Encounter activities conclude this evening, just after 6:00 pm, Pacific Time. Shortly thereafter, the spacecraft begins processing and transmission to Earth pictures and science information acquired and stored on Galileo's onboard tape recorder during the last few days. The first items on the processing and transmission schedule are the pictures of Io's north and south poles taken on Saturday. Early in the morning, flight team members transmit to the spacecraft the set of computer commands that will control most of its activity until the next encounter period in late May. Today's flight path takes the spacecraft past the closest approach point to the last Galilean satellite for this orbit, Callisto, at 8:20 pm and a distance of 205,000 kilometers (128,000 miles).
The final observation of Europa for this encounter is performed this morning by the Near Infrared Mapping Spectrometer (NIMS). This global observation is the last in a set of three distant observations performed by NIMS this orbit. Io returns to the observing schedule today with a joint observation performed by the Ultraviolet Spectrometer and Extreme Ultraviolet Spectrometer as they gather information on Io's plasma torus. The plasma torus is the donut shaped cloud of neutral and ionized gases that is found along Io's orbit. It is believed to be sustained by all of the material that is ejected from Io by way of volcanic eruptions. This observation will allow scientists to keep tabs on and continue to map the activity levels within the torus.
Two observations of Jupiter are performed during today's science gathering. Both are performed in 'real-time', indicating that the data is almost immediately packaged and transmitted to Earth and not stored on the spacecraft's tape recorder for later processing. Both are performed by the NIMS instrument. The first looks at aurora near Jupiter's north pole while the second gathers more information on the makeup and temperatures of Jupiter's atmosphere.
A look at Callisto rounds out the remote sensing observation schedule for this encounter period. Performed jointly by the NIMS instrument and the Ultraviolet Spectrometer (UVS), the global observation will provide more information on the surface properties of this moon. A similar observation will be performed on the Callisto orbit in May 1999. The end of the encounter period also concludes the fields and particles instruments' survey of Jupiter's magnetic and electric field environment. This survey is planned to be repeated each and every encounter and will provide an orbit-to-orbit record of the activity levels within this environment.
Come back tomorrow for the return of This Week on Galileo as the Galileo spacecraft continues its exciting journey of discovery!
Sunday, March 29, 1998
The spacecraft continues to make its way through the Jupiter system for the 14th time since June 1996. Science information has been collected on 11 of the previous 13 orbits, 11 of which occurred during the Galileo primary mission. Today's observation schedule is heavily focused on Jupiter's moon Europa as the spacecraft flies within 1,645 kilometers (1022 miles) of its surface at 5:21 am, Pacific Time. Later in the day, at 4:09 pm, the spacecraft will pass the point of closest approach to Ganymede at a distance from the surface of 918,000 kilometers (571,000 miles).
Many different areas of scientific interest on Europa are covered by today's observing activities. The radio science team, for 20 hours surrounding the point of closest approach, measures changes in Galileo's radio frequency due to Europa's gravitational pull on the spacecraft. By using the Doppler effect, the radio science team will be able to use these measurements to refine the map of the gravity field produced by Europa. Also tied to closest approach, the fields and particles instruments will perform a high time-resolution observation, for just under an hour, of Jupiter's magnetic and electric field environment in the region of space near Europa. This will add to the scientific knowledge of the interaction between Europa and the magnetosphere.
The photopolarimeter radiometer performs three observations of Europa today. Together they are designed to determine the temperature variation across Europa's surface and how it relates to different surface ages, how the surface might have been put together, or the different materials on the surface. Two of these observations are performed at better resolutions than was possible during Galileo's primary mission. The resolution of the third is comparable. Three other global scale observations are performed during the day -- one by the spacecraft's camera, or solid-state imaging (SSI) subsystem, and two by the Near Infrared Mapping Spectrometer (NIMS).
Among the specific regions of Europa that Galileo looks at today, we start with a region characterized by rifts or crevasses in Europa's surface observed by NIMS together with the Ultraviolet Spectrometer (UVS). This is followed by a couple of observations of the Mannann'an crater region performed by the SSI instrument. The pair of images is designed to provide stereo coverage of this region. A region of dark spots is also observed twice by the camera. These two images will also result in stereo coverage of this region, which also lies within regional data obtained during its orbit in November 1997. In addition, a transition from a dark spot region to a region of pull-apart wedge shapes is observed by NIMS and is accompanied by the UVS instrument.
