Today On Galileo - May 30, 1998
The fourth encounter of the Galileo Europa Mission starts today and is scheduled to run for the next 3-1/2 days. The encounter features a close flyby of Jupiter's moon Europa, occurring tomorrow, at an altitude of 2516 kilometers (1564 miles). Gathering of science information is not limited to Europa. Encounter plans include observations of Jupiter and its magnetosphere, and Jupiter's volcanic moon Io.
The set of computer commands that will control the spacecraft's activities through the encounter period start executing at 2 PM PDT*. Prior to the start of the encounter sequence, the flight team finished recovering the spacecraft from a maneuver sequence anomaly. All spacecraft systems are nominal and ready for the encounter. The spacecraft finds itself at a distance of about 770 million kilometers (480 million miles) from Earth during this period which means that it will take radio signals just under 43 minutes to travel to and from the spacecraft.
The start of the encounter initiates the survey of Jupiter's magnetic and electric field environment performed by the fields and particles instruments. The survey is the collection of information at a low rate (a few bits per second). It is repeated every encounter to provide background and context information for higher time resolution observations (a few hundreds of bits per second) that are recorded and returned later. The survey will run for the duration of the encounter period.
The ultraviolet spectrometer starts off the remote sensing activity by taking a look at Europa's atmosphere. The observation looks for gases (oxygen, hydrogen, sulfur) moving around in Europa's atmosphere and possibly into space. A change in the amount of gas that is moving around could be an indication of geologic (earthquakes, geysers) activity on Europa's surface. The observation is repeated 3 hours later.
Several observations of Io are taken by the spacecraft's camera. The first takes a look at how intensely light is being reflected off Io's surface, with the hope of being able to provide information on the size and age of sulfur dioxide grains. Shortly after, Io enters the shadows of Jupiter and the camera takes advantage of the opportunity to take a couple of images. These eclipse images have proven to be the best way to discover and monitor lava temperatures and interactions between volcanic plumes, Jupiter's magnetosphere and Io's tenuous atmosphere. Today's Io observing schedule also includes the ultraviolet spectrometer taking a look at Io's atmosphere. Similar to the Europa observation, the observation looks at gas amounts in Io's atmosphere and the regions of space surrounding Io. The amounts of these gases in Io's atmosphere could be changed by volcanic activity or sublimation (melting and evaporation in one step) of sulfur frost.
Only one observation of Jupiter is performed today. The near infrared mapping spectrometer takes a reading of Jupiter's atmosphere to measure temperatures and material amounts. This observation is repeated several times during the encounter and in almost every encounter to keep track of variations over time.
* All times of day listed correspond to the Pacific Time zone (currently daylight savings) and spacecraft event time. Radio signals indicating that an event has occured on the spacecraft reach the Earth 35 to 50 minutes later, depending on the time of year.