To Galileo Science Instruments Contents PageThe monitor is the Heavy Ion Counter (HIC) (see figure) and the gremlins are heavy ions, which can cause random changes in a spacecraft's electronics. The HIC was conceived in 1984, to answer concerns the Project had about single-event upsets (SEUs) that were then occurring on Earth-orbiting satellites. An SEU is a random, stored, computer bit haphazardly changing its value from a "0" to a "1" or vice versa. Such "bit flips" on a spacecraft could trigger a chain reaction of erroneous commands with disastrous results. An SEU is caused by a heavy ion (such as the nucleus of an oxygen atom) penetrating the delicate electronics in a spacecraft and causing the bit to flip. Heavy ions of oxygen and sulphur are extremely common in Jupiter's environment. The HIC was created to measure and monitor these heavy ions. When SEUs became known in 1984, Galileo was to be shipped to Cape Canaveral in December 1985. That left little more than a year for Team Leader Ed Stone and Tom Garrard of the California Institute of Technology, along with Neil Gehrels and Don Stillwell of the Goddard Space Flight Center, to deliver an instrument that had never before been contemplated. Fortunately, Ed Stone was also the Principal Investigator for the Cosmic Ray Science Team (CRS) on the Voyager Project. He and his colleagues, including the late Al Schardt of Goddard, saw the possibility of reworking the CRS test model into a new Galileo instrument. Ed related, "We repackaged the instrument (which had been in storage), updated the electronics for Galileo's requirements (with assistance from JPL's Don Johnson), and redesigned the sensor system to optimize the detection of heavy ions." Concurrently, the Project was systematically modifying the rest of the spacecraft and its electronics to be resistant to the penetration of heavy ions. Ironically, because the HIC uses older components, it is one of the most SEU-resistant instruments on board Galileo. (The newer, high-tech components in use since the mid-1980s are more delicate than Voyager's mid-1970s parts.)
SEUs are of great concern specifically to Galileo since the Jovian environment is one of the richest in heavy ions. On Jupiter's moon, Io, volcanoes constantly erupt, spewing forth oxygen and sulphur, which then settle on Io's surface. The interaction of Io with Jupiter's magnetosphere strips the oxygen and sulphur from Io's surface at the rate of 900 kg (1 ton) per second, tearing these particles from Io's gravitational influence and expelling them into Jupiter's magnetosphere. These particles become electrically charged and many diffuse outward to 1.5 to 3.6 million kilometers (0.9 to 2.2 million miles) from Jupiter, where they are accelerated by an interaction with the massive Jovian magnetic field. Ed explains, "About 0.2% of the original particles, now highly energized, diffuse back toward Jupiter. The ions may have left Io at one-thousandth the speed of light. By the time they have returned to within 700,000 kilometers (420,000 miles) of the planet (near Europa's orbit), these ions have accelerated to one-tenth the speed of light (about 30,000 kilometers/second (18,000 miles per hour))!" Some of these ions travel along Jupiter's magnetic field lines and spiral into the planet's polar region. In fact, these heavy ions falling into Jupiter's atmosphere may be the single largest contributor to Jupiter's auroras.
Heavy ions are also flung off from the Sun during solar flares. Just after launch, four very large solar flare events occurred. The HIC was turned on for 20 days and detected a great many heavy-ion impacts. However, since no SEUs have happened on Galileo, the radiation-hardening of the spacecraft's components is apparently working well. The heavy ions observed by the HIC during these flare events are analyzed to determine the composition of the Sun. This analysis is a continuation of similar studies from Voyager's CRS. Current indications are that the ejecta from these very large flares are like those observed by Voyager in smaller solar flares. This result was presented to the American Geophysical Union in Baltimore, Maryland, on June 1 and is the first "publication" of scientific results from Galileo.
The HIC and the Extreme Ultraviolet Spectrometer share a communications link on Galileo, and thus must divide observing time between themselves. The HIC will be turned on more frequently once the High-Gain Antenna is unfurled in May 1991. The HIC actually detects heavy ions using five single-crystal silicon wafers, ranging in thickness from 30 to 2000 micrometers (0.001 to 0.08 in.). Each of these wafers has gold and aluminum electrodes on its surface, arranged in arrays called telescopes (as illustrated). A fast-moving ion enters one of two telescopes. It may, for example, pass through the first and second silicon wafers and stop in the third, ionizing the silicon through which it travels. The resulting ionizations are collected as electrical signals that are amplified and then analyzed to determine the particle's charge and speed. A sulphur ion, which has an electrical charge of 16, would create a signal four times as large as would an oxygen ion (with a charge of 8) moving at the same velocity. (In other words, the signal is proportional to the square of the ion's charge.) If there is no signal in the last silicon wafer, the heavy ion, stopped in the preceding wafers and all its energy has been recorded. With two telescopes, the HIC can measure heavy ions with energies as low as 6 million and, incrementally, as high as 200 million electron volts (MeV) per nucleon (that would be 3200 MeV for sulphur's charge of 16). This range includes all atomic substances between carbon and nickel, including, of course, oxygen and sulphur.
"Galileo's HIC can detect ions with energies up to about half the speed of light, nearly three times the capabilities of Voyager instruments," Ed Stone enthusiastically notes. "Its telescope is five times the size of Voyager's. It will especially watch for changes over time, since we will be residing in one environment for so long. This is a great opportunity."
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