January 3 - 9, 2000
Galileo Returns to Europa
Galileo returns to Europa this week in its first post-GEM (Galileo Europa Mission) encounter. The first of the Galileo Millennium Mission, the Europa flyby occurs on Galileo's 27th orbit around Jupiter since arrival at the gas giant in December 1995. The Galileo Europa Mission, a two-year
extension of Galileo's primary mission, began in December 1997 and was comprised of 8 flybys of Europa, 4 flybys of Callisto and 2 flybys of Io. The last flyby of Io occurred on November 25, 1999. Galileo's time at Jupiter has provided a wealth of science information about Jupiter and its many moons.
The spacecraft is beginning to show the effects of 10 years in space and dozens of passages through the radiation belts near Jupiter. The spacecraft is operating and returning valuable data, and the proposed Galileo Millennium Mission will continue Galileo's work at Jupiter into the next millennium.
Encounter commands for the Europa flyby began to execute late Saturday night Pacific time. The start of the encounter brought with it the resumption of the Fields and Particles instruments' survey of Jupiter's magnetosphere. This survey has been performed from orbit to orbit and has allowed scientists to study the long term variations of the inner portions of Jupiter's magnetosphere. Encounter activities continue through Friday evening, January 7.
There are a relatively small number of observations associated with this encounter. This is due to a number of reasons, including having been allocated less antenna time from the Deep Space Network than in the previous two orbits, a decrease in Galileo's orbit period (allowing for less time to prepare commands for the encounter), a desire to return data recorded during the November encounter with Io, and the numerous activities needed to ensure that Galileo's tour will continue at least through another encounter with Io in February 2000.
The Europa flyby occurs on Monday, January 3 at 10:00 am PST. Radio signals indicating that the flyby has occurred, however, won't be received on Earth until 39 minutes later, or 10:39 am PST. The time difference is due to the fact that the spacecraft is approximately 697 million kilometers (433 million miles) from Earth and it will take radio signals just under 39 minutes to travel between the spacecraft and Earth. The spacecraft flies over Europa's surface at an altitude of 343 kilometers (213 miles). That is about the same altitude at which the Space Shuttle orbits around the Earth! During the week, two other flyby events are notable. At 7:33 pm PST (8:22 pm PST Earth receive) on Monday night, the spacecraft flies past Jupiter's cloud tops at a distance of 4.8 Jupiter radii (343,000 kilometers or 213,000 miles). Three hours and 23 minutes later, the spacecraft makes
its closest approach to Io, at an altitude of 214,000 kilometers (133,000 miles) above Io's surface.
The first science activity of the encounter is conducted by the Radio Science team here on Earth. For 45 minutes on either side of the Europa flyby, radio scientists will monitor Galileo's radio signal. The Europa flyby will take the spacecraft behind Europa as seen from Earth. As that occurs, Galileo's radio signal will pass through Europa's tenuous atmosphere, will be weakened and slightly refracted until it is completely blocked by Europa. Signal strength and integrity are reestablished as Galileo emerges from behind Europa. The measurements made by the Radio Science team will allow them to determine how the density of electrons in Europa's atmosphere changes with altitude from Europa's surface.
Starting 30 minutes prior to closest approach to Europa, the Fields and Particles instruments begin recording high resolution measurements of the plasma, dust, and electric and magnetic fields surrounding Europa. The Fields and Particles instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument. The recording lasts a total of 60 minutes and its primary purpose is to determine if Europa has an induced magnetic field. Such a field signature would indicate the presence of a conducting layer inside Europa, another piece of circumstantial evidence that liquid water is present beneath Europa's surface.
Shortly following the Europa flyby, the spacecraft's Solid-State Imaging camera (SSI) snaps a series of images of Europa. The images are designed to help fulfill three different objectives. The first is to validate some models that have been developed to explain the formation of sharp edge ridges on Europa. The second is to look at the ejecta surrounding a multi-ring impact feature named Callanish in hopes of determining if the impact penetrated into Europa's suspected sub-surface ocean. The third
objective is to look at some blotchy-looking, or mottled, terrain in hopes of seeing evidence of ice flows.
Toward the end of Monday, SSI turns its attention to three of Jupiter's smaller moons. SSI snaps an image of each of Amalthea, Thebe, and Metis. These images will provide the best resolution views of these moons, almost a factor of 2 better than the best previous images in the case of Amalthea and Metis. This increased resolution should aid scientists significantly in evaluating the shape and surface conditions of these smaller moons. Two more observations are performed on Tuesday, January 4. These take a look at Io. The first is performed by the Near-Infrared Mapping Spectrometer (NIMS) and captures a near-global observation of the hemisphere of Io that contains the volcanic region of Loki. SSI also snaps a color series of images of this hemisphere.
Outbound from Jupiter, the Extreme Ultraviolet Spectrometer (EUV) makes the final observation of the encounter. Scheduled to last just short of 44 hours starting on Wednesday, January 5, the observation is performed in near-realtime, which means that the EUV data is not stored on the spacecraft's tape recorder, but rather it is directly transmitted to Earth after processing and packaging. The observation takes a look at the Io torus, a doughnut-shaped region with its inner edge bounded by Io's orbit. It is a region of intense plasma and radiation activity, in which there are strong magnetic and electric fields. Similar observations have been performed during Galileo's previous encounters, and the data set will allow scientists to examine long term variations in the torus' size and shape, with the goal of understanding energy transfer between the torus and the overall Jovian magnetosphere.
The return of data stored on the spacecraft's onboard tape recorder is initiated on Wednesday, January 5. The data played back this week were acquired during Galileo's November flyby of Io. The Fields and Particles instruments return portions of a 3-hour high resolution recording of the Io plasma torus. The recording gathered data from 6 Jupiter radii (429,000 kilometers or 267,000 miles) above Jupiter's cloud tops down to an altitude of 5 Jupiter radii (357,000 kilometers or 222,000 miles), making it the third deepest torus recording of Galileo's entire mission to date. The data acquired during the recording will be used to understand the structure and dynamics of plasma, dust, and electric and magnetic fields in the torus region. The data will also be important for understanding the overall dynamics of the Jovian magnetosphere.
Data playback is interrupted twice this week. On Wednesday, the spacecraft performs a standard test on its attitude control gyroscopes, and a test to slew its scan platform. On Friday, the spacecraft performs a flight path adjustment, if necessary.