A simple telescope design, of the kind used by Galileo in the first astronomical telescopes, consisting of two lenses. A long-focus convex converging lens forms the objective and a single concave lens is used as the eyepiece. The resulting image is upright, and the system is still widely used in opera-glasses.

A NASA spacecraft for the exploration of Jupiter, its rings and satellites. It was launched in October 1989 from the Space Shuttle and arrived at Jupiter in December 1995. The flight plan included flybys of the asteroids Gaspra and Ida en route. When Galileo was launched, more than twenty years had elapsed since the mission was first conceived. Initially, the launch had been planned for 1983. Following various problems, including the loss of the Challenger Space Shuttle, it was impossible to obtain sufficient power by means of the Shuttle launch for a direct journey to Jupiter. Engineers therefore designed a gravity-assisted flight plan, whereby the craft was accelerated by close passages by Venus and twice by the Earth during the first three years. A major disappointment was the failure of the craft’s high-gain communications antenna. Having only a low-gain antenna working limited the amount of data that could be transmitted back to Earth. However, Galileo otherwise worked well, despite some problems with its tape recorders. It entered orbit in the jovian system, returning high-resolution images of the Galilean moons. A probe carried by Galileo separated from the craft and entered Jupiter’s atmosphere on 7 December 1995. It parachuted down, returning data on the composition and physical state of the atmosphere for 57 minutes.

Telescopio Nazionale Galileo.

Compton Gamma Ray Observatory.

The study of gamma-rays from astronomical sources. Gamma rays are the most energetic form of electromagnetic radiation, with wavelengths shorter than X-rays i.e. less than about 0.1 nanometres. They are absorbed high in the Earth’s atmosphere; only the most energetic can be detected at ground level and virtually all astronomical gamma-ray observations must be conducted from satellites. The detectors used are scintillation counters, spark chambers and solid-state detectors, and the angular resolution achieved is coarse by astronomical standards. Detectors for gamma-ray bursters have been carried on numerous spacecraft since 1969. Sky surveys have been carried out by the satellites SAS-2 and COS-B. SAS-2 was launched in 1972 and operated for seven months. COS-B was launched in 1975 and operated for over six years. A great advance in gamma-ray astronomy was achieved with the launch of the Compton Gamma Ray Observatory by NASA in April 1991. Within months of launch, many new sources were being identified with greater positional accuracy than was possible previously. Astronomical sources of gamma rays include solar flares, pulsars, X-ray binary stars and quasars, as well as the gamma-ray bursters. Known discrete sources of gamma-rays include the Vela Pulsar, the Crab Pulsar , SS433 and Geminga. The most intense diffuse gamma radiation comes from the galactic plane, generated by interactions between cosmic rays and the interstellar gas. A gamma-ray spectrometer on HEAO-3 in 1979 observed lines produced by electron-positron annihilation from the direction of the galactic centre.

An astronomical source of a transient burst of gamma-radiation and X-rays. The bursts are intense and short, lasting for between a few milliseconds and a few tens of seconds. Gamma-ray bursters were first discovered by chance in the late 1960s by military satellites designed for monitoring nuclear weapons tests and have since been observed by a variety of spacecraft carrying appropriate detectors. In 1979 a single burst, which seemed to come from the Large Magellanic Cloud, was detected simultaneously by nine satellites. Monitoring by the Compton Gamma Ray Observatory GRO showed that bursts occur about twice a day, at random positions all over the sky. It has recorded several thousand. Though the Compton GRO was able to determine the positions of the bursters with greater accuracy than was previously possible, the positions were still not accurate enough to allow optical identification. In 1997, however, the BeppoSAX satellite, with the help of its narrow-field X-ray camera, was able to pinpoint the position of gamma-ray bursters precisely enough for them to be identified optically, and for radio emission to be detected. The first optical spectrum of a gamma-ray burster, obtained at the Keck Observatory, showed it to be at a remote cosmological distance, about halfway to the edge of the observable universe. This implies that the energy output is immense. For a few seconds the burster emits more than a million times more energy than a whole galaxy. Though many theories have been advanced, the precise mechanism is not known. Some of the more favoured theories involve the merger of two neutron stars.

Asteroid 1036, diameter 40 km, discovered in 1924 by W. Baade. It is a member of the Amor group, and is possibly one of the largest asteroids that make close approaches to the Earth.

One of the four Galilean moons of Jupiter number III and the largest natural satellite in the solar system. The first high-resolution images of Ganymede were returned by Voyagers 1 and 2. Images showing even finer detail were obtained by Galileo. There are several different types of terrain, notably dark areas that are heavily cratered and a lighter grooved terrain that constitutes around 60 per cent of the surface photographed. Galileo images of the dark area suggest it is surface that has been changed by various episodes of shearing and furrowing. Galileo also revealed many small craters on the finely grooved areas. One of the most significant discoveries made by Galileo was that Ganymede has a substantial magnetic field, which is stronger at its surface than the fields of Mercury, Venus or Mars. Data from Galileo’s trajectory, combined with the magnetic, field, suggest that Ganymede must have a molten iron-rich core. Overall Ganymede’s density is about twice that of water. It is likely that the core is surrounded by a rocky mantle, overlain by a thick layer of ice.

The overturning of lunar regolith soil by micrometeorite bombardment.

An informal name, first used by William Herschel, for the strikingly red star μ Mu Cephei. It is a red supergiant and a semiregular variable ranging in magnitude between 3.6 and 5.1.

ion tail.

A glowing cloud of gas in interstellar space, which may be either an emission nebula or a reflection nebula. The expression used to be employed more often, when galaxies were referred to as extragalactic nebulae, in order to emphasize the distinction. The adjective gaseous is now generally omitted since nebula is normally used only for such interstellar clouds and not for galaxies.

Asteroid 951, a member of the Flora group, imaged by the Galileo spacecraft, which passed Gaspra at a distance of about 16,000 kilometres on 29 October 1991. It is irregular in shape, measuring about 20 by 12 by 11 kilometres, with a cratered surface. The largest crater is 1.5 kilometres across. Galileo detected a magnetic field, suggesting that Gaspra has a metallic composition.

A lunar crater, 100 kilometres 60 miles in diameter, on the northern border of the Mare Humorum. Clefts cross the crater floor and there are multiple peaks. Gassendi has been particularly associated with the search for transient lunar phenomena.

zodiacal light.