The Galileo spacecraft during preparations for its mating with the IUS booster. |
|
| Operator | NASA |
|---|---|
| Mission type | Orbiter, Fly-by |
| Flyby of | Venus, Earth, 951 Gaspra, 243 Ida |
| Satellite of | Jupiter |
| Orbital insertion date | 1995-12-08 01:20:00 UTC |
| Launch date | 1989-10-18 16:53:00 UTC |
| Launch vehicle | Space Shuttle Atlantis Inertial Upper Stage |
| Launch site | KSC Launch Complex 39B Kennedy Space Center |
| Mission duration | December 8, 1995 - September 21, 2003 (deorbited 2003-09-21 18:57:00 UTC) |
| COSPAR ID | 1989-084B |
| Homepage | Galileo Project Home Page |
| Mass | 2,380 kg (5,200 lb) |
| Power | 570 W (2 RTGs) |
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission. It arrived at Jupiter on December 7, 1995, a little more than six years later, via gravitational assist flybys of Venus and Earth.
Despite suffering from antenna problems, Galileo conducted the first asteroid flyby near 951 Gaspra, discovered the first asteroid moon, Dactyl, around the asteroid 243 Ida, and was the first spacecraft to orbit Jupiter. It furthermore launched the first probe into Jupiter's atmosphere.[1]
The spacecraft measured the atmospheric composition of Jupiter and directly observed ammonia clouds, which seem to be created by an outflow from the lower depths of Jupiter's atmosphere. Galileo also registered Io's volcanism and the plasma interactions between its and Jupiter's atmospheres. Other studies gave support for the popular theory of liquid oceans under the icy surface of Europa. There were also indications of similar liquid-saltwater layers under the surfaces of Ganymede and Callisto, while Ganymede was shown to possess a magnetic field. New evidence was also found for the existence of exospheres around Europa, Ganymede, and Callisto.[1]
It was furthermore discovered that Jupiter's faint ring system is formed by dust from impacts on the four small inner moons. The extent and structure of Jupiter's magnetosphere was also mapped.[1]
Galileo also provided the only direct observation of Comet Shoemaker-Levy 9's impact into the atmosphere of Jupiter in 1994.[1]
On September 21, 2003, after 14 years in space and 8 years of service in the Jovian system, Galileo's mission was terminated by sending the orbiter into Jupiter's 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 potentially life-supporting saltwater ocean beneath its surface.
Galileo's launch had been significantly delayed. Early plans saw it being launched by Space Shuttle Columbia on what was then codenamed STS-23 in January 1982, but delays in the development of the Space Shuttle allowed more time for development of the probe.
Once it was complete, its launch was scheduled for STS-61-G on-board Atlantis in 1986. It was to use the Centaur-G liquid hydrogen-fueled booster stage for a direct trajectory to Jupiter. However, the mission was further delayed by the hiatus in launches that occurred after the Space Shuttle Challenger disaster. New safety protocols introduced as a result of the disaster prohibited the use of the Centaur-G stage on the Shuttle, forcing Galileo to use a lower-powered Inertial Upper Stage solid-fuel booster. The mission was re-profiled to use several gravitational slingshots, referred to as the "VEEGA" or Venus Earth Earth Gravity Assist maneuvers, to provide the additional velocity required to reach its destination. Venus was flown by at 05:58:48 UT on February 10, 1990 at a range of 16,106 km. Having gained 8,030 km per hour in speed, the spacecraft flew by Earth twice, the first time at a range of 960 km at 20:34:34 UT on 8 December 1990 before approaching the minor planet 951 Gaspra to a distance of 1,604 km at 22:37 UT on 29 October 1991. Galileo then performed a second flyby of Earth at 303.1 km at 15:09:25 UT on 8 December 1992, adding 3.7 km per second to its cumulative speed. Galileo performed close observation of a second asteroid, 243 Ida, at 16:51:59 UT on 28 August 1993 at a range of 2,410 km. The spacecraft discovered Ida has a moon Dactyl, the first discovery of a natural satellite orbiting an asteroid. In 1994, Galileo was perfectly positioned to watch the fragments of the comet Shoemaker-Levy 9 crash into Jupiter, whereas terrestrial telescopes had to wait to see the impact sites as they rotated into view. After releasing its atmospheric probe on 13 July 1995, the Galileo orbiter became the first man-made satellite of Jupiter at 00:27 UT on 8 December 1995 when it fired its main engine to enter a 198-day parking orbit.[2]
Galileo's prime mission was a two-year study of the Jovian system. The spacecraft traveled around Jupiter in elongated ellipses, each orbit lasting about two months. The differing distances from Jupiter afforded by these orbits allowed Galileo to 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 prime 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 180 km (110 mi) 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 irreparable radiation damage. NASA engineers were able to recover the damaged tape recorder electronics, and Galileo continued to return scientific data until it was deorbited in 2003, performing one last scientific experiment —a measurement of the moon Amalthea's mass as the spacecraft swung by it.
