This Week on Galileo: Io - And How!
17 Jan 2002
(Source: Jet Propulsion Laboratory)
Thursday, January 17, 2002
Today sees the most intense activities for the spacecraft as Galileo makes its final close flyby of the volcanic satellite Io. Beginning just 20 minutes after midnight PST [See Note 1], the Magnetometer instrument adjusts its sensors to be able to accurately measure the much stronger magnetic fields that will be encountered during the closest approach to Io and to Jupiter.
At 2:58 a.m. PST the Radio Science Team begins an experiment to probe the atmosphere of Jupiter itself as Galileo passes behind the giant planet as seen from the Earth. Telemetry in the transmitted radio signal from the spacecraft is turned off (don't worry, the other science data being collected on the spacecraft is being stored in computer memory, and will be read out later) and the radio signal is changed to a pure tone. As Galileo passes behind the planet, this tone passes through deeper and deeper layers of the atmosphere, and by observing the changes in intensity and frequency of the tone, scientists can determine temperatures, pressures, and electron densities down through the different layers of the atmosphere. Between 3:22 a.m. and 5:19 a.m. PST, the spacecraft will be completely blocked by the planet, and at 5:41 a.m. PST telemetry is restored to the normal configuration, and the bits flow once again. Also during this time, between 3:48 a.m. and 5:42 a.m. PST, the spacecraft finds itself in the shadow of the planet as it passes out of sight of the Sun. Since seeing the Sun is a key element in the spacecraft knowing its orientation in space, the on-board software is informed that the Sun will be invisible during this time, and that sightings of the star Achernar (Alpha Eridani) by the Star Scanner will be the sole means of maintaining attitude knowledge. This technique has worked well on many previous orbits.
As the Sun occultation ends, at 5:42 a.m. PST the Photopolarimeter Radiometer instrument (PPR) again turns it gaze on Io, now only one hour and 31,000 kilometers (19,300 miles) away, and spends 20 minutes studying the temperatures of the Prometheus volcano while that feature is on the night side of the satellite. These night-time studies allow scientists to determine the intrinsic temperatures of features, uncluttered with reflected sunlight.
At 5:46 a.m. PST the Energetic Particle Detector (EPD) performs a power cycle and memory reload. The high radiation environment in previous orbits has caused upsets to the microprocessor that controls the instrument. This pre-emptive reload helps assure us that the instrument is in the proper configuration and operating well for the close flyby to come.
At 5:58 a.m. PST the Fields and Particles instruments [the Heavy Ion Counter (HIC), EPD, the Magnetometer (MAG), the Plasma Subsystem (PLS), and the Plasma Wave Subsystem (PWS)] begin a 5.5 hour stretch of continuous high-rate data collection around the Io closest approach. In addition to the dynamic interactions expected close to Io, this recording will capture data on the Torus, a donut-shaped region of enhanced energetic particles that coincides with the orbit of Io. It will also study a feature known as the "ribbon", a temporary and changing energetically emitting region between the cold and warm portions of the torus.
Between 6:04 a.m. and 6:28 a.m. PST, PPR again studies the temperatures of Io as it scans along the equator of the satellite and then concentrates on the hot spot Marduk, and on the Pillan crater region, both in the southern hemisphere.
At 6:29 a.m. PST the Near Infrared Mapping Spectrometer instrument (NIMS) begins its study of Io with a complementary view of the Marduk region.
The first of the Solid State Imaging camera (SSI) pictures of Io begins at 6:37 a.m. PST with images of the Pele caldera. Even though this feature is in the dark at the time, the lavas glow in the dark, and the brightness of the glow gives a good measure of just how hot the flows are.
At 6:40 a.m. PST PPR directs its line of sight straight down at Io and watches the landscape stream by as Galileo reaches its closest point to the satellite. This occurs at 6:43:53 a.m. PST at a distance of only 100 kilometers (62 miles) above the surface. At the closest point to Io, Galileo is passing over a latitude 43.6 degrees south of the equator. This is equivalent to flying over Hobart, Tasmania, Australia. As Galileo barrels past at 7.72 kilometers per second (17,270 miles per hour!) the landscape is passing too quickly for instruments like SSI to take clear pictures; they would be horribly smeared by the motion of the spacecraft while the shutter was open. It is up to instruments like PPR, that do not directly produce pictures, to provide measurements of the surface at the highest resolution possible. The Radio Science Io gravity experiment begun yesterday also reaches its most important phase at closest approach, where the pull on Galileo is at its peak.
