Galileo nears close encounter with Ganymede this week
NASA/JPL STATUS REPORT
Posted: May 16, 2000
Galileo resumes normal operations late Tuesday night. For a few weeks prior to Tuesday, the orbital motion of the Earth and Jupiter brought the Sun between the two, creating radio interference and making reliable communications between the spacecraft and Earth impossible. This geometric situation is known as superior solar conjunction. As the spacecraft emerges from behind the Sun, the Sun's effect on its radio signal will gradually decrease to the point where communications can again be trusted. If this situation sounds familiar, it is because it has occurred approximately every 13 months while the spacecraft has been in orbit around Jupiter.
The Ganymede flyby is scheduled to occur Saturday morning at 3:10 a.m. PDT. Radio signals indicating that the flyby has occurred, however, won't reach Earth until 50 minutes later, or at 4:00 a.m. PDT, which is denoted as Earth Received Time (ERT). The time difference is caused by the fact that Earth is approximately 6 astronomical units (898 million kilometers, or 558 million miles; 1 astronomical unit is equal to the average distance between the Earth and the Sun) from the spacecraft and it will take radio signals just under 50 minutes to travel between the two. Radio signals travel at a speed of 300,000 kilometers per second (186,000 miles per second), just under 670 million miles per hour.
This is Galileo's fifth return to Ganymede, and its second closest. The spacecraft's previous close flyby of Ganymede occurred in May 1997, but its closest was in September 1996 with an altitude of 262 kilometers (163 miles). During the upcoming flyby, the spacecraft will pass within 808 kilometers (502 miles) of Ganymede's surface. That is about the same as the distance between San Diego and San Francisco.
Events leading up to this weekend's high activity period are geared primarily toward spacecraft preparation. Late Tuesday night, the spacecraft will perform standard maintenance on its propulsion systems. On Wednesday, the spacecraft will perform standard maintenance on its onboard tape recorder. The tape recorder is a key component that allows Galileo to store its valuable science data for later transmission to Earth. Finally, on Friday, the spacecraft will perform a small flight path adjustment, if deemed necessary by flight controllers.
The first science activities also start Thursday night when the Fields and Particles instruments begin a month-long survey of Jupiter's magnetosphere. In most orbits, this survey is performed only in the inner portions of the Jovian magnetosphere, in order to study its variation and to provide context for any recorded high-resolution observations. The current survey, however, will span the entire range from the inner to outer regions of the magnetosphere, and the transition from inside Jupiter's vast magnetic bubble out into the solar wind. The Fields and Particles instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.
Late Friday night, Galileo's radio signal will begin to pass through Jupiter's atmosphere on its way to Earth. Within minutes, the spacecraft will pass behind Jupiter as seen from the Earth, completely blocking its radio signal from reaching Earth. About 40 minutes later, the spacecraft will emerge from behind Jupiter and communications are restored. During this passage, Galileo's radio signal is weakened and refracted by Jupiter's atmosphere. The changes in the signal will be measured by radio scientists here on Earth, which will allow them to gain more knowledge of the structure and electron density of Jupiter's upper atmosphere.