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This Week on Galileo?
Today on Galileo
December 27, 2000
DOY 2000/362

Galileo begins the 29th encounter of its mission since arriving at Jupiter in December, 1995

Today, Galileo begins the 29th encounter of its mission since arriving at Jupiter in December, 1995. This flyby is only the fourth, however, of the Galileo Millennium Mission, a second extension of Galileo's exploration of the Jovian system. The first extension, the Galileo Europa Mission, which ended in December, 1999, followed the primary mission, which concluded in December, 1997.

This passage of the spacecraft through the Jupiter system features a close flyby of Ganymede, which will occur just past midnight tonight PST [see Note 1]. Radio signals indicating that the flyby has occurred, however, will travel for 35 minutes before reaching Earth. It takes that long to hear from Galileo, despite the fact that the spacecraft's signals travel at the speed of light, or 300,000 kilometers per second (186,000 miles per second, or 670 million miles per hour). You see, Jupiter is currently at approximately 4.2 astronomical units from Earth (that is, 630 million kilometers, or 390 million miles; one astronomical unit is equal to the average distance between the Earth and the Sun).

Galileo's science instruments will observe a host of phenomena in the Jupiter system over the next few days. Their data will be complemented by observations performed since October by the Cassini spacecraft. Cassini is passing closest to Jupiter on 30 December 2000 at just past 2 a.m. PST. Cassini will be passing Jupiter at a distance of nearly 10 million kilometers (6 million miles), whereas Galileo is passing within 550,000 kilometers (342,000 miles). Data from Cassini's instruments will provide contextual information for Galileo's more detailed observations. The science investigations for this dual-spacecraft campaign include studies of: interactions of the solar wind with the Jovian magnetosphere, Jupiter atmospheric dynamics, structure of Jupiter's rings, Io, and Jupiter's aurora as influenced by the solar wind. In addition, Galileo will be taking advantage of its flyby of Ganymede to continue its observations of the surface and magnetosphere of the largest of Jupiter's moons.

First on the observation schedule for this encounter is the continuation of the 14-week survey being performed by the Fields and Particles instruments on Galileo. Comparison with data from the Cassini spacecraft will yield information on the interaction between the solar wind and Jupiter's magnetosphere. In addition, data taken near Ganymede closest approach will provide background information for high rate measurements that are planned to be stored on Galileo's onboard tape recorder. Also, data taken near closest approach to Jupiter, which occurs late tomorrow, will provide scientists will information on the interaction between the Io torus and the Jovian magnetosphere. The torus is a ring-shaped region of intense plasma and radiation activity with its inner edge bounded by Io's orbit. The activity in the Io torus is maintained by Jupiter's strong electric and magnetic fields and Io's constant supply of volcanic particles.

Late today, Galileo flies behind Jupiter as seen from Earth. It will take the spacecraft just short of four hours to emerge from behind Jupiter. During that time, the spacecraft's radio signal will be weakened and refracted by Jupiter's atmosphere. The changes in radio signal will be measured by radio scientists here on Earth, which will allow them to learn more about the structure of Jupiter's upper atmosphere. This experiment has been repeated during many of Galileo's encounters. Each time, however, the spacecraft flies behind a slightly different part of Jupiter, giving scientists information on another sample of Jupiter's immense atmosphere.

Come back tomorrow for more flyby news!

Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 35 minutes to travel between the spacecraft and Earth.

 
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