The discovery of Comet Hyakutake, a comet of great brightness that passed very close to Earth, thrilled many and renewed discussions of the likelihood of a large body colliding with Earth at some point in the future.
Human beings have taken note of comets since ancient times. A comet is a celestial body that moves around the Sun; it consists of a central mass surrounded by a misty envelope that often forms a tail that streams away from the Sun. Comets vary considerably in the size of their central masses, and most comets are too small to be observed from Earth. Through the centuries, a few comets, such as Comet Halley, have attracted fame; these comets have been easily seen with the naked eye as they made repeated close passes by Earth.
In the twentieth century, improvements in technology led to the discovery of a number of comets, including Comet Hyakutake, which was discovered by Japanese amateur astronomer Yuji Hyakutake (comets are typically named for their discoverers). As a fifteen-year-old in Fukuoka, Japan, in 1965, Hyakutake saw Comet Ikeya-Seki in the night sky, and the experience led him to a lifelong interest in astronomy. As an adult, Hyakutake, who worked as a photoengraver, moved to Kagoshima, Japan, because the area’s isolation meant that light pollution was low, allowing for relatively clear astronomical observations.
Hyakutake sought to discover a comet with a far orbit. To do so, he traveled to a rural mountaintop about ten miles from his home to get a better view of the night sky. For four nights a month beginning in July, 1995, Hyakutake scanned the sky from 2:00
At 4:50
With the discovery of Comet Hyakutake, sky watchers hoped for a truly big and active comet, but another spectacular comet along the lines of Comet Halley seemed unlikely. However, Hyakutake soon appeared to be brighter than even the original optimistic forecast. It produced roughly as much water vapor as Halley does at a comparable distance from the Sun as it approached Earth. To many experienced observers, Hyakutake qualified as a monster of a comet.
Comet Hyakutake remained visible with the naked eye for about one hundred days after February 27. On March 23, 1996, luminous knots of material—possibly small pieces of the comet’s nucleus—were first observed moving back from its golden, starlike center. The head of the comet had an aquamarine hue. The apparent proportions of the comet’s features were unlike anything the late twentieth century generation of observers had ever experienced, so they often found it difficult to interpret what they saw.
On March 23, the comet passed directly overhead the United States as seen from near 40 degrees north latitude. The comet passed perigee (the point at which it was closest to Earth) on March 25 at a distance of 9.3 million miles. Radar signals that bounced off Hyakutake’s nucleus on that date indicated that the comet was surprisingly small, only about 0.6 to 1.9 miles in diameter. Ions disconnected from the tail from March 24 to March 26, and between April 8 and April 16 great tail lengths were again seen. The tail was detected out to about 60 degrees and possibly even 80 degrees or more. (A gas tail of 20 degrees is considered long.) Gas tails are more difficult to observe than dust tails because the eye is much less sensitive to the wavelengths of light emitted by gas tails than to the light reflected by dust tails. The greatest angular lengths of Hyakutake’s tail were equivalent to about 20 million miles of gas tail.
By April 16, the comet’s nucleus became less active and less bright, as noted in both the light curve and spectrographic observations. Perihelion passage (the point at which the comet was closest to the Sun) took place on May 1, 1997, at which time there was a distance of 21.4 million miles between the comet and the Sun. Lovejoy made the last naked-eye sighting of Comet Hyakutake in late June, 1996. It is not expected to be visible with the naked eye from Earth again for at least 72,000 years.
Yuji Hyakutake spent the remainder of his life searching fruitlessly for another comet. He died of an aneurysm at the age of fifty-one. In addition to the comet he discovered, Asteroid 7291 Hyakutake was named in his honor.
In 1996, Comet Hyakutake became the brightest comet to pass near Earth in more than four hundred years. Before it appeared, perhaps only thousands of living people in all the world had managed to get a good look at a great comet (that is, a comet of particularly great brightness). Hyakutake did not come as close to Earth as 1983’s Comet IRAS-Araki-Alcock, but it captured a far greater amount of public attention. Part of this attention came from increasing fears that a close pass or collision with a comet could have disastrous consequences for the planet. A number of prominent scientists had speculated that a comet strike was responsible for environmental changes that led to the extinction of the dinosaurs, and many people had become concerned that such an event in the future would have similar consequences for humankind.
Although Comet Hyakutake thrilled many people who were interested in comets, the discovery had comparatively little significance within the scientific community. Part of the reason that most recent discoveries of comets had been made by amateur astronomers was that professional astronomers were focusing their attention on other subjects, such as greater understanding of the other planets in our solar system. It is likely, however, that just as Yuji Hyakutake was inspired to study the skies by Comet Ikeya-Seki in 1965, future astronomers were inspired by Comet Hyakutake.
-
Burnham, Robert. Great Comets. New York: Cambridge University Press, 2000. Lavishly illustrated volume provides a brief introduction to the history, nature, and beauty of comets. Includes index. -
Crovisier, Jacques, and Thérèse Encrenaz. Comet Science: The Study of Remnants from the Birth of the Solar System. Translated by Stephen Lyle. New York: Cambridge University Press, 2000. Presents an overview of the forces and processes involved in the origin and evolution of comets. Intended for readers with strong background in science. Includes glossary, bibliography, and index. -
Schaaf, Fred. Comet of the Century: From Halley to Hale-Bopp. New York: Copernicus Springer-Verlag, 1997. Combines solid history on comets and discussion of comet science with a personal account of the author’s fascination with comets. Includes maps, photographs, and index.
Organic Molecules Are Discovered in Comet Kohoutek
Astronomers Discover the Rings of Uranus
First Ring Around Jupiter Is Discovered
Astronomers Discover an Unusual Ring System of Planet Neptune
Space Probes Begin Examination of Comet Halley
Comet Shoemaker-Levy 9 Collides with Jupiter
Asteroid Toutatis Passes Near Earth