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After surviving the most difficult atmospheric entry ever attempted, the Galileo probe
by Richard E. Young, NASA Ames Research Center, Mountain View, Calif.
Arriving at Jupiter on December 7, 1995, the Galileo spacecraft, which consisted of an atmospheric entry probe and a planetary orbiter, conducted one of the most historic planetary exploration missions to date. At 14:04 PST, the probe plunged into Jupi ter's atmosphere, accomplishing the first direct sampling of one of the outer giant planets and surviving the most difficult atmospheric entry ever attempted. On the same date, the orbiter became the first ever to be placed in orbit about Jupiter, and wil l return data about the jovian system for almost 4 years.
To appreciate the achievement and success of the Galileo probe, one has to know a few facts about the entry into the jovian atmosphere. The probe not only entered at 106,000 mph (about 50 times faster than a high-powered rifle bullet) but also had to enter on a flight path that was 8.5E inclined to the horizontal. An error of 1.51E too shallow and the probe would skip out of the atmosphere; 1.5E too steep and the probe would be destroyed.
Once released from the orbiter 50 million miles from Jupiter, the probe's trajectory could not be altered in any way, nor was any communication possible with the probe until its encounter with Jupiter. Once in the atmosphere, t he probe slowed from 106,000 mph to less than 1000 mph in under 2 min. This deceleration caused the probe to experience 228 times the gravitational acceleration on Earth, or 228 g. A jet fighter pilot experiences the same sort of forces while making a hig h-speed turn, but only about 6 g. During the maximum deceleration, the 750 lb probe "weighed" as much as an empty jumbo jet airplane. Due to the high-speed entry, a shock layer, set up in Jupiter's atmosphere, was located about 1 in from the nose of the p robe. Even though the shock layer's temperature reached 28,000EF (about 2.5 times the Sun's surface temperature), the probe survived because of its protective heat shield.
The probe entered the jovian atmosphere at 6.54E N latitude, (just north of the equator). This near equatorial entry was dictated by the probe having to take advantage of the rotational velocity of the jovian atmosphere (which is greatest near the equator) in order to minimize the relative velocity between the atmosphere and probe during the high-speed entry.
Why did we want to go to such great lengths to explore Jupiter? Because we want to understand how the Earth formed and how it has evolved since its formation. In order to accomplish that, we have to understand how planets form in general, and once for med, the processes that affect the way planets change over time. Jupiter influences all the other planets in one form or another, being the largest planet in the solar system. It has more than twice the mass of all the other planets combined. Over 1000 Ea rths could fit inside Jupiter, and in fact, about 2 Earths could fit into Jupiter's famous Great Spot alone. Without understanding the processes that formed Jupiter and affected its evolution, it would be impossible to understand the formation and evoluti on of the Solar System.
What did the Galileo probe learn about Jupiter? First, it found that Jupiter's main constituents, hydrogen and helium (the two most abundant elements in the universe) are very nearly in the same proportion as in the Sun when it was first forming out of a gas and dust cloud. Jupiter, by mass, consists of nearly 75% hydrogen and 24% helium. In other words, Jupiter is made almost entirely of near solar proportions of hydrogen and helium.
Although material constituting Jupiter other than hydrogen and helium is only a very small fraction of the total, it provides important clues as to how Jupiter formed as a planet, and the processes that have affected it since i ts formation. The abundances of heavier elements such as carbon, sulfur, nitrogen, and oxygen, as well as noble gases such as neon and xenon and their isotopic ratios, can be used to trace the evolutionary history of Jupiter and compare it with similar in formation regarding the inner planets. Galileo found that carbon, nitrogen, and sulfur have greater relative abundances to hydrogen on Jupiter than they do in the Sun, by a factor of approximately 2 to 3.
However, oxygen (in the form of water, since Jupiter is much cooler than the Sun), is apparently scarce on Jupiter, at least where the Galileo probe entered. It was anticipated that the Galileo probe would encounter thick water ice clouds, and water would be an important constituent in the atmosphere as it is on Earth. There are observations from the Galileo Orbiter that indicate water does exist in Jupiter's atmosphere, and the probe entry site was dry in the same way that des ert regions are dry on the Earth.
The greater abundances of carbon and other heavier elements on Jupiter as compared to the Sun imply that comets and other small bodies have impacted Jupiter over the age of the Solar System (about 4.6 billion years) and deposit ed extra material. The relevance of such findings to understanding the Earth have to do with the fact that impacts may have played a role for the Earth, bringing in water that we now see in the oceans, and other gases that are now in our atmosphere. Hence , understanding the role of small impacting bodies in the evolution and formation of planets is important, and the information we now have about Jupiter may force new ways of thinking about it.
One of the main objectives of the Galileo probe was to measure winds in the jovian atmosphere. From pictures of Jupiter taken by spacecraft and by Earth-based telescopes, it was known that at cloud levels, winds blew mostly in the east-west direction and reached speeds of greater than 200 mph. It was not known, however, whether these winds extended to great depths below the clouds. The jet streams that occur in the Earth's atmosphere generally have far smaller speeds. The Galil eo probe encountered winds exceeding 400 mph that extend deep below the visible clouds on Jupiter. For reference, the winds in a tornado are typically between 200 and 300 mph, and the highest sustained winds ever recorded in a hurricane were 198 mph. It i s not known exactly what produces the winds on Jupiter, but the probe provided evidence that energy escaping from Jupiter's very hot interior is the ultimate energy source.
The Galileo probe also measured the way temperature and pressure are related in the jovian atmosphere. Temperature and pressure increase with depth. The probe radioed signals to the orbiter to a depth where the pressure was abo ut 22 times Earth's sea level surface pressure. The temperature at that point was about 305E F. The probe first started taking direct measurements where the pressure was about 0.4 that of sea level surface pressure, and the temperature was a chilly -140E F. If the probe had been able to survive to reach the very center of Jupiter (no probe could have done this), temperatures in the range of 60,000E F and pressures millions of times greater than sea level surface pressure would have been measured.
Before the Galileo probe mission, we expected to find three cloud layers in the jovian atmosphere. The probe started direct measurements near the bottom of the top cloud layer, and detected the mostly ammonia ice particles by measuring how sunlight diminished as the probe went through this top cloud. The probe also detected a second tenuous cloud, probably consisting of ammonium hydrosulfide ice particles. But underneath this, no thick water clouds were detected in contrast t o what was anticipated before the mission.
The Voyager spacecraft, which had flown by Jupiter, detected lightning as it took pictures of the night side of Jupiter. Most of this lightning occurred near 45 to 50E N latitude. The Galileo probe was equipped with optical sen
sors to detect nearby lightning flashes; however, no lightning was detected optically. The probe did pick up radio signals emitted by distant lightning bolts. The conclusion is that Jupiter has less-frequent lightning than the Earth on a per unit area bas
is, but individual lightning bolts are approximately 100 times more energetic than on Earth.
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