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 2,564 kilograms (5,653 pounds) and stood seven metres 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 was an antenna, and data were periodically transmitted to it. Back on the ground, the mission operations team used software containing 650,000 lines of programming code in the orbit sequence design process; 1,615,000 lines in the telemetry interpretation; and 550,000 lines of code in navigation.
The CDH subsystem was actively redundant, with two parallel data system buses running at all times.[3] Each data system bus (aka string) was composed of the same functional elements, consisting of multiplexers (MUX), high-level modules (HLM), low-level modules (LLM), power converters (PC), bulk memory (BUM), data management subsystem bulk memory (DBUM), timing chains (TC), phase locked loops (PLL), Golay coders (GC), hardware command decoders (HCD) and critical controllers (CRC).
The CDH subsystem was responsible for maintaining the following functions:
The spacecraft was controlled by six RCA 1802 Cosmac microprocessor CPUs: four on the spun side and two on the despun side. Each CPU was clocked at about 1.6 MHz, and fabricated on sapphire (silicon on sapphire), which is a radiation-and static-hardened material ideal for spacecraft operation. This microprocessor was the first low-power CMOS processor chip, quite on a par with the 8-bit 6502 that was being built into the Apple II desktop computer at that time. Galileo's attitude control system software was written in the HAL/S programming language, also used in the Space Shuttle program.
Memory capacity provided by each BUM was 16K of RAM, while the DBUMs each provided 8K of RAM. There were two BUMs and two DBUMs in the CDH subsystem and they all resided on the spun side of the spacecraft. The BUMs and DBUMs provided storage for sequences and contain various buffers for telemetry data and interbus communication.
Every HLM and LLM was built up around a single 1802 microprocessor and 32K of RAM (for HLMs) or 16K of RAM (for LLMs). Two HLMs and two LLMs resided on the spun side while two LLMs were on the despun side.
Thus, total memory capacity available to the CDH subsystem was 176K of RAM: 144K allocated to the spun side and 32K to the despun side.
Each HLM was responsible for the following functions:
Each LLM was responsible for the following functions:
The HCD receives command data from the modulation/demodulation subsystem, decodes these data and transfers them to the HLMs and CRCs.
The CRC controls the configuration of CDH subsystem elements. It also controls access to the two data system buses by other spacecraft subsystems. In addition, the CRC supplies signals to enable certain critical events (e.g. probe separation).
The GCs provide Golay encoding of data via hardware.
The TCs and PLLs establish timing within the CDH subsystem.
The Propulsion Subsystem consisted of a 400 N main engine and twelve 10 N thrusters, together with propellant, storage and pressurizing tanks and associated plumbing. The 10 N thrusters were mounted in groups of six on two 2-meter booms. The fuel for the system was 925 kg of monomethylhydrazine and nitrogen tetroxide. Two separate tanks held another 7 kg of helium pressurant. The Propulsion Subsystem was developed and built by Daimler Benz Aero Space AG (DASA) (formerly Messerschmitt–Bölkow–Blohm (MBB)) and provided by Germany, the major international partner in Project Galileo.[4]
Solar panels were not a practical solution for Galileo's power needs at Jupiter's distance from the Sun (it would have needed a minimum of 65 square metres (700 sq ft) of solar panels); as for batteries, they would have been prohibitively massive due to the technological limitations of the time. The solution adopted consisted of two radioisotope thermoelectric generators (RTGs). The RTGs powered the spacecraft through the radioactive decay of plutonium-238. The heat emitted by this decay was converted into electricity for the spacecraft through the solid-state Seebeck effect. This provided a reliable and long-lasting source of electricity unaffected by the cold space environment and high radiation fields such as those encountered in Jupiter's magnetosphere.
