Comets

1. General considerations

Comets are among the most-spectacular objects in the sky, with their bright glowing comae and their long dust tails and ion tails. Comets can appear at random from any direction and provide a fabulous and ever-changing display for many months as they move in highly eccentric orbits around the Sun. Comets are important to scientists because they are primitive bodies left over from the formation of the solar system. They were among the first solid bodies to form in the solar nebula, the collapsing interstellar cloud of dust and gas out of which the Sun and planets formed. Comets formed in the outer regions of the solar nebula where it was cold enough for volatile ices to condense. This is generally taken to be beyond 5 astronomical units (AU; 748 million km, or 465 million miles), or beyond the orbit of Jupiter. Because comets have been stored in distant orbits beyond the planets, they have undergone few of the modifying processes that have melted or changed the larger bodies in the solar system. Thus, they retain a physical and chemical record of the primordial solar nebula and of the processes involved in the formation of planetary systems. A comet is made up of four visible parts: the nucleus, the coma, the ion tail, and the dust tail. The nucleus is a solid body typically a few kilometres in diameter and made up of a mixture of volatile ices (predominantly water ice) and silicate and organic dust particles. The coma is the freely escaping atmosphere around the nucleus that forms when the comet comes close to the Sun and the volatile ices sublimate, carrying with them dust particles that are intimately mixed with the frozen ices in the nucleus. The dust tail forms from those dust particles and is blown back by solar radiation pressure to form a long curving tail that is typically white or yellow in colour. The ion tail forms from the volatile gases in the coma when they are ionized by ultraviolet photons from the Sun and blown away by the solar wind. Ion tails point almost exactly away from the Sun and glow bluish in colour because of the presence of CO+ ions.

2. Tails

In 1951 German astronomer Ludwig Biermann studied the tails of comets and showed that the ion tails flowed away from the Sun at speeds in excess of 400 km (250 miles) per second. He suggested that the phenomenon had to be associated with some sort of “corpuscular radiation” flowing outward from the Sun. In fact, he had suggested the existence of the solar wind, which was not directly detected for another 8 years. The outflowing dust and gas in the coma interacts with the solar wind and sunlight. The molecules and free radicals are ionized by charge exchange with the solar wind. Once ionized, they are caught up in the Sun’s magnetic field and flow away at high velocity in the solar wind. The process forms long, narrow, straight trails that glow blue in colour because of the presence of CO+ molecules. However, the major ion in cometary ion tails is H2O+, which does not glow at visible wavelengths. Those tails point almost exactly away from the Sun because the solar wind velocity is typically about 400 km per second, much larger than the orbital velocities of almost all comets. The ion or plasma tails are known as Type I tails. Sometimes the ion tails of comets will disconnect from the coma and slowly fade while the comet grows a new ion tail. That is caused by the comet crossing magnetic sector boundaries in the Sun’s magnetic field. The fine dust suffers a different fate as it is blown away from the Sun by radiation pressure on the tiny grains. That forms a broad, curved, sometimes yellow-coloured tail following the comet in its orbit and pointed generally away from the Sun, which is known as a Type II tail. The grains are blown into a larger orbit than the comet nucleus, and that results in their slowing because of the laws of planetary motion, causing them to lag behind the nucleus. The dust follows the comet around its orbit but eventually disperses into the zodiacal dust cloud. In 1986 American astronomer Mark Sykes and colleagues discovered faint trails of material in images of the sky taken by the Infrared Astronomical Satellite. Sykes showed that those trails matched the orbits of several well-known periodic comets, including Encke’s Comet and 10P/Tempel 2. Further analysis showed that the trails were collections of relatively large particles, from 100 microns to 1 cm in radius, that had been ejected from the comets but whose orbits changed very slowly because they were too big for solar radiation pressure to easily push around. Some comets display anti-tails that are pointed straight at the Sun. These are only seen as Earth passes through the comet’s orbital plane. However, what is seen is a projection effect, and the anti-tails are actually the Type II dust trail curving behind the nucleus into the line of sight.

3. Spacecraft exploration of comets

The latter half of the 20th century saw a massive leap forward in the understanding of the solar system as a result of spacecraft visits to the planets and their satellites. Those spacecraft collected a wealth of scientific data close up and in situ. The anticipated return of Halley’s Comet in 1986 provided substantial motivation to begin using spacecraft to study comets. The first comet mission (of a sort) was the International Cometary Explorer (ICE) spacecraft’s encounter with Comet 21P/Giacobini-Zinner on September 11, 1985. The mission had originally been launched as part of a joint project by the U.S. National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) known as the International Sun-Earth Explorer (ISEE). The mission consisted of three spacecraft, two of them, ISEE-1 and -2, in Earth orbit and the third, ISEE-3, positioned in a heliocentric orbit between Earth and the Sun, studying the solar wind in Earth’s vicinity. In 1982 and 1983 engineers maneuvered ISEE-3 to accomplish several gravity-assist encounters with the Moon, which put it on a trajectory to encounter 21P/Giacobini-Zinner. The spacecraft was targeted to pass through the ion tail of the comet, about 7,800 km (4,800 miles) behind the nucleus at a relative velocity of 21 km (13 miles) per second, and returned the first in situ measurements of the magnetic field, plasma, and energetic particle environment inside a comet’s tail. Those measurements confirmed the model of the comet’s ion tail first put forward in 1957 by the Swedish physicist (and later Nobel Prize winner) Hannes Alfvén. It also showed that H2O+ was the most common ion in the plasma tail, consistent with the Whipple model of an icy conglomerate nucleus. However, ICE carried no instruments to study the nucleus or coma of the comet. In 1986 five spacecraft were sent to encounter Halley’s Comet. They were informally known as the Halley Armada and consisted of two Japanese spacecraft, Suisei and Sakigake (Japanese for “comet” and “pioneer,” respectively); two Soviet spacecraft, Vega 1 and 2 (a contraction of Venus-Halley using Cyrillic spelling); and an ESA spacecraft, Giotto (named after the Italian painter who depicted the Star of Bethlehem as a comet in a fresco painted in 1305–06).