An Etymological Dictionary of Astronomy and Astrophysics
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A joint endeavor of → NASA, → ESA, and the Italian space agency that sent a spacecraft to study the planet → Saturn and its system, including → Saturn's rings and its natural satellites. The spacecraft was 6.70 m × 4 m × 4 m and weighed about 6 tons. Cassini drew its electric power from the heat generated by the decay of 33 kg of → plutonium-238. The spacecraft carried 12 sophisticated observation and measuring instruments. Cassini-Huygens was launched on 15 October 1997. It used several → gravity assist manoeuvres to boost itself toward Saturn. It flew past Venus two times (April 1998 and June 1999), made → flybys of Earth (August 1999), and f Jupiter (December 2000). After 6 years and 8 months, covering about 8 billion km it entered Saturn orbit on July 1, 2004. It stayed there for 13 years and made detailed observations of the planet, its rings, and its moons. A scientific probe called Huygens was released on December 25, 2004 from the main spacecraft to parachute through the atmosphere to the surface of Saturn's largest and most interesting moon, → Titan. The data that Huygens transmitted during its final descent and for 72 minutes from the surface included 350 pictures that showed a shoreline with erosion features and a river delta. Cassini continued to orbit Saturn and complete many flybys of Saturn's moons. A particularly exciting discovery during its mission was that of → geysers of water ice and organic molecules at the south pole of → Enceladus, which erupt from an underground global ocean that could be a possible environment for life. Cassini's radar mapped much of Titan's surface and found large lakes of liquid → methane. Cassini also discovered six new moons and two new rings of Saturn. The mission was ended on September 15, 2017 when the spacecraft was crashed into Saturn's body and destroyed. This was the best way to avoid contaminating Saturn's moons with possible Earth microbes, because the moons may have the potential to support life.

Named after two famous scientists. The Saturn orbiter is named after the Italian/french astronomer Jean-Domenique Cassini, who discovered the Saturnian satellites → Iapetus in 1671, → Rhea in 1672, and both → Tethys and → Dione in 1684. In 1675 he discovered what is known today as the → Cassini division, the narrow gap separating Saturn's rings into two parts. The Titan probe was named Huygens in honor of the Dutch scientist, Christiaan Huygens, who discovered Titan in 1655.

Christiaan Huygens (1629-1695), an outstanding Dutch mathematician, astronomer, physicist, and horologist. → Huygens Division, → Huygens Region, → Huygens' principle, → Huygens-Fresnel principle.

In the system of → Saturn's rings, the gap at the inner edge of the → Cassini division at a distance of 117,680 km from the center of the planet with a width of 285-400 km.

→ Huygens; → division.

The inner bright part of the → Orion Nebula, from which most of the radiation is emitted. It is about 5' across corresponding to 0.7 pc (for a distance of 440 pc). See O'Dell (2001, ARAA 39, 99).

Named after the Dutch astronomer Christiaan → Huygens (1629-1695), who sketched the appearance of the Orion Nebula. His drawing, the first such known sketch, was published in Systema Saturnium in 1659. First named such by O. Gingerich (1982, Ann. NY Acad. Sci. 395, 308); → region.

Every point of a → wavefront may be considered as a center of a secondary disturbance which gives rise to spherical wavelets, and the wavefront at any later instant may be regarded as the envelope of these wavelets. This statement suffices to account for the laws of → reflection and → refraction, and the approximately straight line propagation of light through large apertures, but it fails to account for → diffraction, the deviations from exact straight line propagation of light. Huygens' principle was later extended by Fresnel and led to the formulation of → Huygens-Fresnel principle, which is of great importance in the theory of diffraction.

→ Huygens; → principle.

A development of → Huygens' principle stating that every point on a → wavefront acts, at a given instant, as a source of outgoing secondary spherical waves. The secondary wavelets mutually interfere and the resulting net light amplitude at any position in the outgoing wavefront is the vector sum of the amplitudes of all the individual wavelets. Using this principle, Fresnel calculated with a high accuracy the distribution of light in → diffraction patterns. The Huygens-Fresnel principle was put on a firm theoretical basis by Kirchhoff and expressed as an integral derived from the → wave equation.

→ Huygens; → Fresnel diffraction; → principle.