HALLEY’S COMET F.A.Q.s
We often receive inquiries from students who are assigned projects regarding Halley’s Comet. Here are some of the frequently asked questions and brief responses. We recommend searching elsewhere on our website for more detailed information on these subjects from the various pages and links we have placed here.
1. Where is the best place to view Halley's Comet?
First of all, it depends on where you are located and when the comet is in view. Don't forget, Halley's Comet has a 76 year orbit -- it was last here in 1986 and won't be back again until 2061 -- that's about 57 years from now. Each time it returns, it approaches the vicinity of Earth differently. When it was here in 1985 and 1986, it was very faint, and the best view was from the southern hemisphere. When a comet returns, because it orbits the Sun, you get two chances to see it, once on its way in (as it approaches the vicinity of the Earth and the Sun) and once on its way out (as it leaves the vicinity of the Sun and Earth). Thus, in 1985 it was best visible in November and December on the way in, and then it was visible again in March and April, 1986 on its way out. It is now (March, 2004) approaching the orbit of Neptune on its way to outer space, and will begin its return towards the Sun in 2024, arriving back in 2061. Another factor for viewing the comet is the contrast in the sky. It was best viewed in 1985 and 1986 from Cruise ships in the ocean, far away from the artificial lights of cities.
The orbit of Halley's Comet carries it far south of the plane in which the Earth and other planets revolve. The comet crosses that plane near to the Earth's orbit. The actual orbit of Halley's Comet is an elongated ellipse with a major axis of about 35.6 astronomical units (1 A.U. is the mean distance from the earth to the Sun) and a minor axis of about 9 A.U. The comet's motion is retrograde, that is, in a direction opposite to the motion of the planets around the Sun. The plane of the orbit (shaded area) is inclined 18 degrees from the plane of the earth's orbit. The comet reached its perihelion (point closest to the Sun) about .6 A.U from the Sun on February 9, 1986. Aphelion, or the point furthest from the Sun, was reached in 1948, and will be reached again in 2024, 38 years after it reached perihelion in 1986. After reaching Aphelion, it will begin its return trip to the vicinity of Earth, returning in 2061. The dots dated 1979 through 1985 show the location of of Comet Halley on February 9 in each of those years. Earth is located on this diagram in the exact spot it occupied when the comet reached perihelion on February 9, 1986. It should be noted that the Earth was in a relatively poor position for observing the comet, with the Sun directly between it and the comet. Many people were disappointed with the visibility of the comet both because of the relative positions of Earth and the comet as well as the increase in artificial lighting on Earth, inhibiting the contrast between night darkness and Halley's light. Also, the comet is wearing out!
The diagram and description above has been edited to the past tense, having appeared in The Halley's Comet Watch Newsletter, Volume I, Number 1 - May-June, 1982, three years prior to the return of the comet in 1985.
2. What caused Halley's Comet?
Halley’s Comet, as well as all comets, are simply dusty icebergs that race around the sun at speeds as fast as 600,000 miles per hour. It is composed not only of frozen water, but other frozen gases, such as ammonia and methane. Most comets are always far from the sun and frozen solid. A few comets, like Halley's, periodically travel close to the sun. When one of these comets approaches the orbit of Jupiter (or around 500 million miles) it begins to melt from the Sun's heat. A cloud of gas and dust forms which is called the comet's coma. The coma is usually larger than the Earth and may be as wide as 500,000 miles. The core of the comet remains solid and is called its nucleus. When the comet gets near the orbit of Mars (150 million miles from the sun) the gas and dirt in the coma are blown away. A long tail forms that may be 60 million miles long. It is the tail that has inspired the continual search to understand comets. Halley's comet is only one of at least 100 billion comets. It is currently believed that the comets formed at the same time as the rest of our solar system some 4.5 billion years ago.
3. Has Halley's Comet ever come close to hitting earth?
No. The closest that Comet Halley ever came to earth was in the year 837. It came within less than four million miles of the Earth. This was the 15th recorded visit of the comet to the vicinity of earth. In 1910 Halley's Comet came within 13 million miles of the Earth. In 1986 it only came within 38 million miles of Earth. The closest comet to approach Earth was Lexell's comet in 1770, at a distance of 1.39 million miles. Other close encounters were comet IRAS-Iraki-Alock 1983d, at 2.9 million miles, and comet Sugano-Saigusha-Fujikawa 1983e, which came within 8 million miles. Halley's comet doesn't make the list of Earth Grazing Comets! The 1986 return was the thirtieth RECORDED return of Halley's comet (in other words, we have written records of the 76-year returns of the comet beginning in 239 BC. The 1986 return was only the fourth PREDICTED return of Halley's Comet. You see, Edmond Halley saw the comet in 1682 and speculated that it was the same comet that was here in1607 and on previous occasions counting back an average 76 years each time -- and he predicted, based on an analysis of its orbit and the records of previous comets in history, that it would return in 1759. Although he would not be around to see it (he died in 1742), the comet returned in 1759 as predicted, and was from that point on called "Halley's Comet". In other words, Halley made the scientific discovery that comets had elliptical orbits around the sun, and would return periodically depending on the length of their orbit. It came back again on time in 1835, then in 1910, and then again in 1985.
4. How long has Halley's Comet been in orbit around the sun?
Probably 4.5 billion years! But we only have written records of it since 239 BC, and bearing Halley's name since 1759. Each time it comes back, because the sun melts some of it, it gets smaller. Eventually, as all comets that orbit the sun, it will die out - perhaps several billion years from now!
5. How is research done on Halley's Comet if it isn't seen very often?
For the first time ever, in 1985 and 1986, space craft actually flew through the tail of Halley's Comet (Japan sent two spacecraft, the Soviet Union sent one, and the European Space Agency sent one (the "Giotto")). Scientists learned a lot from these fly-throughs. Also, the Hubble space telescope taught us a lot about the comet after it had left the vicinity of Earth and the Sun, as did other newly developed exploratory tools. Meanwhile, other comets are studied using spacecraft and new advanced telescopes. Learning about ANY comet helps us understand Halley's Comet!
6. Why is Halley's Comet so famous?
I've pretty much answered this question above -- It is only because Edmond Halley, the Royal Astronomer of England, studied the orbits of comets and was the FIRST to determine that comets had elliptical orbits and traveled around the Sun. Because they returned regularly (depending on the size of the orbit) they are called "periodic" comets. In addition, Halley's Comet happens to have an orbit which is similar to the average lifetime of a person -- 76 years. Some people get to see it twice, if they live long enough. It also has a lot of "folk lore" attached to it. In effect, it isn't much different than any other comet, just that it has this "reputation" because of Edmond Halley and his prediction -- and was the first to PROVE that comets actually returned "periodically". Also, because of its predictability and "reputation" we've had some of the fun with it throughout history. Mark Twain talked about “coming in and going out” with the comet, as he was born in 1835 and died in 1910 as the comet was overhead each time. In both 1910 and 1985 there wermany events and souvenirs created to “celebrate” the hype and hoopla about Halley’s Comet.
Material prepared by Joseph M. Laufer, Editor and Publisher of “The Halley’s Comet Watch” Newsletter from 1982 through 1986.