How much does the spacecraft weigh?
At launch, the orbiter had a mass of 2,223 kg (4891 lbs), which includes a 118 kg (260 lbs) science payload and 925 kg (2,035 lbs) of usable propellent. Over 40% of the orbiter's mass at launch is for propellent! The probe's total mass is 339 kg (746 lbs); the probe descent module is 121 kg (266 lbs), including a 30 kg (66 lbs) science payload.
Where is the spacecraft's camera located?
Most people think that the Solid-State Imaging instrument (SSI), which takes photos in visible light, is Galileo's only camera, but there are actually three other cameras on board. A photopolarimeter-radiometer will measure the polarization of light scattered from Jupiter's clouds and the satellites' surfaces, by a process like using polarized sunglasses to cut down on glare. In addition, its infrared channels will sound the atmosphere and measure satellite temperatures. The near-infrared mapping spectrometer will map the satellites, looking for different minerals across their surfaces. It will also study cloud structure and gas composition in the jovian atmosphere. An ultraviolet spectrometer and extreme ultraviolet spectrometer will investigate volatile excape and surface composition of the Galilean satellites, the Io plasma torus, small and large scale properties of the Jupiter clouds, and the composition, structure, and evolution of Jupiter's upper atmosphere.
All four cameras are mounted on a scan platform, located near the bottom of the orbiter. This section of the spacecraft can be "despun," or kept from spinning with the rest of the orbiter--otherwise, all the images would be blurry.
Why is the spacecraft wrapped in black and gold stuff?
Galileo's electronics and science instruments are designed to work in interplanetary space, but, without some sort of insulation, it's too cold for them to operate (just like your camera shutter might freeze if you take it to the North Pole). The black and gold blankets are carefully designed to keep Galileo's innards at a "comfortable" temperature. They also keep micrometeorites from smashing into the spacecraft electronics.
The black blankets, which are made up of 20 different layers, are very efficient insulation. Although only 1/5th of an inch thick, it's three times as good an insulator as the four-inch-thick fiberglass insulation in your attic. The black color is due to carbon in the outer layer, which keeps electrostatic charge from building up in one spot and then shorting out the spacecraft electronics.
Black material in the sun picks up lots of heat, and emits a great deal of infrared light. The "gold" blankets, however, don't absorb a great deal of solar heat, though they do radiate well in the infrared. This material (called "second-surface aluminized kapton") therefore does an even better job of insulation. It's not used on the entire spacecraft because it was developed after Galileo was designed and built. When Galileo's flight path was changed to take advantage of a Venus gravitational assist, there was enough time to use second surface kapton on critical areas of the spacecraft.
That's the science boom. It's 10.9 meters long, and is designed to minimize the effects of orbiter-generated interference on the magnetometer and plasma wave instruments. The fields and particles instruments are mounted at the end of the boom, and also about 3 meters from the spacecraft.
How did the spacecraft fit inside the shuttle bay?
The spacecraft was folded up, like an elaborate piece of origami. The high-gain antenna and booms were furled up.
Unlike previous planetary spacecraft, Galileo features an innovative "dual spin" design: part of the orbiter rotates constantly at three revolutions per minute, and part of the spacecraft remains fixed in inertial space. This means that the orbiter can easily accomodate magnetospheric experiments (which need to take measurements while rapidly sweeping about) while also providing stability and a fixed orientation for cameras and other sensors. The spin rate can be increased to 10 revolutions per minute for additional stability during major propulsive maneuvers.
The Probe is nestled on the "bottom" of the spacecraft, below the area where the camera scan platform is mounted.
How fast is the Probe going when it enters Jupiter's atmosphere?
170,700 kilometers per hour (106,000 miles per hour), or 47 kilometers per second (29 miles per second) That's the highest impact speed of any man-made object ever; it's about 100 times the muzzle velocity of a bullet fired from a .45 caliber gun.
What science will the Probe instruments return?
