The Wind-Up

Well, after twelve years of pre-launch development and planning, six years
of interplanetary cruise, and nearly eight years in orbit, our exciting,
quarter-century odyssey has finally come down to this: the final 19 hours
of existence for the Galileo spacecraft. It began life in October 1977 as
the Jupiter Orbiter Probe mission, was launched in October 1989, and
arrived at Jupiter in December 1995. After circling the solar system’s
largest planet 35 times, it is about to plunge into the atmosphere of
Jupiter, becoming only the second man-made object to do so, following the
smaller Galileo atmospheric probe that accompanied the Orbiter to Jupiter.
From launch to impact, the stalwart spacecraft has travelled 4,631,778,000
kilometers (2,878,053,500 miles) on 925 kilograms of propellant (246
gallons), not counting the fuel for the shuttle. In all that time, and
across all those miles, Galileo has returned over 30 gigabytes of data,
including 14,000 pictures.

One chapter of the volumes of scientific data produced by Galileo over the
years includes the discovery of likely sub-surface water oceans on the icy
satellite Europa. This has fueled speculation about the possibility of life
existing in that environment, and is prompting plans for future spacecraft
to return to Europa to search for life. Since the Galileo spacecraft was
never designed to specifically search for life, it was never subjected to
the rigorous sterilization procedures such as those mandated for craft
going to Mars. To prevent any possible future biological contamination of
Europa, the decision was made to provide a final resting place for the
Galileo Orbiter that guarantees the spacecraft will never collide with any
of the Jovian moons. That resting place is Jupiter itself.

The Pitch

The final day for Galileo begins on Saturday evening at 5:52 p.m. PDT [see
Note 1], when the spacecraft is just over 18 Jupiter radii (1.32 million
kilometers or 822,000 miles) from the center of the planet, and closing
fast. At that time, the instruments that measure the magnetic and electric
fields and the particle environment stop collecting data for a while. These
instruments are the Dust Detector Subsystem (DDS), the Energetic Particle
Detector (EPD), the Heavy Ion Counter (HIC), the Magnetometer (MAG), and
the Plasma and Plasma Wave Subsystems (PLS and PWS). They have been
collecting data continuously for the past six months, storing these data in
a computer memory buffer for later transmission to Earth. By stopping this
collection for 7 hours, this buffer is allowed to drain, and subsequent
data collected can be transmitted to Earth almost immediately.

At 10:52 p.m. the spacecraft attitude control system is told to base its
orientation calculations on the observations of a single star. Normally,
three stars are used, but in the intense radiation environment near
Jupiter, noise in the star sensor circuits overwhelms the signals from
fainter stars, and a single, bright star is selected which can rise above
the noise and still be detected. Today that star is Vega (Alpha Lyrae), the
fourth brightest star in the sky.

At 12:52 a.m. Sunday, the 70-meter-diameter (230 feet) Deep Space Network
tracking station antenna near Madrid, Spain, is listening to the
spacecraft. The science instruments are configured properly, and begin
again to send their data in real-time to Earth. Galileo has closed the
distance to 13.5 Jupiter radii (965,000 kilometers or 600,000 miles).

At 5:55 a.m., the tracking antenna at Goldstone in the Southern California
desert joins the Madrid station, and for the next three hours both stations
are collecting the faint signals sent from a half a billion miles out in
the solar system.

At 5:07 a.m., the distance has closed to 10 Jupiter radii (715,000
kilometers or 444,000 miles) and MAG changes the sensitivity of its
measurements in anticipation of the stronger magnetic fields to come. At
6:24 a.m., all instruments except MAG stop collecting data for just over an
hour. During this time the data collection rate on the spacecraft is
greater than the rate the ground stations can reliably receive. If the
collection continued, the data would accumulate in the storage buffer to
such an extent that the buffer would not have a chance to empty before the
spacecraft is lost from view. This brief pause allows the most valuable
data collected nearest to Jupiter to be sent without buffering.

At 7:22 a.m., the radio signal sent from the spacecraft is changed to
provide more power to the underlying carrier signal. This will help the
ground stations keep track of the signal as Jupiter’s increasing
gravitational pull speeds the spacecraft up and alters the apparent
transmission frequency, due to the familiar Doppler effect.

The Swing

At 9:05 a.m., Galileo crosses the volcanic satellite Io’s orbit at a
distance of 6 Jupiter radii (422,000 kilometers or 262,000 miles). The
spacecraft has spent most of its 8-year travels around Jupiter outside of
this distance, to keep the received radiation dose down. It has ventured
significantly inside this distance only twice. Once, in December 1995, as
Galileo first entered Jupiter orbit, when we reached 4 Jupiter radii (3
radii over the clouds), and again in November 2002, when a flyby of the
small inner moon Amalthea took the craft down to 2 Jupiter radii (1 Jupiter
radius, 71,500 kilometers or 44,400 miles over the cloud tops). This time,
though, it’s a one-way trip. The distance will only get shorter.

By 9:42 a.m., the intensity of the radiation noise has reached a point
where even a bright star like Vega can no longer reliably be seen by the
attitude control star scanner. The software is now told to expect to see no
more stars, ever.