Each GPHS-RTG, mounted on a 5-meter long boom, carried 7.8 kilograms (17.2 lb) of 238Pu.[5] Each RTG contained 18 separate heat source modules, and each module encased four pellets of plutonium dioxide, a ceramic material resistant to fracturing. The modules were designed to survive a range of hypothetical accidents: launch vehicle explosion or fire, re-entry into the atmosphere followed by land or water impact, and post-impact situations. An outer covering of graphite provided protection against the structural, thermal, and eroding environments of a potential re-entry. Additional graphite components provided impact protection, while iridium cladding of the fuel cells provided post-impact containment. The RTGs produced about 570 watts at launch. The power output initially decreased at the rate of 0.6 watts per month and was 493 watts when Galileo arrived at Jupiter.
As the launch of Galileo neared, anti-nuclear groups, concerned over what they perceived as an unacceptable risk to the public's safety from Galileo's RTGs, sought a court injunction prohibiting Galileo's launch. RTGs had been used for years in planetary exploration without mishap: the Lincoln Experimental Satellites 8/9, launched by the U.S. Department of Defense, had 7% more plutonium on board than Galileo, and the two Voyager spacecraft each carried 80% as much plutonium as Galileo did. However, activists remembered the messy crash of the Soviet Union's nuclear-powered Cosmos 954 satellite in Canada in 1978, and the 1986 Challenger accident had raised public awareness of the possibility of explosive spacecraft failures. Also, no RTGs had ever been made to swing past the Earth at close range and high speed, as Galileo's Venus-Earth-Earth Gravity Assist trajectory required it to do. This created a novel mission failure modality that might plausibly have entailed total dispersal of Galileo's plutonium in the Earth's atmosphere. Scientist Carl Sagan, for example, a strong supporter of the Galileo mission, said in 1989 that "there is nothing absurd about either side of this argument." [6]
After the Challenger accident, a study considered additional shielding and eventually rejected it, in part because such a design significantly increased the overall risk of mission failure and only shifted the other risks around (for example, if a failure on orbit had occurred, additional shielding would have significantly increased the consequences of a ground impact).[5]
Scientific instruments to measure fields and particles were mounted on the spinning section of the spacecraft, together with the main antenna, power supply, the propulsion module and most of Galileo's computers and control electronics. The sixteen instruments, weighing 118 kg altogether, included magnetometer sensors mounted on an 11 m boom to minimize interference from the spacecraft; a plasma instrument for 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 multi-spectral images for atmospheric and moon surface chemical analysis; the ultraviolet spectrometer to study gases; and the photo-polarimeter 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 more modern CCD sensor in Galileo's camera was more sensitive and had a broader color detection band than the vidicons of Voyager.
