September 27, 1995
We are finally getting close. Although I feel like one of the newcomers on
Galileo, it has been a very long wait. I joined the Galileo project as an
engineer at NASA Ames in 1980, and became involved with the probe Doppler
Wind Experiment shortly thereafter. I now have principal responsibility for
the experiment, designed to measure the wind profile in the atmosphere of
Jupiter by tracking the probe motions via the Doppler shift of the probe to
orbiter radio signal. During the last fifteen years I've seen Galileo
canceled by Congress and then reinstated, suffered through innumerable
delays in the Space Shuttle development, each causing a redesign of the
Galileo mission. I watched as the launch date was moved from 1984 (with a
1986 arrival) to 1986 (with a 1989 arrival), to 1989. In 1986 Galileo was
at the Cape and preparing for a spring launch when the Challenger accident
occurred. And, the day before launch in 1989, the World Series earthquake
hit the San Francisco bay area and there were concerns that damage to the
facility in Sunnyvale, California responsible for tracking Galileo's upper
stage booster might once again delay the launch.

That was six years ago and, following a long, long journey - not only in
space but also from the drawing board to the launch pad - we are almost
there. Most of my summer was spent at NASA Ames Research Center developing
computer code to analyze the probe radio signal frequency data we expect to
receive shortly after the probe's December 7 arrival at Jupiter. Summers
have always been the hardest time for me, since I must leave my family in
Moscow, Idaho (where I teach during the year at the University of Idaho) and
come to NASA Ames for six weeks. But, although this summer is no different,
it is different. This is the summer of probe release and the countdown to
Jupiter. Once again I left for Ames at the end of May, and moved into an
apartment in Mountain View, California. Mountain View is located about 15
miles north of San Jose in the San Francisco Bay Area and is home to NASA
Ames Research Center. And it was during the first half of July that the
excitement really started building. Following a complex series of
preparations for probe release that started several days after the 4th of
July, it was time to say good-bye to the probe, the orbiter's traveling
companion across the solar system, past Venus, Earth (twice), two asteroids
(Gaspra and Ida) and one asteroid moon (Dactyl). Several minutes past 11:00
in the evening of July 13 we heard the words over the phone from JPL ``clear
indication of probe release'' as the radio signal from the orbiter showed a
action/reaction Doppler shift. As the probe went one way, the orbiter
recoiled slightly in the opposite direction. After working on the project
for fifteen years, it was nice to share this moment with my long time
friends and colleagues on the probe - project manager Marcie Smith, probe
engineer Charlie Sobeck, probe project scientist Rich Young, and fellow
probe experimenters Boris Ragent and Al Seiff. And it was difficult not to
think of some of the probe engineers and scientists who contributed so much
to the success of the probe and who have passed away in the past few years -
Jim Pollack, Carl Privette, Jim Van Ness and Tom Wong.

Following the successful release of the probe I headed to JPL for a probe
science meeting, then went home for what was left of my summer. Two weeks
after the probe release, the orbiter fired its main engines in a maneuver
called the Orbital Deflection Maneuver (ODM). The ODM put the orbiter on the
proper trajectory for its encounter with Jupiter. Several weeks later the
relay antenna on the orbiter was deployed in preparation to receive the
probe signal on December 7.  

And now we wait.

Work in the propulsion area of Galileo mission operations continues
to go very well, although it's perhaps a bit TOO exciting for my
taste! As of this writing, we are only 71 days from the primary use
of the Galileo main rocket engine for the Jupiter Orbit Insertion
(JOI). Along with our myriad of normal duties as propulsion analysts,
we are devoting much time and energy into understanding some surprises
in our data from the first use of the RetroPropulsion Module's (RPM's)
main engine in July. Perhaps you have seen articles in the Los
Angeles or New York Times, or USA Today concerning a leaking valve in
the Galileo propulsion system. The existence of such a leak was
postulated following the reconstruction of the rocket performance
during the first firing in July. Of course, with the spacecraft
nearly half a billion miles from Earth, diagnosing a "sick" valve is a
difficult medical proposition!

By analyzing RPM propellant tank pressure and temperature measurements
that are sent to ground (telemetry), we have determined that it is possible
that a valve (specifically, the oxidizer check valve) may be stuck in the
open position. This is one possible explanation for the discrepancies
in the data; another possibility is an electronic parts drift of two pressure
measurement devices (transducers) that monitor oxidizer tank
pressures. The oxidizer check valve is a one-way valve that allows
high-pressure helium to flow from upstream pressurant tanks in order
to "recharge" the propellant tanks to keep engine performance
"respectable" and consistent throughout the mission. The check valve
is "one-way," meaning that another of its duties is to prevent oxidizer
vapors from moving over to the fuel side of the RPM.

To understand why it's so important to keep the fuel and oxidizer separate, I
should mention here that the RPM is what is known as a "bipropellant" system,
utilizing nitrogen tetroxide as the oxidizer and monomethylhydrazine
as the fuel. This is in contrast with, say, a jet engine which only
requires fuel to be provided on-board the aircraft. This is because the
oxidizer (which is needed in order to burn the fuel) in this case is oxygen,
available from Earth's atmosphere).