At 11:31 a.m., Galileo is two Jupiter radii above the clouds (143,000
kilometers or 89,000 miles) and the Magnetometer instrument has taken its
final data for the mission. At this distance from Jupiter, the magnetic
field is so strong that the instrument, even in its most robust
configuration, would produce a signal that would be completely saturated,
and of no further scientific value.

Seventeen minutes later, at 11:48 a.m., the spacecraft passes the orbit of
the tiny satellite Amalthea, and at about 12:17 p.m., passes the orbits of
the innermost moons, Adrastea and Metis. Galileo is now just 57,500
kilometers (35,700 miles) above the clouds, closing fast, and picking up
speed. As the spacecraft passes Amalthea a special measurement will be
taken using the star scanner. During our previous flyby of this small body
on November 5, 2002, flashes of light were seen by the star scanner that
might indicate the presence of rocky debris circling Jupiter in the
vicinity of the satellite. Though on this final pass, Galileo will not be
near Amalthea, the measurement may help confirm or constrain the extent of
this hypothesized orbital debris.

At 12:26 p.m. the Galileo Orbiter joins the Galileo Probe in going closer
to Jupiter than any other man-made object, passing the 1973 mark that
Pioneer 11 set on its swing through the Jupiter system. At 43,000
kilometers altitude (26,725 miles), the spacecraft is now at a distance
that is 1/9th of the span between Earth and its own Moon. This is also the
approximate altitude that geosynchronous communications satellites orbit
above the Earth’s surface. What seems like a vast expanse when viewed in
terms of the Earth seems like such a small step away when viewed in terms
of Jupiter, which has a diameter that is 11 times that of Earth.

At 12:42 p.m. with 7 minutes and 10 seconds to go, Galileo moves from day
to night as it passes into Jupiter’s shadow, and, one minute later, passes
behind the limb of the giant planet as seen from Earth. Only 9,283
kilometers (5,768 miles) above the clouds, the path of the spacecraft now
takes it out of sight of ground controllers, never to be seen again. The
last data ever to be received from the Galileo spacecraft has now been
sent. The remaining few minutes of the craft will be spent in darkness, and
alone…

The Hit

Finally, at 12:49:36 p.m., Sunday, September 21, 2003, Galileo reaches the
end of its nearly 14 year odyssey through space with a final, glorious
meeting with the king of the solar system’s planetary entourage. This
event, though described as an impact, is actually the gradual, but very
rapid, immersion in the gas giant’s vast atmosphere. The time stated is
when the spacecraft reaches that point in the atmosphere where the pressure
reaches one bar, the equivalent of Earth’s atmospheric pressure at sea
level. For reference, this point is 71,492 kilometers (44,423 miles) from
the center of the planet, at the point where Galileo enters.

The entry point is approximately 1/4 degree south of Jupiter’s equator. If
there were observers floating along at the cloud tops, they could see
Galileo streaming in from a point about 22 degrees above the local horizon.
Streaming in could also be described as screaming in, as the speed of the
craft relative to those observers would be 48.26 kilometers per second
(nearly 108,000 miles per hour!). That is like travelling from Los Angeles
to New York City in 82 seconds. In comparison, the Galileo atmospheric
Probe, aerodynamically designed to slow down when entering, and parachute
gently through the clouds, first reached the atmosphere at a slightly more
modest 47.6 kilometers per second (106,500 miles per hour).

The Galileo Orbiter, with the aerodynamic qualities of a brick, and no
parachute, will not endure nearly as controlled and graceful a fate. It
will rapidly burn up through friction with the atmosphere, returning to its
constituent atoms as it makes its unnoticeable impact on the vast weather
systems of Jupiter. In the life of the giant planet, Galileo will look like
another speck of cosmic debris. It would not be nearly as observable as the
fragments of Comet Shoemaker-Levy 9, which crashed into Jupiter’s
atmosphere in July 1994 and was observed by Galileo. In the life of the
planet Earth, with its inhabitants, Galileo will live on in the memories of
those who worked on the project, as well as those of new generations who
study the astronomy textbooks, rewritten with the reams of data returned
over the years.

Nearly 393 years ago, Galileo Galilei first turned the newly developed
telescope on Jupiter, and spied the four satellites that bear his name;
lonely, silent sentinels that began to show proof that Earth was not the
center of the universe. Mankind has since dreamed, and schemed, and
developed robotic messengers to visit and study our ever more complex
surroundings. What more fitting end to the messenger that is also Galileo’s
namesake, than to find final repose in Jupiter, while overhead circle those
four sentinels, no longer lonely, no longer silent. For we were there. We
listened to what they had to say, and they spoke volumes. They have become
part of our family, our circle of familiar friends. As with all good
friendships, they have inspired us to do more, go farther, look closer and
deeper. To learn.

The future lies before us. Thank you for sharing our journey so far …

===============

Note 1. Pacific Daylight Time (PDT) is 7 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). On the day of impact, it takes Galileo’s radio signals 52 minutes 18
seconds to travel between the spacecraft and Earth. All times quoted above
are in Earth Received Time at JPL in Pasadena.

To summarize, Galileo will impact Jupiter at:

Spacecraft Event Time (SCET) Earth Receive Time (ERT)
11:57:18 PDT 12:49:36 PDT
18:57:18 GMT 19:49:36 GMT