The following information was taken directly from NASA's Galileo legacy site.[7]
The SSI was an 800-by-800-pixel solid state camera consisting of an array of silicon sensors called a "charge coupled device" (CCD). Galileo was one of the first spacecraft to be equipped with a CCD camera. The optical portion of the camera was built as a Cassegrain telescope. Light was collected by the primary mirror and directed to a smaller secondary mirror that channeled it through a hole in the center of the primary mirror and onto the CCD. The CCD sensor was shielded from radiation, a particular problem within the harsh Jovian magnetosphere. The shielding was accomplished by means of a 10 mm thick layer of tantalum surrounding the CCD except where the light enters the system. An eight-position filter wheel was used to obtain images at specific wavelengths. The images were then combined electronically on Earth to produce color images. The spectral response of the SSI ranged from about 0.4 to 1.1 micrometres. The SSI weighed 29.7 kilograms and consumed, on average, 15 watts of power.[8][9]
The NIMS instrument was sensitive to 0.7-to-5.2-micrometre wavelength IR light, overlapping the wavelength range of the SSI. The telescope associated with NIMS was all reflective (using only mirrors and no lenses) with an aperture of 229 mm. The spectrometer of NIMS used a grating to disperse the light collected by the telescope. The dispersed spectrum of light was focused on detectors of indium antimonide and silicon. The NIMS weighed 18 kilograms and used 12 watts of power on average.[10][11]
The Cassegrain telescope of the UVS had a 250 mm aperture and collected light from the observation target. Both the UVS and EUV instruments used a ruled grating to disperse this light for spectral analysis. This light then passed through an exit slit into photomultiplier tubes that produced pulses or "sprays" of electrons. These electron pulses were counted, and these count numbers constituted the data that were sent to Earth. The UVS was mounted on Galileo's scan platform and could be pointed to an object in inertial space. The EUV was mounted on the spun section of the spacecraft. As Galileo span, the EUV observed a narrow ribbon of space perpendicular to the spin axis. The two instruments combined weighed about 9.7 kilograms and used 5.9 watts of power.[12][13]
The PPR had seven radiometry bands. One of these used no filters and observed all incoming radiation, both solar and thermal. Another band allowed only solar radiation through. The difference between the solar-plus-thermal and the solar-only channels gave the total thermal radiation emitted. The PPR also measured in five broadband channels that spanned the spectral range from 17 to 110 micrometres. The radiometer provided data on the temperatures of the Jovian satellites and Jupiter's atmosphere. The design of the instrument was based on that of an instrument flown on the Pioneer Venus spacecraft. A 100 mm aperture reflecting telescope collected light and directed it to a series of filters, and, from there, measurements were performed by the detectors of the PPR. The PPR weighed 5.0 kilograms and consumed about 5 watts of power.[14][15]
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 could detect go from 10−16 to 10−7 grams. The speed of these small particles could be measured over the range of 1 to 70 kilometers per second. The instrument could measure impact rates from 1 particle per 115 days (10 megaseconds) to 100 particles per second. Such data was used to help determine dust origin and dynamics within the magnetosphere. The DDS weighed 4.2 kilograms and used an average of 5.4 watts of power.[16][17]
The Energetic Particles Detector (EPD) was designed to measure the numbers and energies of ions and electrons whose energies exceeded about 20 keV (3.2 fJ). The EPD could also measure the direction of travel of such particles and, in the case of ions, could determine their composition (whether the ion is oxygen or sulfur, for example). The EPD used 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 helped determine how the particles got their energy and how they were transported through Jupiter's magnetosphere. The EPD weighed 10.5 kilograms and used 10.1 watts of power on average.[18][19]
The HIC was in effect a repackaged and updated version of some parts of the flight spare of the Voyager Cosmic Ray System. The HIC detected heavy ions using stacks of single crystal silicon wafers. The HIC could measure heavy ions with energies as low as 6 MeV (1 pJ) and as high as 200 MeV (32 pJ) per nucleon. This range included all atomic substances between carbon and nickel. The HIC and the EUV shared a communications link and, therefore, had to share observing time. The HIC weighed 8 kilograms and used an average of 2.8 watts of power.[20][21]
