FAQ

Q. What happens if the star scanner is pointed at the sun? At Jupiter?

A. The detector is powered down for a few seconds before this happens. Alternately, if the detector senses too much light, the star scanner will close a shutter to protect itself.

Q. Can you provide pictures taken with the star scanner?

A. No. The star scanner is not an imaging instrument. It just records light levels (or radiation levels).

Q. Does the star scanner get tired?

A. Over the years, it has weakened slightly. At the end of the mission, the star scanner sees stars at about 85% of the brightness that it did when it was new. Much of this damage has been caused by radiation taken when Galileo passes through Jupiter’s harsh radiation belts. The damage affects the detector and, secondarily, may be turning some of the glass optical elements slightly darker. That said, the star scanner has performed much better than was required to accomplish its job. As with the incredible resilience of the rest of the Galileo spacecraft, the attention to detail and the money spent in design and testing proved very worthwhile. Had Galileo been a “Faster, Better, Cheaper” spacecraft, it would have died years earlier.

Q. The star scanner must have seen millions of stars by now. It must have seen other variable stars, nova, comets, and all sorts of things. Can I get the whole data set and look for something of interest to me?

A. The star scanner generally only sees about 5 to 10 stars at a time, and then only uses a few of these for attitude reference. There is an intensity threshold – stars dimmer than this will be ignored. This threshold changes all the time, but any star that is not one of the 150 or so brightest will generally never even be processed by the star scanner. Further, it is increasingly difficult to separate out any star dimmer than about magnitude +2.0 from other nearby stars of the same brightness. Also, the spacecraft only holds a given attitude for two days to two months. It is therefore very unlikely that any given star would be in the field of view on any given day of interest. Technically, however, this is taxpayer funded data and is publicly available. For serious queries, contact me and I will try to help as my time allows.

Q. What is the spectral response of the star scanner? Do you have a light curve?

A. It sees from 320 nm to 600 nm with a peak response at 410 nm. In human terms, this is from the near ultraviolet to orange with it seeing violet light the best. A light curve is available from the author.

Q. The star scanner uses a vacuum tube instead of a CCD? Isn’t that ancient technology?

A. The star scanner was developed about 1980. CCDs were available then and, in fact, the Solid State Imager on Galileo uses one. The photomultiplier tube (a vacuum tube) is very sensitive, pretty reliable and, at the time, was a good choice for the star scanner. The technology is largely out of date now although photomultiplier tubes are still used in some specialized area.

Q. Can you locate Jupiter’s moons and determine their position? Can you find new ones?

A. I have occasionally seen Callisto and Ganymede come into the field of view. Their position can be determined in this way but the accuracy is much less than what is known from other sources. This will be true of all the inner moons. Probably at some time or another, a few of the 30 or so small outer moons have come into the Star Scanner’s field of view. These moons tend to be very highly inclined and would move quickly by Galileo. At the usual telemetry rate of one data point per 400 seconds, you would probably get at most one reading. Also, these moons are believed to be quite small and dark and so would have to pass within a few hundreds or thousands of kilometers of Galileo. Overall, detection would be difficult, though it is probably possible if one knew exactly where and when to look.

Note: The above was written before the discovery of the rocks near Amalthea. By some definitions, these count as moons.

Q. What is the difference between a star scanner, a star mapper, a star camera and a star tracker?

A. The terms tend to be used loosely and star tracker is sometimes used generically to mean any of the four. Technically a star scanner scans, that is it sweeps the sky and is mounted on a spinning spacecraft. A star mapper looks at a region of the sky and recognizes multiple stars by their patterns. A star camera does the same thing. A star tracker is just trained on a single star and expected to hold that star’s image in the center of its field of view.

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