Later in the day, a region of triple-bands is observed by all three instruments (SSI, NIMS, UVS). Remember that triple-bands are believed to be formed when Europa's surface cracks, material upwells from below the surface and spills to both sides of the central crack. Coverage of this region is planned to be obtained during the the Europa orbit in February 1999. A transition region between bright plains, pull-apart wedges and dark material is also observed by all three instruments.
A high-resolution picture of the Tyre Macula region is obtained by the SSI instrument. This region was observed at a lower resolution in the primary mission's Ganymede orbit in April 1997, and an observation is planned at even higher resolution during the Europa encounter this coming May. The SSI instrument also performs a photometry observation of the surface of Europa. These photometric measurements will tell us how intensely light is reflected from the surface and provide more information on its makeup.
Two non-Europa observations are performed today. The first is a global color image of Ganymede performed by the SSI instrument. This image will provide data on the radius, shape, color, and photometry of Ganymede as well as the mobility of frost on its surface. Also observed by SSI is Io while eclipsed from the sun by Jupiter. These eclipse images are considered one of the best ways to discover new lava flows, monitor lava temperatures, and study the interaction between volcanic plumes, Io's atmosphere and Jupiter's magnetosphere.
Saturday, March 28, 1998
The second science encounter of the Galileo Europa Mission, the follow on to Galileo's primary mission, started today when encounter-related computer commands began executing at 5 am, Pacific Time. For the next three days, science instruments onboard the Galileo spacecraft will gather more information on the Jupiter system and its intriguing moons. The information will be stored on the onboard tape recorder for later playback. Once again, the bulk of the science observations will focus on Jupiter's moon Europa, but Jupiter, Jupiter's magnetic and electric field environment, and each of the other three Galilean moons: Io, Ganymede and Callisto, are also observed.
` This encounter is being performed without the gyroscopes. That's because of the recent discovery of further degradation of the anomalous behavior of the spacecraft's attitude control subsystem. During data collection by the remote sensing instruments (camera, spectrometers and radiometer), the gyroscopes are used to improve the stability of instrument pointing. They detect small wobbling of the spacecraft and compensate to keep the instruments steady. Without the gyros, no motion compensation is available. This is expected to result in a small degradation in the quality of the data that is gathered.
As the spacecraft flies through the Jupiter system, it passes points of closest approach to each of the bodies of the system. Today, the spacecraft passes within 250,000 kilometers (155,000 miles) of Io and 632,000 kilometers (393,000 miles) of Jupiter at 8:48 pm and 11:59 pm, respectively, both in Pacific Time.
The fields and particles instruments kick off the science data gathering activities by initiating a low-rate survey (information is collected at a rate of only a few bits per second) of Jupiter's magnetic and electric field environment, also known as the magnetosphere. Because of the low rate, this data can be packaged and transmitted to Earth almost immediately. This type of observation is said to be performed in 'real-time'. The survey is performed continuously for the duration of the encounter and the information obtained will provide scientists with a context for data gathered at a higher time resolution (hundreds of bits per second) later in the encounter. The information will also add to the existing record of orbit-to-orbit of activity levels within the inner, most active, portions of the magnetosphere.
The first remote sensing observation of the encounter is performed by the Ultraviolet Spectrometer (UVS). This observation of Europa will provide data on Europa's atmosphere. Also obtained on previous orbits, this information allows scientists to monitor changes in the characteristics of the atmosphere from orbit to orbit. Large changes in these characteristics could be indicative of geologic activity on the surface of Europa. A small observation of Jupiter is performed next by the Near Infrared Mapping Spectrometer (NIMS). The information gathered by this observation is designed to study long term changes in the composition and temperature of Jupiter's atmosphere.
As the spacecraft approaches its point of closest approach to Io for this orbit, the science instruments turn their attention to this fiery moon. The UVS instrument performs an atmospheric monitoring observation similar to the one performed earlier on Europa. This is followed by a series of color pictures taken by the camera of Io's north and south pole regions. These pictures will improve the detail of these regions by providing resolutions of 3 kilometers (1.8 miles) per picture element. The best color resolution obtained during Galileo's primary mission was 10 kilometers (6.2 miles) per picture element. Monochrome images as good as 2.5 kilometers (1.5 miles) per picture element were obtained in the primary mission, but the color in these new images will be critical to identifying surface materials. The pictures will also be useful to scientists as they plan observations for the return to Io at the end of the Galileo Europa Mission.