The magnetometer (MAG) used two sets of three sensors. The three sensors allowed the three orthogonal components of the magnetic field section to be measured. One set was located at the end of the magnetometer boom and, in that position, was about 11 m from the spin axis of the spacecraft. The second set, designed to detect stronger fields, was 6.7 m from the spin axis. The boom was used to remove the MAG from the immediate vicinity of Galileo to minimize magnetic effects from the spacecraft. However, not all these effects could be eliminated by distancing the instrument. The rotation of the spacecraft was used to separate natural magnetic fields from engineering-induced fields. Another source of potential error in measurement came from the bending and twisting of the long magnetometer boom. To account for these motions, a calibration coil was mounted rigidly on the spacecraft to generate a reference magnetic field during calibrations. The magnetic field at the surface of the Earth has a strength of about 50,000 nT. At Jupiter, the outboard (11 m) set of sensors could measure magnetic field strengths in the range from ±32 to ±512 nT, while the inboard (6.7 m) set was active in the range from ±512 to ±16,384 nT. The MAG experiment weighed 7 kilograms and used 3.9 watts of power.[22][23]
The PLS used seven fields of view to collect charged particles for energy and mass analysis. These fields of view covered most angles from 0 to 180 degrees, fanning out from the spin axis. The rotation of the spacecraft carried each field of view through a full circle. The PLS measured particles in the energy range from 0.9 eV to 52 keV (0.1 aJ to 8.3 fJ). The PLS weighed 13.2 kilograms and used an average of 10.7 watts of power.[24][25]
An electric dipole antenna was used to study the electric fields of plasmas, while two search coil magnetic antennas studied the magnetic fields. The electric dipole antenna was mounted at the tip of the magnetometer boom. The search coil magnetic antennas were 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 weighed 7.1 kilograms and used an average of 9.8 watts.[26][27]
The Galileo probe's descent module. |
|
| Operator | NASA |
|---|---|
| Mission type | Atmospheric probe |
| Launch date | July 13, 1995 |
| Launch vehicle | Space Shuttle Atlantis Inertial Upper Stage Galileo Orbiter |
| Launch site | KSC Launch Complex 39B Kennedy Space Center |
| Mission duration | December 7, 1995 (57.6 minutes) |
| COSPAR ID | 1989-084E |
| Homepage | Galileo Project Home Page |
| Mass | 339 kg (750 lb) |
| Power | 580 W (LiSO2 battery) |
| Orbital elements | |
| Eccentricity | 1.03116 |
| Inclination | 8.151° |
| Periapsis | 0.9928 RJ |
The 339-kilogram atmospheric probe, built by Hughes Aircraft Company[28] at its El Segundo, California plant, 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 into the Jovian atmosphere, travelling 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. The probe was slowed from its arrival speed of about 47 kilometers per second to subsonic speed in less than two minutes.
This was by far the most difficult atmospheric entry ever attempted; the probe had to withstand 230 g[29] and the probe's 152 kg heat shield, making up almost half of the probe's total mass, lost 80 kg during the entry.[30][31] NASA built a special laboratory, the Giant Planet Facility, to simulate the heat load, which was similar to that of an ICBM-style straight-down reentry through a thermonuclear fireball.[32][33] It then deployed its 2.5-meter (8 ft) parachute, and dropped its heat shield.
As the probe descended through 150 kilometers of the top layers of the Jovian atmosphere, it collected 58 minutes of data on the local weather. It only stopped transmitting when ambient pressure exceeded 23 atmospheres and temperature reached 153 °C (307 °F).[34] The data was sent to the spacecraft overhead, then transmitted back to Earth. Each of 2 L-band transmitters operated at 128 bits per second and sent nearly identical streams of scientific data to the orbiter. All the probe's electronics were powered by lithium sulfur dioxide (LiSO2) batteries that provided a nominal power output of about 580 watts with an estimated capacity of about 21 ampere-hours on arrival at Jupiter.
The probe included six instruments for taking data on its plunge into Jupiter:
Total data returned from the probe was about 3.5 megabits (~458752 bytes). The probe stopped transmitting before the line of sight link with the orbiter was cut. The likely proximal cause of the final probe failure was overheating, which sensors indicated before signal loss.
The atmosphere through which the probe descended was somewhat hotter and more turbulent than expected. The probe was eventually completely destroyed as it continued to descend. The parachute would have melted first, roughly 30 minutes after entry,[35] then the aluminum components after another 40 minutes of free fall. The titanium structure would have lasted 6.5 hours more before disintegrating. Due to the high pressure, the droplets of metals from the probe would finally have vaporized once their critical temperature had been reached, and mixed with Jupiter's liquid metallic hydrogen interior.