Galileo's Probe incorporates experiments to measure temperature and pressure along the descent path, locate major cloud decks, and analyze the chemistry of atmospheric gases. In addition, the Probe will attempt to detect and study jovian lightning both by looking for optical flashes and by listening for the radio "static" they generate. The latter detector will also measure high-energy electrons close to Jupiter just prior to atmospheric entry.
Once the Probe separates, what wakes it up many months later just before
entry? Does it just have a built-in programmable timer, or does the Orbiter
beam the Probe a command to wake up? Or does it remain fully "awake," doing the
same thing (that is, taking data) from the time it separates until the end of
its mission?
Yes, the Probe does have a built-in, programmable timer, which is set by ground
command shortly before the Probe separates from the orbiter. The Probe designers
wanted to have some flexibility in starting the timer to accomodate any late
changes in the Probe release schedule. If Probe release is delayed for any
reason, the timer is reset appropriately. The timer is the only thing running
on the Probe during its five month long cruise to Jupiter.
Like an alarm clock, the timer is set to wake up the Probe 6 hours before entry
into Jupiter's atmosphere so that the Probe can 1) take measurements of the
inner magnetospheric energetic particle environment, and 2) listen for radio
emissions characteristic of lightning (these actually sound like long,
descending whistles).
This "pre-entry" phase ends when the Probe's accelerometers detect signs that
the Probe is being decelerated by Jupiter's atmosphere. At this point, the
Probe starts its entry/descent phase.
Although Jupiter is a planet, it is very different from Earth. In fact,
scientists refer to hard and rocky planets like Earth, Mercury, Venus, and
Mars as "terrestrial," while planets like Jupiter, Saturn, Neptune, and
Uranus are called "gas giants," since they seem to be, essentially, huge balls
of gas and liquid with a small rocky core. So, Jupiter doesn't really have a
"surface" in the sense of its being something that humans could walk
around on, or that a spacecraft could land on.
Galileo's atmospheric Probe will travel between 130 and 160 kilometers
below Jupiter's cloud tops, deep enough to help answer questions such as
what's in Jupiter's yellow clouds, or how strong are the winds below the
cloudtops. However, the Probe won't come anywhere near seeing the
"surface" of Jupiter's rocky core, buried roughly 60,000 kilometers
underneath the cloud tops.
Why aren't we taking an image of the Probe as it drifts
away from the Orbiter after separation?
Imaging the Probe as it drifts away from the Orbiter was contemplated both
for engineering assessment (that is, looking for any problems with the
Probe hardware) and
optical navigation. In order to assess the external
condition of the Probe, detailed pictures would be desireable, but, because
the SSI is focused on infinity, objects up close would be out of focus. In
fact, by the time the Probe is in focus (at about 18 km away from the
Orbiter), it would only be about 2-3 resolution elements
(about 4-6 pixels) across--not
very detailed!
Taking a picture of the Probe for Optical Navigation purposes appears to be
feasible
, but, because the Probe delivery knowledge requirements are being
met, and because of the operational costs of trying to return large data sets
(such as images), it was decided not to pursue optical navigation.
The umbrella-like high gain antenna is located at the top of the spacecraft, and is 4.8 meters (16 feet) in diameter. It was designed to transmit data back to earth at rates of up to 134,000 bits of information per second (the equivalent of about one television picture each minute). Compare this to a fast home modem, which only manages to send 14,400 bps!
The antenna, which is made of gold-plated metal mesh, was stowed behind a sun shield at launch, in order to avoid heat damage from the sun while the spacecraft flew "inside" the earth's orbit.
Why didn't the antenna open all the way up? Is JPL still trying to open it?
On April 11, 1991, the Galileo spacecraft began to deploy its high-gain antenna under computer-sequence control.
The antenna -- a 16-foot mesh paraboloid stretched over 18 umbrella-like ribs -- had been furled and hiding behind a small sunshade for the almost 18 months since launch, in which the spacecraft came closer to the sun than Earth and briefly closer even than Venus. Communications, including Venus and Earth-moon science data return, had been using the low-gain antennas.
Within minutes, Galileo's flight team, watching spacecraft telemetry 37 million miles away on Earth, could see that something was wrong: The motors had stalled, something had stuck, the antenna had opened only part way.
Within weeks, a tiger team had thoroughly analyzed the telemetry, begun ground testing and analysis, and presented its first report.
They attributed the problem to the sticking of a few antenna ribs due to friction between their standoff pins and their sockets. The first remedial action was taken -- turning the spacecraft to warm and expand the central tower, in hopes of freeing the stuck pins.
In addition to thermal cycling, the tiger team developed other ideas for loosening the stuck ribs: retracting the second low-gain antenna (on a pivoting boom), pulsing the antenna motors, and increasing the spacecraft spin rate to maximum 10 rpm (normally about 3 rpm).
After a nearly two-year campaign to try to free the stuck ribs there is no longer any significant prospect of deploying the HGA, though one last attempt will be made in March of 1996. The Project is proceeding to perform the Galileo Mission with the Low-Gain Antenna.
For additional information, see "Galileo's antenna:
the anomaly at 37 million miles," an article by Jim Wilson that appeared
in JPL's newspaper on July 3, 1992, or Unfurling the
HGA's Enigma in the August, 1991 Galileo Messenger.
What's that big black thing at the top of the spacecraft? Is it a mirror for the antenna?
That's one of the sunshades for the spacecraft, designed to protect the HGA from the sun's heat while the spacecraft was travelling inside of Earth's orbit.
The project briefly studied this option, but quickly determined that, given the short amount of time in which the relay satellite would need to be designed and launched, a relay would be prohibitively expensive.
There were also significant unresolved technical problems related to both orbital mechanics and telemetry. Engineers first proposed having a relay satellite that would follow Galileo closely, like a well-trained dog. But, the amount of propellant needed made the mission too expensive. Even keeping the relay satellite within a few million kilometers of the spacecraft throughout the entire two-year orbital tour would be a difficult (if not impossible) engineering problem, given the limited amount of propellant on board a relay satellite. Finally, the relay antenna itself would need to be comparable in size to Galileo's original high gain antenna, which would add to the expense and complexity of a relay mission.
What is the low-gain antenna, and where is it located?
The low-gain antenna is located on the tip of the cone (the "feed") sticking up over
the High Gain Antenna.
It broadcasts its signal over a wide cone (the half-angle of the cone is
almost 120 degrees), which allows it to remain in communication with Earth
even when it's not pointed directly at the Earth. This is unlike the
high-gain antenna, which sends out a much more narrowly directed signal
(with a half-angle of 1/6th of a degree). In both antennas, the same
amount of power is being transmitted, but the low-gain spreads that
power out over a much larger area of the sky than does the high gain.
This is somewhat like the difference between a bare bulb and a spotlight,
both operating at the same power. Since the ground antennas that are
receiving Galileo's signal need to receive a certain amount of power
in order to actually "hear" that signal, the low gain can't handle
nearly as high a data rate as the high gain antenna--during Jupiter
operations, the low gain's top data rate will be 160 bits per second,
compared to the high gain's 134,400 bits per second.
There is a second low-gain antenna on the spacecraft--the
long, thin antenna that hangs off of the short boom that holds the
radioisotope thermoelectric generators--but it is stowed away permanently.
The geometrical situation that you describe with occurs when the Sun lies
directly between Jupiter and Earth (scientists refer to this as "conjunction").
The Sun is a strong source of electromagnetic activity, and it wreaks havoc
with the spacecraft's radio signal, essentially reducing the spacecraft's data
rate to Earth to 0 for the two and a half weeks centered around conjunction.
Mission planners and telemetry engineers define this problem area as
occurring when the Sun-Earth-Galileo angle is less than 7 degrees
(see figure below); a
relatively "quiet" Sun can mean that data can be successfully
returned at angles as small as 3-5 degrees.
Galileo's two conjunction periods will run from December 11-28, 1995,
and January 11-28, 1997. Conjunction lowers the amount of data that can
be returned to Earth. However, Galileo still has roughly two years in which
to investigate the Jovian system, so not being able to return data for 18 days
out of those two years is not a serious difficulty.
How does the spacecraft know how to orient itself?
When spacecraft engineers refer to attitude, they're
discussing how
the spacecraft is oriented, or pointed, with respect to some unmoving
reference, like the stars. This is one of the jobs of the Attitude and
Articulation Control Subsystem, or AACS, which performs a number of
functions for the Spacecraft, including:
If you're interested in more information, look at the
article on attitude and
articulation found in JPL's
Basics of Space Flight Workbook.
There is also an older
Galileo Messenger article on the AACS.
How does the Galileo spacecraft's software work? What language was
the software written in?
Before we start to discuss software, we need to know a little about the
hardware, since there can be up to 18 microcomputers running on Galileo
at any given time.
First off, although all eleven science instruments use microprocessors,
only eight of the instruments are actually reprogrammable in flight (some technical details are available;
additional information on the science instruments can be found in
back
issues of the Galileo Messenger).
Only two of the major engineering subsystems--the
Attitude and Articulation Control Subsystem
(AACS) and Command and Data Subsystem
(CDS)--are programmable in flight. These two use very different
computer architectures.
Each of the AACS's attitude functions requires a tremendous amount of
mathematical computation, all of which is the job of the AACS software.
Just like the word processor or spreadsheet on a home computer system
gets revised into newer, sometimes better versions, the AACS software
has evolved from what was originally launched with the spacecraft in
1989--except that the AACS improvements really are better! The
software will continue to evolve for the remainder of the mission.
An additional technical description of the
AACS software discusses the AACS hardware, programming
language, and operating system. There is also a
Galileo
Messenger article on the AACS available.
The Command and Data Subsystem (CDS) and its software perform a
number of functions for the spacecraft. For example:
Spacecraft software listings are not available electronically. In paper
form, there are a lot of them and they take up a lot of shelf
space (volumes and volumes). Without a lot of supporting documentation,
they would be of little use since one needs to know a great deal about the
hardware in which software operates in order to understand what the
software is doing. Nor would such listings be of much value to anyone
other than someone working directly on the Galileo spacecraft team itself.
How is the spacecraft powered?
Galileo uses two Radioisotope Thermoelectric Generators (or RTGs for short) to generate electrical power. The RTGs power the spacecraft through the radioactive decay of plutonium-238. The decay emits heat, which is converted into electricity for the spacecraft to "see, sense, hear, and speak." Each RTG is mounted on a 5 meter long boom.
The spacecraft was able to generate 570 watts of electric power at launch;
we expect that it will be able to produce 480 watts when the spacecraft actually
arrives at Jupiter in December of 1995.
What type of fuel do Galileo's engines use?
Galileo uses monomethylhydrazine for fuel. The fuel has to be oxidized in order to ignite, so nitrogen tetroxide is mixed into the fuel right before a burn. There are two separate tanks of helium pressurant. In all, Galileo carried 932 kg (2,050 lbs) of propellant at launch.
Why do we have to use plutonium? Why not use solar panels?
Spacecraft that travel at or within Earth's orbit can use solar energy to power their instruments. However, at the great distance of Jupiter, the only feasible power source means using Galileo's Radioisotope Thermal Generators (or RTGs for short). Galileo would need a minimum of 700 to 1,600 square feet of solar panels--a solar panel about the size of a house!
Unlike other power sources, the RTGs are insensitive to the freezing cold of space, and are virtually invulnerable to high radiation fields, such as Earth's Van Allen belts and Jupiter's magnetosphere.