The photopolarimeter radiometer (PPR) contributes further to the Io data set for this orbit with an observation designed to characterize the different temperatures on Io's surface. This is followed by a joint observation of Io's surface performed by the NIMS instrument and the UVS instrument. The observation is designed to keep track of any changes due to volcanic activity.
It is now 2 days to the Galileo spacecraft's next encounter with Europa.
A special Countdown to Europa home page is now available on the Galileo Home Page.
Launched in October 1989, Galileo entered orbit around Jupiter in December 1995, and completed its primary 2 year orbital tour around the solar system's largest planet. Galileo has embarked on a two-year extended mission, called Galileo Europa Mission (GEM). During GEM, Galileo will make 8 close flybys of Europa, four flybys of Callisto, and two close encounters with Io provided the spacecraft is still alive.
The third encounter for GEM is scheduled for Europa on March 29, 1998 at 13:23 UT. With a diameter of 3,138 km, Europa is slighty smaller than our own Moon and is the smoothest object in the solar system. On Galileo's previous encounters with Europa, evidence of a possible ocean were found, including the discovery of ice vocalnoes (non active), probable icebergs, and salt deposits on the surface. Galileo will continue its attempt to find additional evidence of a liquid ocean underneath Europa's icy crust and look for signs of active volcanism on the moon's young surface. On the upcoming encounter, the spacecraft will pass by Europa at a distance of 1,649 km, which is 124 times closer than Voyager's closest approach. Galileo will focus on Mannann'an crater and Tyre macula. Observations of Io, Ganymede and Callisto will also be taken.
Highlights of the Countdown to Europa home page:
o A virtual flyby of Europa with computer-generated approach images of Jupiter and Europa displayed at the top of the home page. These images are all updated every 5 minutes. o Live Doppler plots of Galileo spacecraft radio signal as it received on Earth. Watch the gravity of Europa change the frequency of the radio signal in real-time. The Doppler plots will be updated every minute on encounter day (March 29). o Flyby animation of the Europa 14 flyby. o The latest Galileo status reports reporting on the Europa 14 encounter. o Fact sheets and Europa, Callisto and Io. o A detailed timeline of events and sequences that the spacecraft will perform for the Europa 14 encounter. o Voyager 1 & 2 images of Callisto, Ganymede, Europa and Io. o Hubble Space Telescope images of the Galilean satellites. o Pioneer 10 & 11 images of Callisto, Ganymede, Europa and Io.
Ron Baalke
Galileo Webmaster
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
March 25, 1998
The flight team is preparing for Galileo's next Europa flyby, scheduled for Sunday, March 29, at an altitude of about 1,645 kilometers (1,022 miles). A flight path correction was performed on Friday, March 13, and an attitude update was performed Thursday, March 19. Both events went well, even though they used the gyroscopes, the known cause of recent anomalous behavior of the attitude control system. Precautions were taken to prevent the gyroscope anomaly from affecting activities. The flight team has decided the upcoming Europa flyby will be performed without the gyros. This means there will be no way to compensate for any wobble that may be present in the spacecraft's spin axis, and instrument pointing and stability are likely to be degraded somewhat. Only very minimal effects on images taken by the spacecraft's camera are expected, with a somewhat greater impact anticipated for another instrument, the near infrared mapping spectrometer.
On Thursday, March 26, Galileo will perform its final flight path correction before Sunday's Europa flyby, and the flight team will send computer commands to control all spacecraft activity during the encounter period. Regular maintenance of Galileo's onboard tape recorder will be performed Friday.
Galileo's flight team is nearing completion of modifications to the attitude control system flight software that would allow the spacecraft to operate with only one gyro. Although this won't be a complete "fix," it will eliminate the need for workarounds currently being used for maneuvers and attitude updates.
The recent anomalies may be caused by Galileo's long-term exposure to Jupiter's intense radiation. The spacecraft successfully completed its primary mission in December 1997 and is now in its two-year extension, the Galileo Europa Mission. The flight team will continue to monitor the radiation's impact, but current plans include five more Europa flybys, four Callisto encounters, and one or two of Io, depending on spacecraft health.
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