To Galileo Mission Science Contents PageGalileo's December 8, 1990, flyby of Earth (see photo) was an outstanding success. The spacecraft's instruments gathered a good deal of information about our planet, its Moon (see photo), and their environment in space. Galileo also received a gravity assist that will help send it on its way to Jupiter.
"Everything has worked superbly, both the spacecraft and the team," noted Project Manager Bill O'Neil. In fact, delivery accuracy was better than 99 percent in aiming point and time of closest approach for both the last two TCMs--TCM 7 and TCM 8. The targeted altitude at Galileo's closest approach to Earth was 952 kilometers (590 miles), and the actual altitude was 960 kilometers (595 miles). The spacecraft was expected to arrive at that point at 12:34:34:00 Pacific Standard Time (PST) and arrived within a half-second -- a highly accurate pinpointing.
This gravity assist increased Galileo's speed around the Sun by about 5.2 kilometers per second (or 11,600 miles per hour) and substantially redirected Galileo as required for its flybys of the asteroid Gaspra in October 1991 and Earth in 1992; the second gravity assist from the Earth in 1992 will add an additional 3.71 kilometers per second (8280 miles per hour) to its speed.
The solar wind is a continuous stream of ionized gas (called a plasma) blowing radially outwards from the Sun. It travels at speeds between 200 and 800 kilometers per second, that is, from 450,000 up to almost 2,000,000 miles per hour. This high-speed wind carries the Sun's magnetic field outwards and into the solar system. At the Earth and also at Jupiter, the solar wind is deflected by the planet's magnetic field, forming a magnetosphere. The Earth's magnetosphere protects the planet from the solar wind, deflecting it around the Earth instead of impinging onto the planet's atmosphere. On the Earth's day side, a shock wave forms, much like the shock wave caused by a supersonic aircraft. The night side of the Earth's field is dragged back to form a long tail (over 1000 times longer than the planet's radius). The magnetized plasma that sweeps by the Earth causes magnetic storms and aurorae (sometimes called the northern, and southern, lights).
The plasma that comprises the solar wind is extremely tenuous, with a typical density of only a few particles per cubic centimeter. Plasmas of this sort are difficult to study on Earth. As Galileo's Plasma Detector Principal Investigator Dr. Louis Frank pointed out, though, excellent "laboratories" to study plasmas do exist--in the space near the planet. The Earth's magnetosphere has been studied by spacecraft for 25 years, and although much has been learned, it still holds many of its secrets. The Galileo flyby allowed us to use one of the most sophisticated scientific spacecraft to help further unravel these mysteries, and provided information that will help in the study of Jupiter's magnetosphere.
The Earth's bow shock slows the solar wind down prior to its impinging on the Earth's magnetosphere. The region between the bow shock and the magnetosphere is called the magnetosheath, and it is this region that the Galileo spacecraft first encountered in its December flyby. The Galileo spacecraft approached the Earth from the night side, where the solar wind sweeps the magnetosphere back to form a long tail, called the geotail, surrounded by the magnetosheath. Galileo flew along the magnetosheath and entered the magnetotail about 560,000 kilometers (348,000 miles) behind the Earth. Fortunately for the scientists involved, the Galileo-Earth encounter took place when the magnetosphere was in a very dynamic state and so, once inside the geotail, the spacecraft detected a number of geomagnetic substorms.
Powered by disturbances in the Earth's field, charged particles traveling along magnetic field lines are scattered into the atmosphere, creating the aurorae and also forming the intense regions of radiation above the Earth called the Van Allen radiation belts. Galileo detected energetic particles throughout the magnetosphere. The spacecraft's observations will help scientists understand the relationship of the aurorae to magnetospheric processes such as magnetic storms.
Another phenomenon observed by Galileo involved the detection of lightning. A lightning stroke emits a broad range of electromagnetic waves, including visible and radio frequencies. Galileo's Plasma Wave experiment can detect the radio frequency emissions as "whistlers," so named because they sound like a whistle decreasing in frequency with time. The reason for this effect is that higher frequencies travel fastest, thus reaching the spacecraft first, followed by the lower frequencies. The result, when played back at corresponding audible frequencies, sounds much like a falling artillery shell. At Galileo's Earth flyby, many lightning whistlers were clearly detected and can be compared with those Galileo may detect at Jupiter, as Voyager's Plasma Wave instrument did in 1979.
