Today on Galileo
Thursday, January 17, 2002
Io, And How!
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
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
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
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.