. The UVS team currently believes all or most of this hydrogen is associated with the extension of the Earth's geocorona, rather than an aspect of the Moon's tenuous atmosphere.Galileo bid adieu to the Earth on December 8, 1992, at 7:09 a.m. PST. The spacecraft swept within 303.1 kilometers of the South Atlantic Ocean, its point of closest approach to the Earth. This, the last of three planetary gravity assists, added 3.7 kilometers per second to the spacecraft's speed in its solar orbit. In addition, the gravity assist changed the spacecraft's direction slightly, so its elliptical orbit will intersect the orbit of Jupiter, about 780 million kilometers from the Sun. The navigation, as in previous gravity assists, was impeccable. Galileo was within a kilometer of its intended path, and was just 0.1 second early.
The gravity assist was the essential item on the Project's agenda at this latest Earth flyby. However, with a constant eye to gleaning knowledge from every opportunity, the Project planned several scientific investigations during this flyby. Much of the analysis of these investigations continues, but initial results have been announced by the Near-Infrared Mapping Spectrometer (NIMS) and Ultraviolet Spectrometer (UVS) teams.
During the first Earth flyby, in December 1990, the NIMS instrument observed stratospheric clouds over Antarctica. Scientists now know that these clouds play a significant role in the complex sequence of events leading to the formation of an ozone hole over the South Pole. The extreme cold in the Antarctic stratosphere during the winter produces clouds composed of ice crystals rather than water droplets. On the surfaces of these ice crystals, reactions occur that result in the release of large amounts of ozone-destroying chlorine. The arrival of the Sun in the Antarctic spring triggers photochemical processes, resulting in the dramatic depletion of stratospheric ozone. As the ozone hole comes and goes each year, it appears to be growing larger. This trend follows the increase in chlorine released into the atmosphere through industrial activity.
At the time of the first Earth flyby, scientists were surprised to find stratospheric clouds (see photo). Galileo's NIMS instrument again found these clouds at the second Earth flyby. The clouds consist of large (20-micron) ice crystals and cover a large geographic area. What had been hoped to be an anomaly is now suspected to occur more frequently. Dr. Robert Carlson, Principal Investigator for NIMS, notes, "We have good evidence for these high stratospheric clouds, which may have been elusive before. These clouds may be a common phenomenon over Antarctica."
Along with the NIMS team, the UVS team, led by Principal Investigator Dr. Charles Hord of the University of Colorado, discovered a wealth of information about the Earth's corona, the geocorona.
The geocorona is the outermost part of the Earth's atmosphere and consists of hydrogen that has escaped (or "evaporated") from the Earth's mesosphere. This atomic hydrogen, when exposed to the Lyman-a radiation of the Sun, absorbs and re-emits this radiation. (Lyman-a radiation occurs at a wavelength of 121.5 nanometers.) This absorption process leads to a force on the hydrogen atom in the anti-Sun or geotail direction. The atomic hydrogen in the geocorona and geotail (see figure) scatters the Lyman-a radiation from the Sun, and some of this scattered radiation is then detected by Galileo's UVS.
During this Earth flyby, Galileo's UVS conducted 11 scans of the space around the Earth and Moon. The geocorona had previously been measured at its geotail to distances of 95,000 kilometers. Galileo detected a huge hydrogen corona bulge surrounding the Earth to approximately 400,000 kilometers at the geotail, nearly to the Moon's orbit and four times the thickness of the traditional geocorona model. In fact, Galileo actually detected atomic hydrogen near the Moon at a level of approximately 1 atom/cm. The UVS team currently believes all or most of this hydrogen is associated with the extension of the Earth's geocorona, rather than an aspect of the Moon's tenuous atmosphere.
The NIMS and UVS will continue their studies of planetary atmospheres when Galileo reaches Jupiter in December 1995.
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