Hidden in the outer reaches of our solar system are chunks of frozen gas, rocks and dust — the nuclei of comets. As these unassuming, icy lumps travel toward the sun, they’re transformed into brilliant celestial objects.
In ancient civilizations, the unexpected presence of these glowing objects in the heavens inspired fear, but we now know that comets are billions of years old and an intricate part of our solar system.
How Comets are Formed
Comets were created when the solar nebula collapsed to form the sun and planets. The debris froze together in clumps in distant regions of the solar system, creating a comet’s nucleus.
As a comet orbits near the sun, the ice heats and vaporizes, generating a thin atmosphere around the nucleus called a coma. Loose bits of rock and dust are blown by solar winds to form two tails illuminated by the sun. One tail contains dust particles and shines yellow and white. The second tail consists of ionized molecules and glows blue.
Where Do Comets Come From?
There are two types of comets. Short-period comets have an orbit of less than 200 years and are believed to travel from the Kuiper Belt outside of Neptune’s orbit. Long-period comets take more than 200 years for a return visit and originate in the Oort cloud, the most distant part of our solar system.
Comets have been cited as early as 1000 BC and were named kometes by Greek philosophers, which means a head of long hair. Individual comets are generally named for the persons who discovered them.
Notable Comets in Recent History
These recent comets generated scientific and public interest:
- Halley: Astronomer Edmond Halley proposed that a comet appearing in 1531, 1607 and 1682 was the same comet with a 75-year orbit. He accurately predicted its return in 1758. Halley’s Comet appeared in 1986 and will return in 2061.
- Iyeka-Seki: The brightest comet of the 20th century, Iyeka-Seki is a Kreutz Sungrazer — a comet whose orbit takes it close to the sun. In 1965, Iyeka-Seki approached to within 744,000 miles of the sun’s center. It’s next expected in 3000.
- Hyakutake: In 1996, a spacecraft called Ulysses unexpectedly crossed the tail of long-period comet Hyakutake. Scientists realized the accidental interaction was due to the surprising length of the comet’s tail. Approximately 350 million miles long, it was twice the length of any other known comet. Hyakutake is not expected for another 14,000 years.
- Hale-Bopp: In 1997, Hale-Bopp became the most widely-viewed comet, visible to the naked eye for 18 months. Its nucleus measured 60 miles, compared to an average nucleus which ranges from 300 feet to 30 miles. Hale-Bopp released dust streams more than eight times that of previously observed comets. Its next approach is in 4385.
- McNaught: After McNaught appeared in 2007, scientists examined the space disturbed by its presence and realized the comet’s scale. Ulysses took 2.5 days to cross the shocked solar winds around Hyakutake, but it took 18 days for it to cross those of McNaught.
There have been a dozen international space missions studying short-period comets. The European Space Agency is planning a future three-spacecraft mission to visit a long-period comet as it enters our inner solar system.
Astronomer Alan Hale, co-discoverer of Hale-Bopp, regularly documents comets. He shares information about observable comets and incoming ones.
Comets have been roaming our solar system for billions of years. Whether they’re quietly travelling distant regions or making a spectacular splash in our sky, they’re a beautiful reminder of the vastness of our galaxy and our ancient origins.