VI.

Observation and Exploration of Comets

A.

Early Observations of Comets

Thousands of comets have been observed and recorded over the past 2,500 years. Before comets were understood as natural objects, they were seen as supernatural signs. For example, a bright comet that appeared in 44 bc after the murder of the Roman leader Julius Caesar was hailed as a sign that Caesar had become a god.

Although comets appeared to move through the sky like planets, the 4th-century bc Greek philosopher Aristotle described comets as objects in Earth’s atmosphere. Appearances of large comets were regarded as atmospheric phenomena until 1577, when Danish astronomer Tycho Brahe, working before the invention of the telescope, proved that they were celestial bodies distant from Earth. In the 17th century British scientist Isaac Newton demonstrated that the movements of comets are subject to the same laws of gravitation and motion that control the planets in their orbits.

By comparing the orbital elements of a number of earlier comets, British astronomer Edmond Halley showed the comet of 1682 to be identical with the two that had appeared in 1607 and 1531. Minor variations in the period could be accounted for by gravitational interactions. Halley successfully predicted the comet’s next return, which occurred in 1758 after his death. Many earlier appearances of what came to be known as Halley’s Comet have now been identified from records dating from as early as 240 bc, and it is probable that the bright comet observed in 466 bc was also this famous comet.

Halley’s Comet most recently passed around the Sun again early in 1986, but was not especially bright compared to past appearances. Its future reappearance in 2061 is not predicted to be especially bright either. The Hubble Space Telescope is now able to study Halley’s Comet as a faint spot throughout its orbit, though it does not currently have a tail

B.

Modern Study of Comets

The scientific study of comets was greatly aided by the invention of the telescope in the early 17th century. Detecting and cataloging comets became an important task for professional astronomers. However, amateur astronomers with telescopes have been the first to detect a number of notable comets. A comet is usually detected after it starts to form a coma that reflects sunlight, making the object brighten. Most comets found since the late 20th century were discovered with the aid of astronomical instruments such as the Large Angle Spectroscopic Coronagraph (LASCO) on the SOHO satellite that observes the Sun, or with Earth-based computer-controlled telescope systems such as the Lincoln Near Earth Asteroid Research (LINEAR) and Near Earth Asteroid Tracking (NEAT) projects that scan the skies and detect small objects that move.

A number of spacecraft have provided scientists with important data about comets. In 1974 the crew of Skylab, the first U.S. space station, used a solar telescope to observe Comet Kohoutek as it approached the Sun. In 1986 Halley’s Comet was visited by two probes, Vega 1 and 2, which were launched by the Soviet Union, and by another spacecraft called Giotto, which was launched by the European Space Agency (ESA). Giotto made the closest approach to the comet, coming within about 600 km (375 mi) of its nucleus. Two Japanese spacecraft observed Halley’s Comet at a great distance as it passed.

Giotto and the Vega spacecraft were equipped with cameras. Their images confirmed that Halley’s nucleus was very black, reflecting only a small percent of the sunlight that strikes it. Its dark color probably comes from the presence of hydrocarbons. Images also showed that the nucleus had an elongated, irregular outline shaped somewhat like a potato. Several bright, localized jets of escaping gas and dust spurted from the nucleus, which was about 15 km (9 mi) long and 7 km (3.6 mi) wide.

In January 2004 a United States spacecraft called Stardust, which was launched in 1999, became the first spacecraft to gather sample dust grains from a comet as it flew through the coma of Comet Wild (pronounced vilt) 2. The spacecraft encountered the comet as it orbited the Sun about 390 million km (240 million mi) from Earth. Stardust’s cameras also took closeup images of the comet’s nucleus from a distance of about 240 km (149 mi). As the spacecraft passed through the coma, it used a special device to gather a tiny amount of microscopic dust grains and sealed them in a canister containing an extremely low-density material known as aerogel, which trapped the particles.

Stardust jettisoned a capsule containing the canister when the spacecraft flew by Earth on its return journey in January 2006. The capsule successfully reentered Earth’s atmosphere, its final descent slowed by a parachute, and was recovered on January 15 at a landing site in Utah.

Scientists with the National Aeronautics and Space Administration (NASA) then examined the canister. The lead scientist for the mission, astronomer Donald Brownlee, calculated that it contained more than a million microscopic specks of dust. Later analysis of the dust revealed that the composition of Comet Wild 2 was surprisingly similar to material found in asteroids. The dust in the comet had been heated and chemically altered from the primitive material that first coalesced into the early solar system. Much of the material apparently formed close to the early Sun and not in the cold, icy outer regions of the solar system. Some process may have mixed material in the early solar system.

In July 2005 NASA successfully engineered the first collision between a human-launched object and a comet in an effort to penetrate a comet’s outer crust and thereby expose chemical compounds located within the comet’s nucleus. NASA’s Deep Impact spacecraft, which was launched from Earth in January 2005, rendezvoused with Comet Tempel 1 about 134 million km (83 million mi) from Earth. As the spacecraft approached the comet, it released a smaller craft known as an impactor that slammed into the comet’s nucleus on July 4 at 1:52 am Eastern Daylight Time. The impactor was destroyed and the impact sent a plume of debris from the comet billowing into space.

Both Earth-based and space-based telescopes, along with cameras and other scientific instruments onboard the Deep Impact spacecraft and the impactor itself, observed the approach and collision, and recorded data for later analysis. Recent study of the data indicates that the comet contains a wide range of chemicals, including carbonates, clays, metal sulfides, crystalline silicates, and aromatic hydrocarbons. Some of the compounds must have formed in the presence of liquid water, while others require the extreme high temperatures found near the Sun, findings in line with the Stardust results.

The ESA’s Rosetta spacecraft is planned to be the first spacecraft to go into orbit around a comet and to place a lander on its nucleus. The lander is named Champollion in honor of the famous 19th-century French scholar who decoded the Egyptian hieroglyphs on the Rosetta stone. The 100-kg (220-lb), box-shaped lander carries a variety of instruments to measure the composition of the nucleus and return both panoramic and microscopic images. Rosetta was launched in March 2004 and is expected to reach Comet 67P/Churyumov-Gerasimenko in 2015.