After arriving on December 7, 1995 and completing 35 orbits around Jupiter throughout a nearly eight year mission, the Galileo Orbiter was destroyed during a controlled impact with Jupiter on September 21, 2003. During that intervening time, Galileo forever changed the way scientists saw Jupiter and provided a wealth of information on the moons orbiting the planet which will be studied for years to come. Culled from NASA's press kit, the top orbiter science results were:
The star scanner was a small optical telescope used to provide the spacecraft with an absolute attitude reference. It was also able serendipitously to make scientific discoveries.[36] In the prime mission, it was found that the star scanner was able to detect high energy particles as a noise signal. These data were eventually calibrated to show the particles were predominantly > 2 MeV electrons that were trapped in the Jovian magnetic belts.
A second discovery occurred in 2000. The star scanner was observing a set of stars which included the second magnitude star Delta Velorum. At one point, this star dimmed for 8 hours below the star scanner's detection threshold. Subsequent analysis of Galileo data and work by amateur and professional astronomers showed that Delta Velorum is the brightest known eclipsing binary, brighter at maximum than even Algol.[37] It has a primary period of 45 days and the dimming is just visible with the naked eye.
A final discovery occurred during the last two orbits of the mission. When the spacecraft passed the orbit of Jupiter's moon Amalthea, the star scanner detected unexpected flashes of light that were reflections from moonlets. None of the individual moonlets were sighted twice, hence no orbits were determined and the moonlets did not meet the International Astronomical Union requirements to receive designations.[38] It is believed that these moonlets most likely are debris ejected from Amalthea and form a tenuous, and perhaps temporary, ring.
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 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 absorption by chlorophyll in photosynthesizing plants, absorption bands of molecular oxygen which is also a result of plant activity, infrared absorption bands caused by the ~1 micromole per mole (µmol/mol) of methane in Earth's atmosphere (a gas which 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. [39]
In December 1992 during Galileo's second gravity assist planetary 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,[40] 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-Aluminium Garnet (Nd:YAG) laser operating at 532 nm with a repetition rate of ~15 to 30 Hz and a pulse power (FWHM) in the tens of megawatts range, which was coupled to a 0.6 meter Cassegrain 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. Adverse weather conditions, restrictions placed on laser transmissions by the U.S. Space Defense Operations Center (SPADOC) and a pointing error caused by the scan platform acceleration on the spacecraft being slower than expected (which prevented laser detection on all frames with less than 400 ms exposure times) all contributed to the reduction of the number of successful detections of the laser transmission to 48 of the total 159 frames taken. Nonetheless, the experiment was considered a resounding success and the data acquired will likely be used in the future to design laser "downlinks" which will send large volumes of data very quickly, from spacecraft to Earth. The scheme is already being studied (as of 2004) for a data link to a future Mars orbiting spacecraft.[41]
On October 29, 1991, two months after entering the asteroid belt, Galileo performed the first ever asteroid encounter by passing about 1,600 kilometers (990 mi) from 951 Gaspra at a relative speed of about 8 kilometers per second (18,000 mph). Several pictures of Gaspra were taken along with measurements using the NIMS instrument to indicate composition and physical properties. The last (and best) two images were played back to Earth in November 1991 and June 1992. The imagery revealed a cratered and very irregular body about 19 by 12 by 11 kilometers (12 by 7.5 by 7 miles). The remainder of data taken, including low resolution images of more of the surface, were transmitted in late November 1992.[42]
Twenty-two months after the Gaspra encounter, on August 28, 1993, Galileo flew within 2,400 kilometers (1,500 mi) of asteroid 243 Ida. The probe discovered that Ida had a small moon, dubbed Dactyl, only 1.4 km in diameter which was the first asteroid moon discovered. Measurements using Galileo's solid state imager, magnetometer and NIMS instrument were taken. From subsequent analysis of data, Dactyl appears to be an SII subtype S type asteroid and is spectrally different from 243 Ida. It is hypothesized that Dactyl may have been produced by partial melting within a Koronis parent body (Ida belongs to the "Koronis" family of asteroids that travels in the main Asteroid Belt between Mars and Jupiter) while the 243 Ida region escaped such igneous processing.