One minute after closest approach, however, SSI can look to the side, where the range from the spacecraft to the viewpoint on the surface is about 1,200 kilometers, and will image a region of enigmatic circular rises called "tholi". This is our first look at these unusual features at this high resolution. This observation is followed in rapid succession by views of the Mbali Patera and the Kanehekili volcanic area. The Mbali pictures provide an opportunity to see the actual source of the lava flow there at a resolution of about 20 meters per picture element (65 feet per pixel).
At 6:53 a.m. PST NIMS provides a complementary thermal study of the Kanehekili hot spot. By combining observations of the same features taken by different instruments whose strengths lie in different regions of the electromagnetic spectrum, scientists can extend their knowledge of the body past mere form, and can deduce detailed structure, texture, temperature, and composition of the surface.
By 6:59 a.m. PST, a scant 15 minutes past closest approach, the distance to the tholi region has increased to 8,300 kilometers (5160 miles), and SSI provides wider-angle views of the region to supply broader context for the high-resolution pictures taken earlier. This is followed by broader context pictures of the Mbali and Kanehekili locations as well, with the Mbali pictures taken in color.
During the course of the next hour, SSI continues to capture views of several areas on Io. The Hi'iaka area is suspected of showing some strike-slip faulting, and there is the possibility that such a fault is tearing a mountain in two! The Pan Mensa area is a mountain with extensive fracturing and bright basins of lava (pateras) on either end. The Gish Bar region also has mountain-patera interactions and has been studied on previous orbits. This area also contains a mysterious Y-shaped crack. Finally, at 7:48 a.m. PST a strip of images ranging from far southern latitudes to just south of the equator stretches across the surface, capturing the Masubi and Kanehekili regions, as well as another hot spot that has shown some dramatic changes in appearance in the past.
During this time, PPR and NIMS are also studying the thermal emissions in the Kanehekili region. NIMS also views the Hi'iaka hot spot region. Then at 8:14 a.m. PST NIMS begins a 52 minute map of the entire Jupiter-facing hemisphere of Io.
The geometric closest approach to Jupiter occurs at 8:57 a.m. PST, when Galileo reaches in to 4.5 Jupiter radii (322,000 kilometers or 200,000 miles) above the cloud tops. This is the closest we've come to Jupiter since the 24th full orbit, which was also a close Io flyby, in November 1999.
At 9:17 a.m. PST PPR begins a 1.5-hour-long map of the entire visible disk of Io, which is now more than 80,000 kilometers distant (50,000 miles). Attention is then briefly torn away from Io as PPR takes a polarimetry measurement of the icy satellite Europa.
The focus returns to Io at 11:33 a.m. PST, when SSI acquires a color map of approximately half of the visible Io face. At 11:42 a.m. PST NIMS begins another hour-long mapping of the entire visible Jupiter-facing hemisphere of Io. Our attention again wanders from Io, as SSI captures our second-best ever picture of the small inner satellite Thebe. In this picture, one pixel in the camera image spans 3 kilometers (1.9 miles) on the surface of the satellite. During our 26th orbit, our best resolution picture had a pixel span of only 2 kilometers (1.25 miles).
PPR now shifts the focus to Jupiter itself. Between 12:50 p.m. and 4:10 p.m. PST the instrument scans the giant planet from east to west, then from north to south, both through the Great Red Spot, then focusing on a long-lived white oval storm in the atmosphere, followed by a scan off of the northern limb studying atmospheric structure, finishing with another north to south pole-to-pole scan.
At 4:40 p.m. PST SSI steps up again with a color map of the side of Io that faces away from Jupiter. This view will cover many of the most dramatic features studied by Galileo to date. These include Prometheus, Amirani, Tvashtar, and the site of the giant new volcanic plume discovered during a previous flyby in August. NIMS follows this set of pictures with our final Io observation of the mission! This global map will search the Jupiter-facing hemisphere of the satellite for new hot-spots. By 5:02 p.m. PST, this observation is finished, and, a mere 10 hours after our closest brush with the volcanic fury of the most geologically active body in the solar system, we bid a fond farewell to Io forever! It has been an exciting and tumultuous ride over the past 6 years in orbit, and Io has never once failed to surprise and delight us! Thank you, old friend!
Though the Io observations have concluded, there is still good science to be done. PPR spends the next hour studying hot spots in the north equatorial boundary region of Jupiter's atmosphere, followed by a final instrument calibration. At 6:42 p.m. PST PPR performs one final polarimetry measurement of Europa.
About 10 p.m. PST SSI captures an image of the inner satellite Amalthea. This picture will be used for optical navigation, where the positions of the satellite and of distant stars are compared to provide the Navigation team an accurate idea of the relative positions of Galileo and Amalthea. This will be used to guide our trajectory to a close flyby of that satellite in November of this year.
What? Has it only been one day? The Spirit of Science Present is an extremely ambitious task master! And there's more to come...
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. All times quoted above are in Earth Received Time.