Jupiter's moons

13 September, 2009

Two continent-sized storms that erupted in Jupiter's atmosphere in March 2007 shows that Jupiter's internal heat plays a significant role in generating atmospheric disturbances.
Two continent-sized storms that erupted in Jupiter's atmosphere in March 2007 shows that Jupiter's internal heat plays a significant role in generating atmospheric disturbances. Image taken on May 11, 2007 by NASA's Hubble Space Telescope.
Image: NASA's Hubble Space Telescope
Source: NASA/ESA

Question: Why do different sources give different figures for the number of Jupiter’s moons? How many moons does Jupiter really have?

Answer: The figures given for the number of Jupiter’s moons vary because of new discoveries that are happening all the time.
 
What has enabled the ongoing discovery of new moons has been the advance of CCD (charge-coupled device) cameras during the early 2000s. Before then, the number of moons in the Solar System had been the same for 20 years or so, with most of the moons of the outer planets discovered by the Voyager spacecrafts. But at the beginning of the new century, CCD cameras had improved so that we could look at more of the sky in a single image (a wide field of view). Also, as bigger telescopes were being built (8m and 10m), it meant that the smaller telescopes (2m and 4m) were now available for such projects – re-observing the same part of the sky every 30 to 60 minutes.

The moons were found by comparing these images, and this was made much easier because CCD cameras produce digital images and computer programs could be made to do "blinking" of images, looking for objects that moved differently relative to the background stars.
 
In this way the number of moons for the outer planets all increased by a huge leap. The only planet still with new moon discoveries is Saturn, because the Cassini spacecraft currently remains in orbit around the planet. The last tiny little moon it found (too small to be imaged from Earth) was discovered in March 2009.
 
The Nine Planets website generally has the most uptodate material, including a list of moons for each planet and their dates of discovery.

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