History of cometary exploration

Stargazer · June 14, 2005

With the Deep Impact spacecraft moving closer to 9P/Tempel 1, I thought it might be a good time to look into the history of cometary exploration.

Ancient Era

Comets have fascinated humans since we first saw them in the sky. Early civilisations had different explanations on what they might be. Aristotle, for example, thought that they were slowly burning shooting stars, or "slowly burning gas in the atmosphere". In some cultures, it was thought that comets were signs of something that would happen in the future, good or bad. It is said that around the same time as Augustus Caesar became the emperor of the Roman Empire, a comet appeared in the sky. This was thought to be a blessing from the gods.

Modern Science

During the 16th century, astronomers began to seriously study the comets as their equipments improved and the telescope was invented.

In 1531, the German astronomer Petrus Apianus and the Italian physician and scholar Girolamo Fracastoro discovered, apparently independently of each other, that a comet's tail always point away from the sun.

By this time it was still not completely accepted that comets were celestial bodies. Many still thought that they were atmospheric phenomena or in any case within the radius of the moon's orbit. This changed after a great comet appeared in 1577. Many astronomers observed this comet, and one of them was the Danish astronomer Tycho Brahe, who made measurements on its movement. The idea that comets were celestial bodies began to become more accepted among scientists, and the view of the solar system and the universe had to change, not for the first time and certainly not for the last time.

There was still much debate on what comets really were. The Italian astronomer Galileo Galilei thought that the comets were optical phenomena, an idea he published in Assayer in 1623. René Descartes, the French philosopher, believed the comets travelled between solar systems. Other philosophers had their own ideas and suggestions on the true nature of the comets.

Sir Isaac Newton, one of the most famous of scientists, expanded upon Johannes Kepler's laws of planetary motion, showing that orbits could be parabolic and hyperbolic, and not only elliptic. His fellow countryman Edmond Halley discovered that the comet sightings in 1456, 1531, 1607 and 1682 were actually the same comet that had returned several times. He predicted it would return once again in 1758 - and it did. That comet is known as Halley's comet, not surprisingly perhaps.

Space Age

With the space age and humankind's ability to launch objects to other worlds, scientists could get closer to comets than ever before. Several spacecrafts with scientific instruments has been launched to study different comets. With six spacecrafts sent to study the Halley's comet, it's now the comet that has recieved the most attention from that point of view.

All the cometary spaceprobes:

ISEE 3, a NASA spacecraft. It first investigated comet 21P/Giacobini-Zinner and later went between the sun and comet Halley in 1986.

Vega 1 and Vega 2, from the Soviet Union. These two were first sent to Venus to deploy landers and balloons, and then sent on flyby trajectories towards Halley's comet. The closest approach for Vega 1 was 8,890 km, and for Vega 2 it was 8,030 km. Data from these spacecrafts was used to plan for Giotto's encounter with the comet.

Sakigake and Suisei, from Japan. The Sakigake went on a flyby mission to Halley's comet, which it passed with a distance of 6.99 million km. The Suisei probe came much closer at a distance of 150,000 km.

Giotto, from the European Space Agency. Giotto approached the comet closer than the other spacecrafts, with merely 596 km of distance. In 1992 it also visited the comet 26P/Grigg-Skjellerup. The Giotto mission revealed that Halley's comet was a porous and icy object, though with large amounts of dust. Several huge jets shooting out matter were observed as well.

Deep Space 1, a NASA technology demonstration mission, that also visited comet 19P/Borrelly in 2001 during the extended mission, and sent back the best images of a comet so far at the time.

Stardust, a NASA comet sample return mission to comet 81P/Wild. It came within 250 km from the comet on the flyby course in 2004, and captured many particles that will be sent back to Earth for analysis. The Stardust also captured some very detailed imagery of the surface of the comet, revealing gas vents and craters.

Deep Impact, a NASA comet mission to 9P/Tempel 1. Launched in January 2005 it will arrive on July 4, 2005. The objective is to release a 370 kg copper projectile to smash into the comet, creating an artificial crater and ejecting internal matter of the comet, which will then be studied.

Rosetta, an ESA comet mission to 67P/Churyumov-Gerasimenko. It was launched in 2004 and will arrive in 2014. The objectives are to study the comet from orbit, and to release a small lander called Philae.

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