The objective of the Nephelometer (NEP) instrument was to detect and characterize cloud particles in the immediate vicinity of the Probe as it descended to different levels. This objective was achieved by shining a laser beam across a short distance to a small mirror deployed just outside the Probe. By studying the scattered and transmitted light, cloud particles could be detected and characterized. This experiment has found several surprising initial results. No thick dense clouds were found, in contrast to expectations based on analysis of telescopic and flyby spacecraft observations of the planet and simple theoretical models. In fact only very small concentrations of cloud and haze materials were found along the entire descent trajectory. Only one well-defined distinct cloud structure was found, and this layer appears to correspond to a previously postulated ammonium hydrosulfide cloud layer. The observed cloud structure is very different than that expected by astronomers, and they may have to revise ideas of cloud formation on Jupiter. One important question which has arisen from these as well as other observations is whether the Probe's entry location is representative of most other regions of Jupiter.

Thick cloud detected some distance away from the Probe entry site.
The variation of the amount of sunlight with depth and the variation of infrared ("thermal") radiation with depth, which were measured by the Net Flux Radiometer (NFR) experiment, can aid in the detection of cloud layers, the understanding of the power sources for the winds, and the detection of water vapor. On a clear day on Earth the brightness of the sky is quite bright in the direction of the sun and less bright in other directions. On a very cloudy day, the sky is nearly equally bright in all directions and determining the direction to the sun can be difficult. The Net Flux Radiometer instrument has used this effect along with the Probe's spin to locate an important cloud layer on Jupiter. Large variations in the brightness of the sky in different directions were noticed until an abrupt drop-off in the variation occurred below a pressure level of 0.6 bars, indicating a cloud layer which is most likely the previously postulated ammonia cloud layer-- believed to correspond to the uppermost cloud layer on Jupiter. No other significant cloud layers were found-- in particular the tenuous cloud layer detected by the NEP was not seen by this experiment. Moreover, the cloud seen by the NFR was not seen by NEP. This apparent contradiction can be understood by noting that the NEP measures cloud particles in the immediate vicinity of the Probe while the NFR measures clouds over a long distance. The simplest explanation for the results from these two cloud-detecting experiments appears to be that the clouds are patchy and that the Probe went through a relatively clear area. Heating of the NFR's cloud layer by heat escaping from the interior of Jupiter appears to also be occurring and may affect the nature of Jupiter's winds. Once again the cloud structure at the Probe entry site appears to be very different than expected for Jupiter as a whole. More analysis will be required before it is known whether models of cloud formation on Jupiter must be revised.

Strong winds persist to great depth:
Previous studies of Jupiter's cloud motions show that it has a very unusual wind system consisting of strong alternating east-west jetstreams quite unlike Earth's wind systems. The origin of Jupiter's winds is not clear, in large part due to our inability to see below the uppermost clouds in the atmosphere. The Doppler Wind Experiment used changes in the frequency of the radio signal from the Probe due to its motion (called the Doppler effect, this phenomenon can also be observed by listening to the changing pitch of a train whistle as the train goes by) to evaluate the vertical variation of winds in the atmosphere, thus providing a key clue to understanding the origin of the winds. Initial results from this experiment indicate that the winds below the clouds are 540 km/hour (330 mph) and roughly independent of depth. These wind speeds are the same as the wind speeds at the cloud-tops as determined from observations by the Hubble Space Telescope. These results have profound implications. One implication of this result is that winds on Jupiter do not appear to be produced by heating due to sunlight or by heating due to condensation of water vapor-- two heat sources which power winds on Earth. A likely mechanism for powering the winds now appears to be the heat escaping from Jupiter's deep interior.

Lightning on Jupiter very different than on Earth:
Lightning activity in an atmosphere can provide evidence of thunderstorm-like activity which would be indicative of regions of strong atmospheric updrafts and regions of precipitation. Production of certain chemical species, including organic molecules such as those that are the building blocks of life on Earth, can also depend on the amount of lightning activity. On Earth we are accustomed to lightning discharges between the clouds and the ground. However, lightning discharges between clouds are quite common as well. On Jupiter, where no solid surface exists, lightning is expected to be of the cloud-to-cloud variety. The Lightning and Radio Emission Detector searched for optical flashes and radio waves emitted by lightning discharges. No optical lightning flashes were observed in the vicinity of the Galileo Probe. Many discharges were observed at radio frequencies. The form of the radio signals indicates discharges are far away (roughly one Earth diameter away), and the lightning bolts are much stronger than Earth's. Radio wave intensity suggests the lightning activity is 3-10 times less than on Earth. Therefore, the initial analysis implies that lightning activity on Jupiter is very different than on Earth. The unusual form of the radio signals from lightning indicates more work on lightning discharge physics on Jupiter is needed. Ideas of water cloud distribution and heat escape from Jupiter may need revision.

Several key elements and compounds appear to be less abundant than expected:
For the reasons stated earlier, the accurate determination of Jupiter's composition can not only have implications for understanding Jupiter today but can also provide clues to the planetary formation and evolution process. The Neutral Mass Spectrometer (NMS) experiment's objective was to accurately determine the composition of the atmosphere. Initial results indicate the atmosphere has less than expected amounts of water vapor, which is a measure of the amount of the element oxygen. The atmosphere appears to have less than expected carbon in the form of methane gas. Also, slightly less sulfur than expected in the form of hydrogen sulfide appears to be present. Puzzlingly, the amount of the element nitrogen in the form of ammonia gas appears to be greater than expected. Noble ("inert") gas concentrations differ from expectations as well, including a notable depletion of the gas Neon. Little evidence for organic molecules was found. The Helium Abundance Detector experiment very accurately measured the abundance of Helium. The abundance of Helium was found to be significantly less than that in the Sun. Moreover, this amount is less than that expected for Jupiter based on previous observations. These results suggest our ideas about the formation and evolution of Jupiter must be revised. In particular, fractionation or "raining out" of Helium appears to have occurred in the atmosphere. The role of local meteorology in producing the dryer atmosphere must also be considered.

The Probe apparently entered a rather special location on a quite nonuniform world.
Groundbased telescopic observations were undertaken to determine the appearance of the Galileo Probe entry site (6.5 degrees North Latitude, 4.5 degrees West Longitude) at the time of entry and to determine the variability of this location on the planet. An important goal of these observations was to place the Galileo Probe results in the context of Jupiter as a whole. Initial results indicate the entry site is a quite variable region. The Probe entered Jupiter near the edge of a so-called infrared "hot spot" believed to be a region of reduced clouds. The results imply that the Galileo Probe apparently entered a rather unique location on this highly heterogeneous planet. This uniqueness may account for some of the apparent surprises found by the Probe during its descent. However, the major findings regarding composition and winds are unlikely to be affected by the unique location of the probe entry.

There are more results to come!
This summary of scientific findings from the Galileo Probe mission is the result of a quick analysis of the returned data and thus these results should be viewed as preliminary. The process of converting the data returned from the Galileo Probe into useful physical scientific measurements requires time and careful analysis. Much additional work needs to be done in the coming months and years to refine and expand upon this initial work. Scientific and popular publications with formal reports on the Galileo Probe Mission results will be produced. Additional information on the status of the data reduction and the latest results will be listed on the World Wide Web at The Galileo Probe Project has been and is managed by NASA's Ames Research Center, Mountain View, CA. Hughes Space and Communications built the Galileo Probe spacecraft. NASA's Jet Propulsion Laboratory, Pasadena, CA built the Galileo Orbiter spacecraft and manages the overall mission.

(The above narratives and pictures are copied from NASA's JPL Galileo Site.}








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