Discovery of Enceladus, Voyager observations, and Cassini's arrival		Printable Version (PDF)
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Contents The Cassini-Huygens mission and the Enceladus flybys Discovery of Enceladus, Voyager observations, and Cassini's arrival Enceladus: Name and characteristics Cassini sheds new light on Enceladus Conclusion

Discovery. Enceladus, the sixth-largest moon of Saturn, was discovered by astronomer and telescope-maker Frederick William Herschel (1738-1822). In 1789, after about two years of work, Herschel completed his largest telescope. It had a 48-inch (1.2-meter) aperture and was the world's largest telescope for over 50 years. With it, Herschel discovered Saturn's sixth known moon, Enceladus, on August 28th,1789. On September 17th of that same year he also discovered Saturn's seventh known moon, Mimas.

Enceladus [NASA photo]

Voyager observations. Until the two Voyager spacecraft and the Pioneer 11 spacecraft passed near it in the early 1980s, very little was known about this small moon besides the identification of water ice on its surface. The two Voyagers showed that Enceladus is only about 505 kilometers (505 km, about 314 miles) in diameter, seven times smaller than the Earth's Moon, and reflects almost 100% of the sunlight that strikes it [1]. Voyager 1 was the first spacecraft to obtain images of Enceladus and was the first spacecraft to fly past it. It did so on November 12, 1980, at a distance of 202,000 km [2]. The distant images had very poor spatial resolution, but they did reveal a highly reflective surface that seemed devoid of impact craters, indicating a youthful surface [3]. Because impact cratering is common to most solar system bodies, a smooth surface seems to indicate a geologically active exterior undergoing occasional renewal, which would tend to cover over or erase evidence of past cratering. Voyager 1 also confirmed that Enceladus was embedded in the densest part of Saturn's diffuse E Ring, leading Voyager scientists to suggest that Saturn's E Ring consisted of particles vented from volcanic activity on Enceladus [3].

Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981, allowing much higher resolution images [2]. They revealed a surface with different regions of vastly different ages, with a heavily cratered mid- to high-northern latitude region, and a lightly cratered region closer to the equator. This geologic diversity contrasts with the ancient heavily cratered surface of Mimas, which is slightly smaller than Enceladus. The geologically youthful terrains came as a great surprise to the scientific community, because no theory was then able to predict that such a small (and cold, compared to Jupiter's highly active moon Io) celestial body could bear signs of such activity. However, Voyager 2 failed to determine whether Enceladus was currently geologically active, with occasional surface renewal, or whether it was the source of the E Ring [1].

Cassini's arrival. The answer to these and other mysteries would have to wait until the arrival of the Cassini spacecraft on July 1, 2004, when it went into orbit around Saturn. Given the results from the Voyager 2 images, Enceladus was considered a priority target by the Cassini mission planners, and several targeted flybys within 1,500 km of the surface were planned, as well as numerous "non-targeted" opportunities within 100,000 km of Enceladus. These encounters are listed in the tables below:

Successful Cassini flybys of Enceladus			Planned Cassini flybys of Enceladus (as of July 2006)
Dates	Closest distance from Enceladus (km)		Dates	Closest distance from Enceladus (km)
February 17, 2005	1,264		September 9, 2006	40,000
March 9, 2005	500		November 9, 2006	95,000
March 29, 2005	64,000		June 28, 2007	90,000
May 21, 2005	93,000		September 30, 2007	98,000
July 14, 2005	175		March 12, 2008	23
October 12, 2005	49,000		June 30, 2008	101,000
December 24, 2005	94,000
January 17, 2006	146,000

So far, three close flybys of Enceladus have been performed (those of February 17th, March 9th, and July 14th, 2005), yielding significant information concerning Enceladus' surface, as well as the discovery of water vapor venting from the geologically active South Polar Region. These discoveries have prompted the adjustment of Cassini's flight plan to allow closer flybys of Enceladus, including an encounter in March 2008, which will take the probe to within 23 km of the moon's surface [5].

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List of Visuals

1. Enceladus from Voyager
   http://en.wikipedia.org/wiki/Image:Enceladus_from_Voyager.jpg
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