Galileo’s intrepid flight controllers take the spacecraft back to Io in the next few days as part of the
second encounter of the Galileo Millennium Mission, the extension to Galileo’s mission at Jupiter.
Dubbed as I27 (“I” for Io), the encounter is the 27th flyby of Galileo’s orbital tour, which started
back in June 1996 with Ganymede. This Io flyby will mark the fourth time the spacecraft has flown
past Io since arriving at Jupiter in December 1995, and is the closest-ever approach, with a flyby
altitude of only 200 kilometers (124 miles) above Io’s fiery surface. That is the same distance at
which Galileo flew over Europa in December 1997, and is about the same as the distance between
Los Angeles and San Diego!

As has been the case for recent encounters, Galileo’s flyby of Io places the spacecraft at risk of
being affected by Jupiter’s intense radiation belts. Galileo’s components have already survived more
than twice the radiation they were originally designed to withstand, and any passage through the
Jupiter system that can reach Io also adds significantly to the total radiation dose experienced by
the spacecraft. The risk associated with this endeavor is well worth the promise of new information
on Io and its unique status as the most volcanically active body in the solar system.

The Io flyby is scheduled to occur on Tuesday morning at 5:47 a.m. PST [see Note 1]. Radio signals
indicating that the flyby has occurred, however, won’t be received on Earth until 45 minutes later, or
at 6:32 a.m. PST, which is denoted as Earth Received Time (ERT). The time difference is due to the
fact that the spacecraft is approximately 5.4 astronomical units (811 million kilometers, or 504
million miles, 1 astronomical unit is equal to the average distance between the Earth and the Sun)
from Earth and it will take radio signals just over 45 minutes to travel between the spacecraft and
Earth. Encounter commands for the Io flyby begin to execute late Saturday night at 8:00 p.m. PST
(8:45 p.m. PST-ERT). They will continue to execute through mid-Wednesday when data playback will
be initiated. Prior to the start of the encounter, the spacecraft performs standard maintenance on
its onboard tape recorder. The majority of the observations performed during the encounter will be
stored on this device for later processing and transmission to Earth.

The encounter also features distant flybys of the other Galilean moons. Although no observing is
done during those flybys, the only one close enough to mention is the flyby of Europa that occurs on
Monday at 7:51 pm PST (8:36 pm PST-ERT) at a distance of 409,000 kilometers (254,000 miles).

Around noon Sunday, Galileo’s radio signal begins to pass through Jupiter’s upper atmosphere on its
way to Earth. The signal is weakened and refracted by the extremely tenuous atmosphere
approximately 25,000 kilometers (15,500 miles) above Jupiter’s cloud tops. The small changes in
the signal will be measured by the Radio Science Team here on Earth, and will allow scientists to gain
more knowledge of the structure and electron density of Jupiter’s upper atmosphere.
Approximately 12 hours later, the spacecraft passes behind Jupiter as seen from the Sun. The
eclipse lasts just short of two hours.

Late Sunday, the Fields and Particles instruments resume their survey of the Jovian magnetosphere.
This survey has been performed from orbit to orbit and allows scientists to study the long term
variations of the inner portions of Jupiter’s magnetosphere. The survey data also provide context
for higher resolution recordings performed by the instrument suite. The Fields and Particles
instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter,
Magnetometer, Plasma Detector, and Plasma Wave instrument.

Monday sees the first remote sensing observations of the encounter. The Photopolarimeter
Radiometer (PPR) takes two observations of Jupiter’s atmosphere. The polarimetric observations
will allow scientists to learn more about the vertical cloud structure of Jupiter, including cloud particle
shape and size. This particular pair of observations will provide the best PPR resolution from
Galileo’s mission to date. PPR is also the first to look at Io. Its first observation captures the dark
side of the volcanic moon. The map will be used to describe night time thermal emissions on Io and
will aid scientists in the development of heat flow models.

Anticipating the flyby of Io early Tuesday morning, the Radio Science team begins to carefully
measure changes in the frequency of Galileo’s radio signal just after 8:32 p.m. PST-ERT on Monday.
The changes are caused by Io’s gravitational pull on the spacecraft. If this sounds familiar, it is
because similar experiments have been conducted on each of the previous four Io flybys, with
differences only in the spacecraft’s flight path. Radio scientists will track these changes for 20 hours,
centered on the point of closest approach to Io. The data will be added to the existing repository
that provides the basis for a model of Io’s gravity field and internal structure.

Come back tomorrow for flyby day!

Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Mean Time (GMT).

For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo
home page at one of the following URL’s: