Online Encyclopedia
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance man Galileo Galilei, it was launched on October 18 1989 by the Space Shuttle Atlantis on the STS-34 mission and arrived at Jupiter on December 7 1995.
The Galileo spacecraft conducted the first asteroid flyby, discovered the first asteroid moon, was the first Jupiter orbiter and launched the first probe into Jupiter's atmosphere.
On September 21, 2003, after 14 years of flight time and 8 years of service in the Jovian system, Galileos mission was terminated by sending the orbiter into Jupiter's crushing atmosphere at a speed of nearly 50 kilometres per second to avoid any chance of it contaminating local moons with bacteria from Earth. Of particular concern was the ice crusted moon Europa, which, thanks to Galileo, scientists now suspect harbors a salt water ocean—and possibly microbial life—beneath its surface.
Mission overview
Galileo's launch had been significantly delayed by the hiatus in Space Shuttle launches that occurred after the Space Shuttle Challenger disaster. New safety protocols that were implemented as a result of the explosion forced Galileo to use a lower powered upper stage booster rocket to send it from Earth orbit to Jupiter; several additional gravitational slingshots (once by Venus and twice by Earth), commonly called a "VEEGA" or Venus Earth Earth Gravity Assist maneuver, was required in order to give it enough velocity to reach its target. Along the way, Galileo performed close observation of the asteroids 951 Gaspra (October 29, 1991) and 243 Ida, and discovered Ida's moon Dactyl. In 1994, Galileo was perfectly positioned to watch the fragments of comet Shoemaker-Levy 9 crash into Jupiter. Earth based telescopes had to wait to see the impact sites as they rotated into view.
Galileo's prime mission was a two year study of the Jovian system. Galileo traveled around Jupiter in elongated ellipses; each orbit lasted about two months. By traveling at different distances from Jupiter, Galileo could sample different parts of the planet's extensive magnetosphere. The orbits were designed for close up flybys of Jupiter's largest moons. Once Galileo's primary mission was concluded, an extended mission followed starting on December 7 1997; the spacecraft made a number of daring close flybys of Jupiter's moons Europa and Io. The closest approach was 112 miles (180 km) on October 15, 2001. The radiation environment near Io in particular was very unhealthy for Galileo's systems, and so these flybys were saved for the extended mission when loss of the spacecraft would be more acceptable.
Galileo's cameras were deactivated on January 17 2002 after they had sustained irrecoverable radiation damage. NASA engineers were able to recover the damaged tape recorder electronics, and once more Galileo continued to return other scientific data until it was deorbited on September 21 2003 by impacting Jupiter in order to avoid an uncontrolled collision of the unsterilized probe and a Jovian moon. This was done because it is thought that some Jovian moons might harbor microbial life and a crash of Galileo on one of these moons would contaminate any future investigation and analysis.
The Galileo spacecraft
The Jet Propulsion Laboratory built the Galileo spacecraft and managed the Galileo mission for NASA. Germany supplied the propulsion module. NASA's Ames Research Center managed the probe, which was built by Hughes Aircraft Company.
At launch, the orbiter and probe together had a mass of almost 2,700 kilograms and was seven meters tall. One section of the spacecraft rotated at 3 rpm, keeping Galileo stable and holding six instruments that gathered data from many different directions, including the fields and particles instruments. The other section of the spacecraft held steady for cameras and the four instruments that had to point accurately while Galileo was flying through space. This was the job of the attitude control system (see below).
The spacecraft was controlled by an RCA 1802 microprocessor CPU fabricated on sapphire (Silicon on Sapphire) which is a radiation and static hardened version, ideal for spacecraft operation. Galileo 's attitude control system software was written in the HAL/S programming language, also used in the Space Shuttle program. The 1802 CPU had previously been used onboard the Voyager and Viking spacecraft.
Instrumentation overview
Scientific instruments to measure fields and particles, together with the main antenna, the power supply, the propulsion module, most of the computers and control electronics, were mounted on the spinning section. The instruments included magnetometer sensors, mounted on an 11 meter boom to minimize interference from the spacecraft; a plasma instrument detecting low energy charged particles and a plasma wave detector to study waves generated by the particles; a high energy particle detector; and a detector of cosmic and Jovian dust. It also carried the Heavy Ion Counter, an engineering experiment added to assess the potentially hazardous charged particle environments the spacecraft flew through, and an added Extreme Ultraviolet detector associated with the UV spectrometer on the scan platform.
The despun section's instruments included the camera system; the near infrared mapping spectrometer to make multispectral images for atmospheric and moon surface chemical analysis; the ultraviolet spectrometer to study gases; and the photopolarimeter radiometer to measure radiant and reflected energy. The camera system was designed to obtain images of Jupiter's satellites at resolutions from 20 to 1,000 times better than Voyager's best, because Galileo flew closer to the planet and its inner moons and because the CCD sensor in Galileos camera was more sensitive and had a broader color detection band than the vidicons of Voyager.
Instrumentation details
The following information was taken directly from NASA's Galileo legacy site.
Dust Detector Subsystem (DDS)
The Dust Detector Subsystem (DDS) was used to measure the mass, electric charge, and velocity of incoming particles. The masses of dust particles that the DDS can detect go from 10-16 to 10-7 grams. The speed of these small particles can be measured over the range of 1 to 70 kilometers per second. The instrument can measure impact rates from 1 particle per 115 days to 100 particles per second. These particles will help determine dust origin and dynamics within the magnetosphere. The DDS weighs 4.2 kilograms and uses an average of 5.4 watts of power.[1] DDS Team site:[2]
Energetic Particles Detector (EPD)
The energetic particles detector (EPD) is designed to measure the numbers and energies of ions and electrons whose energies exceed about 20 keV (3.2 fJ). The EPD can also measure the direction of travel of such particles and, in the case of ions, can determine their composition (whether the ion is oxygen or sulfur, for example). The EPD uses silicon solid state detectors and a time-of-flight detector system to measure changes in the energetic particle population at Jupiter as a function of position and time. These measurements will tell us how the particles get their energy and how they are transported through Jupiter's magnetosphere. The EPD weighs 10.5 kilograms and uses 10.1 watts of power on average.[3] EPD Team site:[4]
Ultraviolet Spectrometer / Extreme Ultraviolet Spectrometer (UVS/EUV)
The Cassegrain telescope of the UVS has a 250 mm aperture and collects light from the observation target. Both the UVS and EUV instruments use a ruled grating to disperse this light for spectral analysis. This light then passes through an exit slit into photomultiplier tubes that produce pulses or "sprays" of electrons. These electron pulses are counted, and these count numbers are the data that are sent to Earth. The UVS is mounted on the scan platform and can be pointed to an object in inertial space. The EUV is mounted on the spun section of the spacecraft. As Galileo spins, the EUV observes a narrow ribbon of space perpendicular to the spin axis. The two instruments combined weigh about 9.7 kilograms and use 5.9 watts of power.[5] EUV Team site:[6]
Heavy Ion Counter (HIC)
The HIC is really a repackaged and updated version of some parts of the flight spare of the Voyager Cosmic Ray System. The HIC detects heavy ions using stacks of single crystal silicon wafers. The HIC can measure heavy ions with energies as low as 6 MeV (1 pJ) and as high as 200 MeV (32 pJ) per nucleon. This range includes all atomic substances between carbon and nickel. The HIC and the EUV share a communications link and, therefore, must share observing time. The HIC weighs 8 kilograms and uses an average of 2.8 watts of power.[7] HIC Team site:[8]
Magnetometer (MAG)
The magnetometer (MAG) uses two sets of three sensors. The three sensors allow the three orthogonal components of the magnetic field section to be measured. One set is located at the end of the magnetometer boom and, in this position, is about 11 meters from the spin axis of the spacecraft. The second set, designed to detect stronger fields, is 6.7 meters from the spin axis. The boom is used to remove the MAG from the immediate vicinity of the spacecraft to minimize magnetic effects from the spacecraft. However, not all these effects can be eliminated by distancing the instrument. The rotation of the spacecraft is used to separate natural magnetic fields from engineering induced fields. Another source of potential error in measurement comes from bending and twisting of the long magnetometer boom. To account for these motions, a calibration coil is mounted rigidly on the spacecraft and puts out a reference magnetic field during calibrations. The magnetic field at the surface of the Earth has a strength of about 50,000 nT (nanoTesla). At Jupiter, the outboard (11 meter) set of sensors can measure magnetic field strengths in the range from +/- 32 to +/- 512 nT while the inboard (6.7 m) set is active in the range from +/- 512 to +/- 16,384 nT. The MAG experiment weighs 7 kilograms and uses 3.9 watts of power.[9] MAG Team site:[10]
Near-Infrared Mapping Spectrometer (NIMS)
The NIMS instrument is sensitive from 0.7 to 5.2 micrometre wavelength IR light, overlapping the wavelength range of SSI. The telescope associated with NIMS is all reflective (uses mirrors and no lenses) with an aperture of 229 mm. The spectrometer of NIMS uses a grating to disperse the light collected by the telescope. The dispersed spectrum of light is focused on detectors of indium antimonide and silicon. The NIMS weighs 18 kilograms and uses 12 watts of power on average. [11] NIMS Team site:[12]
Plasma Subsystem (PLS)
The PLS uses seven fields of view to collect charged particles for energy and mass analysis. These fields of view cover most angles from 0 to 180 degrees, fanning out from the spin axis. The rotation of the spacecraft carries each field of view through a full circle. The PLS will measure particles in the energy range from 0.9 eV to 52 KeV. The PLS weighs 13.2 kilograms and uses an average of 10.7 watts of power.[13] PLS Team site:[14]
Photopolarimeter-Radiometer (PPR)
The PPR has seven radiometry bands. One of these uses no filters and observes all the radiation, both solar and thermal. Another band lets only solar radiation through. The difference between the solar- plus-thermal and the solar-only channels gives the total thermal radiation emitted. The PPR also measured in five broadband channels that span the spectral range from 17 to 110 micrometres. The radiometer provides data on the temperatures of the Jovian satellites and Jupiter's atmosphere. The design of the instrument is based on that of an instrument flown on the Pioneer Venus spacecraft. A 100 mm aperture reflecting telescope collects light, directs it to a series of filters, and, from there, measurements are performed by the detectors of the PPR. The PPR weighs 5.0 kilograms and consumes about 5 watts of power.[15] PPR Team site:[16]
Plasma Wave Subsystem (PWS)
An electric dipole antenna is used to study the electric fields of plasmas, while two search coil magnetic antennas studied the magnetic fields. The electric dipole antenna is mounted at the tip of the magnetometer boom. The search coil magnetic antennas are mounted on the high-gain antenna feed. Nearly simultaneous measurements of the electric and magnetic field spectrum allowed electrostatic waves to be distinguished from electromagnetic waves. The PWS weighs 7.1 kilograms and uses an average of 9.8 watts.[17] PWS Team site:[18]
Solid State Imager (SSI)
The SSI is an 800 by 800 pixel solid state camera consisting of an array of silicon sensors called a "charge coupled device" (CCD). The optical portion of the camera is built as a Cassegrain telescope. Light is collected by the primary mirror and directed to a smaller secondary mirror that channels it through a hole in the center of the primary mirror and onto the CCD. The CCD sensor is shielded from radiation, a particular problem within the harsh Jovian magnetosphere. The shielding is accomplished by means of a 10 mm thick layer of tantalum that surrounds the CCD except, of course, where the light enters the system. An eight position filter wheel is used to obtain images of scenes through different filters. The images may then be combined electronically on Earth to produce color images. The spectral response of the SSI ranges from about 0.4 to 1.1 micrometres. The SSI weighs 29.7 kilograms and consumes, on average, 15 watts of power.[19] SSI Imaging Team site:[20]
Galileo's atmospheric entry probe
The 320 kilogram atmospheric probe measured about 1.3 meters across. Inside the heat shield, the scientific instruments were protected from ferocious heat during entry. The probe had to withstand extreme heat and pressure on its high speed journey at 47.8 km/s. The probe was released from the main spacecraft in July 1995, five months before reaching Jupiter, and entered Jupiter's atmosphere with no braking beforehand. It slowed, deployed its parachute, and dropped its heat shield. As the probe descended through 150 kilometers of the top layers of the atmosphere, it collected 58 minutes of data on the local weather. The data were sent to the spacecraft overhead, then transmitted back to Earth. Total data returned from the probe was about 3.5 megabits. The probe was likely eventually crushed by the increasing extreme pressure of the atmosphere and then quickly melted and vaporized by the intense heat of the atmospheric "hot spot" it fell into.
Unique non-Jupiter related science done with Galileo
The late Carl Sagan, pondering the question of whether life on earth could be easily detected from space, devised a set of experiments in the late 1980s using Galileo's remote sensing instruments to determine if life indeed could be detected during the first earth flyby of the mission in December of 1990. After data acquisition and processing, Sagan et. al. published a paper in Nature in 1993 detailing the results of the experiment. Galileo had found what are now referred to as the "Sagan criteria for life"; these were: strong absorption of light at the red end of the visible spectrum (especially over continents) which was caused by chlorophyll in photosynthesizing plants, absorption bands of molecular oxygen which is also a result of plant activity, infrared absorption bands caused by the ~1ppm of methane in earth's atmosphere, a gas that must be replenished by either volcanic or biological activity, and modulated narrowband radio wave transmissions uncharacteristic of any known natural source. Galileo's experiments were thus the first ever controls in the newborn science of astrobiological remote sensing.
In December of 1992 during Galileo's second gravity assist flyby of earth, another groundbreaking yet almost entirely unpublicized experiment was done using Galileo to assess the possibility of optical communication with spacecraft by detecting pulses of light from powerful lasers which were to be directly imaged by Galileo's CCD. The experiment, dubbed Galileo OPtical EXperiment or GOPEX, used two separate sites to beam laser pulses to the spacecraft, one at Table Mountain Observatory in California and the other at the Starfire Optical Range in New Mexico. The Table Mountain site used a frequency doubled Neodymium:Yttrium Aluminum Garnet (Nd:YAG) laser operating at 532 nm with a repetition rate of ~15 to 30 Hz and a pulse power (FWHM) in the megawatt range, which was coupled to a 0.6 meter Caassegrain telescope for transmission to Galileo, the Starfire range site used a similar setup with a larger transmitting telescope (1.5 m). Long exposure (~0.1 to 0.8 s) images using Galileo's 560 nm centered green filter produced images of earth clearly showing the laser pulses even at distances of up to 6,000,000 km.
