Dictionary of Space Technology by Joseph Angelo

Author:Joseph Angelo [Angelo, Joseph]
Language: eng
Format: epub
Tags: vl-nfspace
ISBN: 9781579581497
Google: Ho9UAAAAMAAJ
Amazon: 1579581498
Publisher: Routledge
Published: 1998-12-02T00:00:00+00:00

Figure 76. The mosaic of several hundred images from the Mariner 10 provides an enhanced view of Mercury’s southern hemisphere. (Photograph courtesy of NASA/JPL)

NASA’s Mariner 10 spacecraft provided the first close-up views of Mercury. (See Figure 76.) This spacecraft was launched from Cape Canaveral in November 1973. After traveling almost five months (including a flyby of Venus), this spacecraft passed within 805 kilometers (km) (483 miles [mi]) of Mercury on March 29, 1974. Mariner 10 then looped around the Sun and made another rendezvous with Mercury on September 21, 1974. This encounter process was repeated a third time on March 16, 1975, before the control gas used to orient the spacecraft was exhausted. This triple flyby of the planet Mercury by Mariner 10 is sometimes referred to as Mercury I, II, and III in the technical literature.

The images of Mercury transmitted back to Earth by Mariner 10 revealed an ancient, heavily cratered world that closely resembled Earth’s Moon. Unlike the Moon, however, huge cliffs (called lobate scarps) crisscross Mercury. These great cliffs apparently were formed when Mercury’s interior cooled and shrank, compressing the planet’s crust. The cliffs are as high as 2 km (1.2 mi) and as long as 1,500 km (900 mi).

To the surprise of scientists, instruments on board Mariner 10 discovered that Mercury has a weak magnetic field. It also has a wisp of an atmosphere—a trillionth of the density of Earth’s atmosphere and made up mainly of traces of helium, hydrogen, sodium, and potassium.

Temperatures on the sunlit side of Mercury reach approximately 700° Kelvin (427° Celsius)—a temperature that exceeds the melting point of lead; on the dark side, temperatures plunge to a frigid 100° Kelvin (minus 173° Celsius). Quite literally, Mercury is a world seared with intolerable heat in the daytime and frozen at night.

In the late 1960s, scientists on Earth bounced radar signals off the surface of Mercury. Analysis of the scattered radar signals indicated that the planet actually rotated slowly on its axis with a period of about 59 days. Consequently, the “days” and “nights” on this planet are quite long by terrestrial standards, 1 Mercurian day equals 59 Earth-days. It takes the planet approximately 88 days to orbit around the Sun.

As shown in Figure 101, Mercury’s surface features include large regions of gently rolling hills and numerous impact craters like those found on the Moon. Many of these craters are surrounded by blankets of ejecta (material thrown out at the time of a meteoroid impact) and secondary craters that were created when chunks of ejected material fell back down to the planet’s surface. Because Mercury has a higher gravitational attraction than the Moon, these secondary craters are not spread as widely from each primary crater as occurs on the Moon. One major surface feature discovered by Mariner 10 is a large impact basin called Caloris, which is about 1,300 km (780 mi) in diameter. Scientists now believe that Mercury has a large iron-rich core—the source of its weak, but detectable, magnetic field. The table

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