(100-mile-diameter) meteor that punched through to the Moon's mantle. One scientist described the impact "like a small state coming at you from space." Such a large impact would have caused large perturbations and may have reoriented the Moon's axis. Certainly the Moon's mass would have been redistributed.
The spacecraft's images also show evidence of a mare underlying the ejecta from the later impact that created the Orientale basin, pushing back the estimate of lunar volcanism by 100 million years, to 4 billion years ago.
While we have studied the Moon extensively in the past, Galileo discovered several new things. Galileo confirmed the earlier hypotheses from Apollo 15's analyses of altimetry and composition of the far side and the existence of an impact basin in the south polar region. Galileo's findings clarified characteristics of unexplored regions, improved our coordinates of lunar features, and enabled us to see entirely new parts of Moon. Much of this mapping covers "territory virtually unexplored by modern sensors," noted Project Scientist Torrence Johnson.
The dynamic movie of the Earth's rotation was taken over a 25-hour period as Galileo retreated from the Earth. Taken from a vantage point below the equator, the movie centers on the southern hemisphere, although the frames extend as far north as Florida and the Persian Gulf. The movie gives scientists a unique view of global weather patterns.
Observations of very high clouds in the mesosphere were made by the NIMS. Located above the stratosphere, the mesosphere is the coldest part of the atmosphere (130 K) and plays a significant role in the ozone chemistry cycle. Such high mesospheric clouds in northern and southern latitudes have been seen only since 1885. These clouds are indirectly caused by increasing amounts of methane released from industry. The methane, in turn, causes an increase in the warm air that flows into the polar regions. Coupled with natural springtime heating, the warm air increases the temperature dramatically and the ice pack melts more quickly, releasing more water vapor into the atmosphere. The water vapor, now in the form of mesospheric clouds, reacts with the ozone and breaks it apart, thereby depleting the ozone and creating an ozone hole.
Although these clouds are occasionally seen in September and October and are rarely seen as late as December, Galileo's instruments revealed several clouds. Such an anomaly may indicate a change in the ozone population. A fundamental problem in ozone depletion centers on the fact that since ozone helps shield the Earth's surface from the Sun's heat, a decrease in ozone yields an increase in polar heating. In turn, this creates more mesospheric clouds, destroying more ozone. To create good models of how ozone depletion occurs, scientists need highly accurate data regarding the amount of mesospheric water. This had always been a missing factor in the equation, but Galileo's observations supplied that vital information.
Due to the fact that Galileo encountered Venus in February 1990, Project Manager Bill O'Neil has characterized the spacecraft as "the first confirmed interplanetary visitor to Earth." Encountering the Earth as a planet for the first time enabled scientists to evaluate their methods of interpreting data with a critical eye by comparing what they could surmise from Galileo's data with what they actually knew.
Scientists were able to verify the Earth's mass, diameter, com-position, magnetic field, aurorae, and atmospheric composition.
One objective note of interest was the detection of a high oxygen level combined with a low carbon dioxide level. The proportions of these gases were not at equilibrium, indicating an additional driving factor--could there be life? Interestingly enough, Galileo's observations could not objectively prove that life exists on Earth. Indications were strong that life did exist--some unusual radio waves and the levels of oxygen, carbon dioxide, methane, and nitrous oxide all pointed to biological life. To be fair, Galileo's instruments were not constructed to detect civilization on the Earth. Because of Galileo's trajectory and distance at the Earth flyby, its imaging resolution was 1 kilometer (0.62 mile). A resolution of 100 meters (330 feet) would have been necessary to image human constructions. However, at its best, Galileo will achieve a 50-meter (165-foot) resolution on Jupiter's satellites.
Using the spacecraft's performance of the 7000 commands given and the 58 billion bits of data collected during the encounter, Project engineers now have a better understanding of how to finely control the scan platform and operate the science instruments. The Solid-State Imaging System, Galileo's "camera," was particularly tested, delivering 2,675 frames during the encounter.
"This encounter was a valuable experience of working with the instruments and the teams. We had seven days of detailed observations of the Earth and Moon," Project Scientist Torrence Johnson reflected. "This was just a small preview of what Galileo will do in 1995 on every one of its orbits of Jupiter."
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