Spacecraft malfunctions
Main antenna failure
For reasons which are not currently known, and in all likelihood will never be known with certainty, Galileos High Gain Antenna failed to fully deploy after its first flyby of Earth. Investigators speculate that during the time that Galileo spent in storage after the Challenger disaster lubricants evaporated, or the system was otherwise damaged. Engineers tried thermal cycling the antenna, rotating the spacecraft up to its maximum spin rate of 10.5 rpm, and "hammering" the antenna deployment motors - turning them on and off repeatedly - over 13,000 times; all attempts failed to open the high gain antenna. Fortunately Galileo had an additional Low Gain Antenna that was capable of transmitting information back to Earth, though since it transmitted a signal isotropically the Low Gain Antenna's bandwidth was significantly less than the high gain antenna's would have been; the high gain antenna was to have transmitted at 134 kilobits per second whereas the low gain antenna was only intended to transmit at about 8 to 16 bits per second. Galileo's low gain antenna transmitted with a power of about 15 to 20 watts, which, by the time it reached earth, and had been collected by one of the large aperture (70 m) DSN antennas, had a total power of about -170 dBm or 10 zeptowatts (10 × 10−21 watts).[21] Through implementation of sophisticated data compression techniques, arraying of several Deep Space Network antennas and sensitivity upgrades of receivers used to listen to Galileo's signal, data throughput was increased to a maximum of 160 bits per second. The data collected on Jupiter and its moons were stored in the on board tape recorder, and transmitted back to Earth during the long apogee portion of the probe's orbit using the low gain antenna. At the same time, measurements were made of Jupiter's magnetosphere and transmitted back to Earth. The reduction in available bandwidth reduced the number of pictures that were transmitted significantly; in all, only 14,000 images were returned.
Tape recorder anomalies and remote repair
Since Galileo's high gain antenna failed to open in 1991 the mission was forced to use the low gain antenna for all communication to earth. This meant that data storage to Galileo's tape recorder for later compression and playback was absolutely crucial in order to obtain any substantial information from the planned Jupiter and moon flybys. In October of 1995 Galileo's 109 Megabyte tape recorder, after recording an image of Jupiter, remained stuck in rewind mode for 15 hours before engineers learned what happened and sent commands to shut it off. Though the recorder itself was still in working order the malfunction possibly damaged a length of tape at the end of the reel. This section of tape was subsequently declared "off limits" to any future data recording and was covered with 25 more turns of tape to secure the section and reduce any further stresses, which could tear it. Because it happened only weeks before Jupiter Orbit Insertion, the anomaly prompted engineers to sacrifice data acquisition of almost all of the Io and Europa observations during Jupiter Orbit Insertion in order to focus solely on recording data sent from the Jupiter probe descent.
In November of 2002, after completion of the mission's only encounter of Jupiter's moon Amalthea, problems with playback of the tape recorder would again plague the spacecraft. About 10 minutes after closest approach of the flyby Galileo stopped collecting data, shut down all of its instruments, and went into "safe mode"; apparently as a result of exposure to Jupiter's extremely high radiation environment. Though most of the Amalthea data was already written to tape, it was found that the recorder refused to respond to commands telling it to play back data. Through careful analysis [22] after weeks of troubleshooting of an identical flight spare of the recorder on the ground, it was determined that the cause of the malfunction was a reduction of light output in 3 infrared Optek OP133 [23] light emitting diodes located in the drive electronics of the recorder's motor encoder wheel. The GaAs LEDs had been particularly sensitive to proton irradiation induced atomic lattice displacement defects, which greatly decreased their effective light output and caused the drive motor's electronics to falsely believe the motor encoder wheel was incorrectly positioned. Galileo's flight team then began a series of "annealing" sessions, where current was passed through the LEDs for hours at a time to heat them to a point where some of the crystalline lattice defects would be shifted back into place, thus increasing the LED's light output. After about 100 hours of annealing and playback cycles, the recorder was able to operate for up to an hour at a time. After many subsequent playback and cooling cycles, the complete transmission back to earth of all recorded Amalthea flyby data was successful.
Future of Jupiter exploration
After the end of the Galileo mission and in the light of the discoveries Galileo made, NASA is planning a future Jupiter mission called JIMO: Jupiter Icy Moons Orbiter. The JIMO mission is in its early planning stage and liftoff is not to be expected before 2012.
External links
- Galileo home page
- Jupiter Icy Moons Orbiter
- Site explaining the LGA bandwidth upgrades from the Parkes Observatory
- GOPEX site from JPL
- NASA site on Galileo life detection experiments