Nitrogen tetroxide and monomethylhydrazine are hypergolic--meaning, they
ignite on upon physical contact--so keeping them from mixing except in
the rocket engines (for which they were designed to mix in a
controlled manner) is an important safety consideration for the whole
spacecraft! In the last two months or so, I have been primarily
concerned with keeping the spacecraft safe by minimizing propellant
tank temperature excursions in case this oxidizer check valve really
is stuck open. This is of concern because an increase in propellant
tank temperature will cause the oxidizer tank pressure to increase
more than the fuel tank pressure (due to higher vapor pressure of
nitrogen tetroxide--forgive the foray into chemistry!), which could
transport oxidizer vapor over to the fuel check valve or even into the
fuel propellant lines and propellant tanks. We are confident that we
can successfully execute JOI and the remainder of the mission even if
the valve is failed open, but it will require even harder work from
the already busy flight team. Also, I have been spending much of my
time performing "what if" calculations to determine how much oxidizer
and fuel could react through various phases of the mission. This is a
large team effort, because the effect of the amounts that I calculate
is not able to be interpreted without help from experts (in propellant
chemistry, for example).

We are still finishing the design for the computer sequence to be sent up to
the spacecraft to perform the Jupiter Orbit Insertion and to obtain the science
data associated with our first close pass by the solar system's giant
among planets. What a tremendous day December 7 will be! First we fly
by the Jovian satellite Europa at a distance much closer than the
Voyager or Pioneer spacecraft closest approach distances, allowing more
detailed images of this intriguing, icy body. Then it is on to Io,
perhaps the most interesting moon in the solar system, with its teeming
sulfurous volcanoes. We will by flying only 600 miles above the
surface! The eventual science return should be phenomenal, but this
closest approach to Io actually is dictated by the requirement for a
"gravity assist," whereby Io will actually slow us down and reduce our
propulsive requirements to get into orbit about Jupiter. You might ask
yourself (especially if you have had physics) how we can get "something
for nothing" from this gravity assist. The answer is that energy is
conserved, and Io will actually speed up imperceptibly from the flyby!
This technique was used by Galileo once at Venus and twice at the Earth
just to enable Galileo to get to Jupiter. Indeed, without gravity
assist, it would have been impossible to get the massive Galileo
spacecraft to Jupiter!

Following the Io flyby, a few hours later the orbiter should lock on to the
signal from the atmospheric entry probe, just beginning its grandiose plunge
into the cloud tops of Jupiter.  A full 75 minutes of data on the pressure,
temperature, and composition of the Jupiter cloud layers should be gleaned
from this spectacular event.  Then just an hour or so later, the main rocket
will fire for about 48 minutes to place Galileo in orbit around Jupiter for an
exciting two-year tour of the Jovian system--its magnetosphere, collection
of satellites, and of course the gas giant Jupiter itself.

Headquarters, Washington, DC           October 26, 1995
RELEASE:  95-193

NASA's Galileo spacecraft is proceeding toward its
December rendezvous with Jupiter, with spacecraft engineers
greatly relieved at last weekend's test results showing
that its onboard tape recorder remains functional.

On Tuesday, Oct. 24, a revised spacecraft command
sequence radioed to Galileo began issuing instructions
ordering the spacecraft to resume regular readouts of data
from the memories of several science instruments.  The
spacecraft also returned to normal housekeeping duties,
executing scheduled engineering operations such as flushing
of rocket thrusters.

The new command sequence replaced the one ground
controllers stopped after the Oct. 11 tape recorder
problem, in which the data tape recorder failed to cease
rewinding after recording an image of Jupiter.

The tape recorder had remained in a standby mode until
Friday, Oct. 20, when it was tested and proved still
operational.  Detailed study of engineering data from the
spacecraft indicates that the tape recorder can be
unreliable under some operating conditions, project
officials said.  However, the problem appears to be
manageable, and should not jeopardize return of the nearly
2,000 images of Jupiter and its moons that are to be stored
on the recorder for playback over the course of Galileo's
two-year tour in orbit around the planet.

Tuesday's work on the spacecraft included commands for
the tape recorder to wind 25 extra times around a section
of tape possibly weakened when the recorder was stuck in
rewind mode with the tape immobilized for about 15 hours.
Due to uncertainty about its condition, spacecraft
engineers have declared that this portion near the end of
the tape reel is "off-limits" for future data recording.
The extra tape wound over it secures that area of tape,
eliminating any stresses that could tear the tape at this
potential weak spot.  Unfortunately, the approach image of
Jupiter that Galileo took Oct. 11 is stored on the portion
of tape that is now unavailable, and it will not be played back.

With only weeks to go before Galileo's Dec. 7 arrival
at Jupiter, project engineers are busy analyzing the tape
recorder's condition to fully understand its capabilities
and weaknesses.  "We need to be sure we fully understand
the system that we have now," said Galileo Project Manager
William J. O'Neil.

The tape recorder is a key link in techniques
developed to compensate for the loss of use of Galileo's
high-gain antenna, which is stuck in a partially open
position.  Data must now be sent at a much lower data rate
through Galileo's low-gain antenna.  The tape recorder is
to be used to store information, particularly imaging data,
until it can be compressed and edited by spacecraft
computers and radioed back to Earth.

Since the tape recorder incident, Galileo project
officials have decided to not take pictures of Io and
Europa on the day the spacecraft arrives at Jupiter.
Instead, they will devote the tape recorder that day to
gathering data from Galileo's Jupiter atmospheric probe as
the cone-shaped probe descends into the giant planet's
swirling atmosphere over the course of its 75-minute
mission. During that time, the probe will collect the
first-ever direct measurements of the chemical makeup and
weather of the solar system's largest planet.

"Our priorities are clear," said O'Neil.  "We have to
get all the probe data."  Other flybys of the Jovian moons,
including frequent "volcano watch" monitoring of Io, occur
throughout the mission, giving ample opportunity to collect
data on all the moons.  Late in the mission, O'Neil said, a
close flyby of Io might be made to make up for the Io flyby
data that will be sacrificed on Dec. 7.