Figure 1: Enceladus as seen by Voyager 2, August 26, 1981
Planned/previous Cassini encounters with Enceladus[1][2]
Date
Distance (km)
February 17, 2005
1,264
March 9, 2005
500
March 29, 2005
64,000
May 21, 2005
93,000
July 14, 2005
175
October 12, 2005
49,000
December 24, 2005
94,000
January 17, 2006
146,000
September 9, 2006
40,000
November 9, 2006
95,000
June 28, 2007
90,000
September 30, 2007
98,000
March 12, 2008
52
June 30, 2008
84,000
August 11, 2008
54
October 9, 2008
25
October 31, 2008
200
November 8, 2008
52,804
November 2, 2009
103
November 21, 2009
1,607
April 28, 2010
103
May 18, 2010
201
August 13, 2010
2554
November 30, 2010
48
December 21, 2010
50
January 30, 2011
60000
February 20, 2011
68000
September 13, 2011
42000
October 1, 2011
99
October 19, 2011
1231
November 5, 2011
496
November 23, 2011
35000
December 11, 2011
20000
Enceladus was discovered by Fredrick William Herschel on August 28, 1789, during the first use of his new 1.2 m telescope, then the largest in the world.[3][4] Herschel first observed Enceladus in 1787, but in his smaller, 16.5 cm telescope, the moon was not recognized.[5] Due to Enceladus's faint apparent magnitude (+11.7m) and its proximity to much brighter Saturn and its rings, Enceladus is difficult to observe from Earth, requiring a telescope with a mirror of 15–30 cm in diameter, depending on atmospherical conditions and light pollution. Like many Saturnian satellites discovered prior to the Space Age, Enceladus was first observed during a ring crossing, when Earth is within the ring plane during Saturnian equinox. During these periods, Enceladus is easier to observe due to the reduction in glare from the rings.
Prior to the Voyager program, the view of Enceladus improved little from the dot first observed by Herschel. Only its orbital characteristics, along with an estimation of its mass, density, and albedo, were known.
The two Voyager spacecraft obtained the first close-up images of Enceladus. Voyager 1 was the first to fly past Enceladus, at a distance of 202,000 km on November 12, 1980.[6] Images acquired from this distance had very poor spatial resolution, but revealed a highly reflective surface devoid of impact craters, indicating a youthful surface.[7]Voyager 1 also confirmed that Enceladus was embedded in the densest part of Saturn's diffuse E-ring. Combined with the apparent youthful appearance of the surface, Voyager scientists suggested that the E-ring consisted of particles vented from Enceladus's surface.[7]
Dramatic plumes, both large and small, spray water ice out from many locations along the famed "tiger stripes" near the south pole of Saturn's moon Enceladus. From right to left, the four major stripes are Damascus, Baghdad, Cairo and Alexandria sulci.
Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981, allowing much higher resolution images of this satellite.[6] These images revealed the youthful nature of much of its surface, as seen in Figure 1.[8] They also revealed a surface with different regions with vastly different surface 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, another moon of Saturn 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 active or whether it was the source of the E-ring.
False color Cassini image of jets in the southern hemisphere of Enceladus
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 at right. So far, four close flybys of Enceladus have been performed, yielding significant information concerning Enceladus's surface, as well as the discovery of water vapor and complex hydrocarbons 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 took the probe to within 52 km of the moon's surface.[1] A planned extended mission for Cassini includes seven close flybys of Enceladus between July 2008 and July 2010, including two passes at only 50 km in the later half of 2008.[9]
The discoveries Cassini has made at Enceladus have prompted several studies into follow-up missions. In 2007, NASA performed a concept study for a mission that would orbit Enceladus and would perform a detailed examination of the south polar plumes.[10] The concept was not selected for further study.[11] The European Space Agency also recently explored plans to send a probe to Enceladus in a mission to be combined with studies of Titan.[12]
The Titan Saturn System Mission (TSSM) is a joint NASA/ESA proposal for exploration of Saturn's moons, including Enceladus. TSSM was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009 it was announced that ESA/NASA had given the EJSM mission priority ahead of TSSM,[13] although TSSM will continue to be studied for a later launch date.
↑Herschel, W. (1795) Description of a Forty-feet Reflecting Telescope, Philosophical Transactions of the Royal Society of London, Vol. 85, pp. 347–409 (reported by M. Arago (1871), Herschel, Annual Report of the Board of Regents of the Smithsonian Institution, pp. 198–223)