For additional details, and a discussion of the safety review conducted on
the RTGs, see What's in an RTG?
What spinoff technology has Galileo produced?
A few of the inventions we now enjoy were originally developed for Galileo. Charge-coupled devices like those in Galileo's television systems are used in some of our home video cameras, yielding sharper images than ever conceived of in the days before the project began. In addition, radiation-resistant components developed for Galileo are now used in research, businesses, and military applications where radiation environment is a concern. Another advance, integrated circuits resistant to cosmic rays, has helped to handle disturbances to computer memory that are caused by high-energy particles; these disturbances plague extremely high-speed computers on Earth and all spacecraft.
Imaging data will make up less than 25% of all of the "bits" of scientific data
returned ("downlinked") to Earth during the orbital tour. The remainder of the
downlink budget will be used to return data from other scientific
instrumentation, such as infrared, ultraviolet, and fields and particles
observations.
It can be argued that more has been learned about the solar system from images
than from any other type of instrumentation. Most astronomical discoveries
through the ages have been made through either visual observations or using
cameras attached to powerful telescopes. The same is true for solar system
exploration using robotic spacecraft. That is why virtually every planetary
exploration mission has included a camera in its payload. The value of images
for public information is a wonderful bonus, but if it were not for the
scientific utility of these cameras, few would be flown.
A simple review of some of the major discoveries and scientific studies that
have been made possible through spacecraft images illustrates the valuable
contributions pictures have made to our understanding of the solar system.
So why aren't we sending back even more pictures, since they add so much to
scientist's understanding of the solar system? As the question noted, it's
expensive to send back pictures, compared to fields and particles data.
However, it is the mix of both types of data that will yield the most complete
and scientifically interesting picture of the jovian system.
Consider some of the goals of the imaging team: long-term studies of Io's active
volcanoes and specific features in Jupiter's atmosphere, for starters. Both of
these phenomena show changes from day to day, but the quick Voyager flybys
didn't allow scientists to see how these changes evolved. We need
pictures in order to, for example, map out the surface of the Galilean
satellites (Jupiter's four largest moons), or to detect ring particles.
Pictures are also used to support other types of scientific observations. For
example, the atmospheric Probe will send back data on the temperature, pressure,
and composition of the atmosphere, but it will be images from the Orbiter that
will help scientists to put the Probe observations into context. In another
example, the fields and particles instruments on board the orbiter will map and
characterize the distribution of magnetic fields, plasma, and particles, but
pictures of Jupiter's auroral phenomena, which are intimitely connected with
fields and particles, will add additional insight into the magnetic field's
interaction with Jupiter's atmosphere.
Galileo images will show up on the Galileo Home Page throughout the mission.
If you're interested in hard copies, the following vendors make JPL/NASA photographic and video press-release products available to the general public.
CD-ROM images from Galileo are available from the
National Space Science Data Center .
Press-released videotapes (for example, a time lapse movie of the earth rotating) are also available; please contact the JPL Public Information Office for further information.
I'm a teacher who needs pictures for my classroom.
The Teaching Resource Center can be of help.
How can I watch Galileo press conferences?
If you have a satellite dish, you can find NASA TV on Spacenet 2, transponder 5, channel 9, 69 degrees West,. Transponder frequency is 3880 MHz, audio subcarrier is 6.8 MHz, polarization is horizontal.
Where can I get pictures of Comet Shoemaker-Levy-9 impacting Jupiter?
Galileo's pictures of the impact
are available online. Also, JPL has a SL-9 homepage . There are literally hundreds of downloadable images available from this site.
Return to Project Galileo Homepage Galileo's Antennas
Navigation
Galileo's Computers
Power
Of what scientific value are the pictures that are sent back from Galileo?
Or are they only used for public relations? After all, it takes so much time to
send back one picture!
Sources for Pictures
Where can I get pictures from the Spacecraft? Can I get images on CD?
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