An observatory in Nantucket, Massachusetts, founded in 1908 as a memorial to Maria Mitchell 1818-88, a pioneering scientist and teacher in an era when very few women took on such academic work. She won international fame after discovering a comet in 1847. The observatory was established by the Maria Mitchell Association. It houses 18-centimetre 7-inch and 20-centimetre 8-inch telescopes used primarily for educational purposes. Variable-star work is undertaken.

A series of spacecraft launched by the USA during the 1960s and 1970s in a programme to explore the planets Mercury, Venus and Mars. Mariner 2 in 1962 achieved the first successful flyby of Venus, to be followed by Mariner 5 in 1967. Mariner 4, launched in 1964, was the first successful probe to Mars and revealed the presence of craters on that planet. Mariners 6 and 7 followed in 1969. Mariner 9 was put into orbit around Mars in 1971 and returned over 7,000 images. Mariner 10 in 1974 was the first two-planet mission. Its orbit allowed three separate encounters with Mercury, providing 10,000 images, as well as a flyby of Venus. There were a total of seven successful missions in the Mariner series. Numbers 11 and 12 were renamed Voyagers 1 and 2.

Any of the galaxies in the list drawn up by the Soviet astronomer B. E. Markarian in the 1970s, characterized by strong continuum emission of ultraviolet light.

The fourth major planet from the Sun, often known as the Red Planet because of its distinctive colour, noticeable even to the naked eye. Mars is one of the terrestrial planets with a diameter just over half that of the Earth. It had long been regarded as the planet other than Earth most likely to have life, a view encouraged by the presence of polar ice caps and observations of seasonal changes. Nineteenth-century observers, notably Percival Lowell, convinced themselves that they could make out systems of straight channels, canals, that might be artificially constructed. Exploration of the planet by spacecraft has virtually eliminated the possibility that life exists currently on Mars. However, studies of meteorites believed to be of martian origin have fuelled speculation that microscopic life at least may have existed on Mars in the remote past when the climate was wetter and warmer. Successful US probes to Mars include: Mariner 4 in 1965, Mariners 6 and 7 in 1969, Mariner 9 in 1971, and Vikings 1 and 2 in 1976. Following the failure of Mars Observer in 1993, the USA launched Mars Global Surveyor and Mars Pathfinder for arrival in 1997. Mars is considered to be a realistic target for a manned landing in the early twenty-first century. The relatively low density of Mars 3.95 times that of water suggests that 25 per cent of its mass is contained in an iron core. There is a weak magnetic field, about 2 per cent the strength of the Earth’s. The crust is rich in olivine and ferrous oxide, which gives the rusty colour. The tenuous martian atmosphere is composed of 95.3 per cent carbon dioxide, 2.7 per cent molecular nitrogen and 1.6 per cent argon, with oxygen as a major trace constituent. The atmospheric pressure at the surface is only 0.7 per cent that at the surface of the Earth. However, strong winds in the atmosphere cause extensive dust storms, which occasionally engulf the entire planet. A variety of clouds and mists occur. Early-morning fog forms in valleys and orographic clouds, formed when air masses cool as winds drive them from low to high ground, appear over the high mountains of the Tharsis region. In winter, the north polar cap is swathed in a veil of icy mist and dust, known as the polar hood. A similar phenomenon is seen to a lesser extent in the south. The polar regions are covered with a thin layer of ice, thought to be a mixture of water ice and solid carbon dioxide. High-resolution images show a spiral formation and strata of wind-borne material. The north polar region is surrounded by stretches of dunes. The polar ice caps grow and recede with the seasons, which arise - as they do on Earth - because the planet’s rotation axis is tilted by 25° to the orbital plane. The martian year is about twice the length of the Earth year, so the seasons are also longer. However, the relatively high eccentricity of Mars’s orbit makes them of unequal duration: southern summers, which occur when Mars is near perihelion, are shorter and hotter than those in the north. Seasonal changes in the appearance of features as observed from Earth are explained as physical and chemical changes. There is a marked difference in the nature of the terrain between the two halves of Mars divided roughly by a great circle tilted at 35° to the equator. The more southerly part consists largely of ancient, heavily cratered terrain. The major impact basins - the Hellas, Argyre and Isidis planitiae - are located in this hemisphere. The north is dominated by younger, more sparsely cratered terrain, lying 2-3 kilometres lower. The highest areas are the large volcanic domes of the Tharsis and Elysium planitiae. Both areas are dominated by several huge extinct volcanoes, the largest of which is Olympus Mons. These volcanic areas are located at the east and west ends of an immense system of canyons, the Valles Marineris, which stretches for more than 5,000 kilometres 3,000 miles around the equatorial region and has an average depth of 6 kilometres. It is believed to have been caused by faulting associated with the upthrust of the Tharsis dome. There is evidence, in the form of flow channels, that liquid water once existed on the surface of Mars. Channels from the Valles Marineris appear to have been created in some kind of sudden flood. There are also sinuous, dried-up river beds with many tributaries, found only in the heavily cratered terrain. Mars has two small natural satellites, Phobos and Deimos, which are in near-circular orbits in the equatorial plane, close to the planet. They are very difficult to see from Earth. They are so different from Mars that it seems likely they are captured asteroids.

