Space is filled with the ripped up remnants of atoms. Negatively charged electrons get separated from the protons and neutrons that make up the nuclei of atoms. The remaining atomic nuclei become positively charged ions when they have a deficit of electrons. Particles which carry an electric charge can be grabbed by magnetic fields and flung to very high speeds. When these atomic bits blast outward from the sun, they are referred to as the solar wind. Radiation belts can form above a planet where concentrations of these particles are trapped by a planet’s magnetic field.  Engineers often loosely refer to all these particles as “radiation” although they are not radioactive in the way of, say, uranium.


The Galileo spacecraft was launched from earth in 1989 and arrived at Jupiter in late 1995. By the end of its mission in 2003, Galileo had made 35 orbits. At the low point (periapsis) in each orbit, the spacecraft entered Jupiter’s intense radiation belts. Generally, the radiation worsens as the spacecraft goes toward Jupiter. By the end of the mission the spacecraft electronics, hidden by an average of 8 mm of aluminum, had absorbed roughly 6 kiloGrays of radiation – enough to kill a man 1500 times over.




Galileo Mission Trajectory. The sun is towards the top of the page. GEM and GMM are the Galileo Europa Mission and Galileo Millenium Mission, respectively. Each orbit is given a unique identifier such as A34. The letter represents the body most closely approached by Galileo, the moon Amalthea in this case. The number is a counter for the number times the spacecraft has orbited Jupiter.



The “torso” of Galileo is composed of two segments, a rotating part and a stationary one. The part that contains the antenna, main computer, power sources and the star scanner spins about 3 times per minute. The star scanner looks outward to the heavens and sweeps across the sky. It has a telescope that focuses light onto a slit. Behind the slit is a light sensitive device called a photomultiplier tube. As the spacecraft rotates, stars briefly flash through the slit and the star scanner recognizes the stars. The on-board attitude control computer is then able to compute Galileo’s orientation relative to the stars.


Even before the final assembly of the star scanner, it was recognized by engineers that it might, by accident, be able to detect the radiation that is trapped by Jupiter’s immense magnetic field. They worked hard to prevent this from happening, partly by hiding the sensitive parts of the star scanner under blankets of protective metal. While they did their job well enough to prevent this radiation from blinding the star scanner, it is still able to perceive this radiation as if it were a diffuse background of  light. Thus, unintentionally, the star scanner is measuring very energetic electrons in the Jovian environment. These particles move at the nearly the speed of light and are especially dangerous to spacecraft. They are able to penetrate spacecraft walls and hit the sensitive electronics and optical components. Given enough of these electrons, bad things start to happen [12]. Galileo has experienced glass lenses glowing blue, noisy detectors and probably even arcing as a result of this barrage of electrons.


In the case of the star scanner, these electrons penetrate to its light sensor and some lenses and are mistakenly perceived as starlight. However, the star scanner electronics are smart enough to separate a real pulse of star light from the more or less continuous flux of electrons. The information on both star light and radiation are sent to the ground in telemetry.


We have been able to use the data on star light to monitor stars for unusual activity – such as unexpected bursts of brightness. The radiation data provide information on how many electrons there are buzzing about near Jupiter which, besides being of scientific interest, is important information for future spacecraft venturing near Jupiter.


Galileo’s 11th instrument, the star scanner, may be the only example of a scientific instrument added to a spacecraft after its launch.