For reasons which in all likelihood will never be known with certainty, Galileo's 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 1986 Challenger disaster, its lubricants evaporated, damaging the system. 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 possessed an additional low-gain antenna that was capable of transmitting information back to Earth, although 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).[43] Through the implementation of sophisticated technologies, the arraying of several Deep Space Network antennas and sensitivity upgrades to the receivers used to listen to Galileo's signal, data throughput was increased to a maximum of 160 bits per second.[44] By further using data compression, the effective data rate could be raised to 1000 bits per second.[44][45] The data collected on Jupiter and its moons was stored in the spacecraft's onboard tape recorder, and transmitted back to Earth during the long apozene 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 total amount of data transmitted throughout the mission, although 70% of Galileo's science goals could still be met.[46]
The failure of Galileo's high-gain antenna 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 flybys of Jupiter and its moons. In October 1995, Galileo's 114-megabyte (914,489,344 bits[47]), four-track digital tape recorder, which was manufactured by Odetics Corporation, remained stuck in rewind mode for 15 hours before engineers learned what had 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 Galileo entered orbit around Jupiter, the anomaly prompted engineers to sacrifice data acquisition of almost all of the Io and Europa observations during the orbit insertion phase, in order to focus solely on recording data sent from the Jupiter probe descent.
In November 2002, after the completion of the mission's only encounter with Jupiter's moon Amalthea, problems with playback of the tape recorder again plagued Galileo. About 10 minutes after the closest approach of the Amalthea 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 intense 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. 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 three infrared Optek OP133[48] 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.
Jupiter's uniquely harsh radiation environment caused over 20 anomalies over the course of Galileo's mission, in addition to the incidents expanded upon above. Despite exceeding its radiation design limit by at least a factor of three, the spacecraft survived all these anomalies. Several of the science instruments suffered increased noise while within about 700,000 kilometres (430,000 mi) of Jupiter. The quartz crystal used as the frequency reference for the radio suffered permanent frequency shifts with each Jupiter approach. A spin detector failed, and the spacecraft gyro output was biased by the radiation environment. The SSI camera began producing totally white images when the spacecraft was hit by the exceptional 'Bastille Day' coronal mass ejection in 2000, and did so again on subsequent close approaches to Jupiter. The most severe effect was a CDS despun bus reset of the onboard computers which occurred when the spacecraft was either close to Jupiter or in the region of space magnetically downstream of the Earth. Work-arounds were found for all of these problems, and Galileo was never rendered entirely non-functional by Jupiter's radiation.
The atmospheric probe deployed its first parachute about one minute later than anticipated, resulting in a small loss of upper atmospheric readings. Through review of records, the problem was later determined to likely be faulty wiring in the parachute control system. The fact that the chute opened at all was attributed to luck.[49]
It is believed today that the accelerometer controlling the parachute's pyrotechnics was installed backwards. The same thing happened to the Genesis probe's sample return capsule when it returned to Earth in September 2004, causing the capsule to crash in the Utah desert.[50]
Once its fuel supply was nearly depleted, Galileo was intentionally commanded to crash into Jupiter to eliminate any chance of a future impact with Europa that could contaminate the icy moon with terrestrial bacteria. At the completion of its 35th and final circuit around the Jovian system, Galileo impacted the gas giant in darkness just south of the equator on September 21, 2003, at 18:57 GMT, at a speed of approximately 48.26 kilometers per second (nearly 108,000 mph).[51] In order to crash into Jupiter, Galileo flew by Amalthea on November 5, 2002,[52] during its 34th orbit, allowing a measurement of the moon's mass as it passed within 163.0 ± 11.7 kilometres (7.3 mi) of its surface. On April 14, 2003, Galileo reached its greatest distance from Jupiter for the entire mission, 26,000,000 kilometres (16,000,000 mi), before plunging back into the gas giant, taking a little less than nine months to do so.[53]
|
||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||