A series of Soviet spacecraft intended to return data about the planet Mars. Useful information was returned by Mars 2 and 3 in 1971 and Mars 5 in 1974, but the others were not successful.

A Russian-led, international Mars mission which was lost when the launch failed in November 1996.

A NASA mission to Mars, launched on 7 November 1996 for arrival in September 1997, effectively as a replacement for the failed Mars Observer. It was successfully put into a high elliptical orbit around Mars on 11 September 1997. In the following months, it was gradually manoeuvred into an almost circular near-polar orbit from which to carry out systematic mapping between March 1998 and January 2000. The technique used to change the spacecraft’s orbit was aerobraking, in which the drag of the martian atmosphere was used to reduce the orbital speed of the craft. Mars Global Surveyor was designed to serve as a communications satellite for future missions after completion of its mapping mission.

An unmanned NASA mission to Mars, successfully launched in 1992, but lost presumed destroyed on its arrival at Mars in September 1993. The probe had been intended to orbit around Mars, undertaking detailed mapping and remote sensing for one martian year. Replacements for six of the eight instruments lost were subsequently carried on Mars Global Surveyor.

A NASA Mars mission which was launched on 4 December 1996 and arrived at the planet on 4 July 1997. The main objective was to test a low-cost means of sending a spacecraft and a surface rover vehicle to land on the martian surface. It carried a 10-kg 22-lb miniature rover, named Sojourner, equipped to measure the elemental composition of the surface rocks and soil and take images around the landing site in Ares Vallis. In a new technique, the lander’s impact was cushioned by airbags, which bounced several times before coming to rest. The performance of both the lander and the rover exceeded expectations and they were able to continue operating beyond the minimum target of 7 martian days sols originally envisaged. Panoramic views of the landscape were returned and Sojourner successfully travelled on expeditions covering about 80 metres. Instruments on the lander monitored atmospheric conditions at the surface. Measurements of the martian atmosphere also were made during the parachute descent. After arrival at Mars, the lander base station was renamed the Sagan Memorial Station in honour of the American planetary scientist, Carl Sagan, who died in 1997.

A two-spacecraft NASA Mars mission scheduled for launch in December 1998 and early January 1999. It comprises both an orbiter, which will be launched first, and a lander, to be launched about a month later. The orbiter is intended to continue the global reconnaissance started by Mars Global Surveyor, while the second craft is to land near the south polar region. The emphasis of the mission is on studying the martian climate and interactions between the atmosphere and the surface.

An area of anomalously strong gravitational field on the Moon. The word is a contraction of mass concentration. Mascons are presumed to indicate the presence of rocks denser than average, though there is no consensus about exactly how they formed. The areas are roughly circular and are associated with mare areas.

In astronomy, an emission process in molecular clouds whereby certain spectral lines in the microwave emission of particular molecules are strongly amplified by natural processes similar to those exploited in lasers. Maser action in an astronomical source was first discovered in 1965 in the emission from hydroxyl OH molecules in a source in the Orion Nebula. Other molecules subsequently found to show a similar effect include water H2O, silicon monoxide SiO, formaldehyde H2CO and methyl alcohol CH3OH. The word maser is an acronym for microwave amplification by the stimulated emission of radiation. See also:

For a group of related objects, such as a cluster of stars or galaxies, a graph or mathematical expression describing the relative numbers of objects found in different ranges of mass. See also: initial mass function, Salpeter function.

The flow of material from one star to another in a close binary system. The process can occur when a star has expanded during its evolutionary process to the extent that its outer layers are pulled towards the companion. The material transferred may stream directly on to the star’s surface or form an accretion disc. See also: contact binary, dwarf nova, Roche lobe, semi-detached system, X-ray binary.

The simple relation between the mass M and luminosity L of a star on the main sequence that takes the form L ~ Mn where the index n has the value 3.5 for stars of 7 solar masses or less. For stars with masses in the range 7 to 25 solar masses, n falls to 3.0 and is nearer 2.7 for masses in excess of 25 times the Sun’s.