Halley's Comet

After observing the Eta Aquarids Meteor Shower recently I was curious to learn more about Halley’s Comet. I had not paid it much attention, other than being aware that it existed. I was aware that this meteor shower was as a result of Earth passing through the comets trail of dust and debris.

There is not intention to list names of who discovered this and who deduced that, but a general more lay, (my perspective). Well I suppose one name has to be mentioned, Edmond Halley 1656 to 1742, the comet although know and seen repeatedly for centuries, in fact going as far back back as 240BC. Thought apparently to be different objects, as one view was that comets travelled in straight lines. Halley was the first comet to be recognised as periodic.

It was Halley who using Newtons Laws proved that the Planets Jupiter and Saturn affected cometary orbits, concluding that a comet seen in 1682 was the same as one seen in 1531 and also 1607. the same object returning every 76 years. Last seen in modern times in 1986 ( wow I was 36). It is due to return in 2061 ( I would be 111). The famous Halley himself saw it in 1682 and forecast its return in 1758, did not live to see it again. His prediction was correct, there it was illuminating our skies once again.

Numbers, times / dates, distances speed are all extreme and mind blowing in astronomy. Well I find them so, how an object passing through the far flung reaches of space can follow, virtually, the same path or should I say orbit and return to pass Earth on such a regular frequency. Distances so great they had to define a new unit to deal with the universe and beyond. 1AU (Astronomical Unit) is the average distance from Earth to the Sun about 93 Million Miles.

Halley’s Comet at its closest pass by the Sun is 0.6 AU (55,800,000 Miles) at its furthest before swooping back on its Elliptical Orbit its 35AU (3,255,000,000Miles) Roughly as far a Pluto, wow my mind struggles to comprehend those distances, taking only approx. 38 years there and 38 years back. The other unusual fact is I gleaned, is that Halley’s comet is retrograde travelling in the opposite direction to the planets. This makes it the fastest (known) object in the universe. 157,838 miles per hour. At this speed it could orbit the earth in 9.5 minutes. The speed of the comet is not constant as it approaches the Sun its speed increases dramatically and as it recedes it slows down.

Halley is classified as a periodic or short-period comet (defined as a comet with an orbit lasting 200 years or less. This contrasts with long-period comets whose orbits can last for thousands of years. It is believed that Halley was originally a long-period comet, perturbed by the gravity of the large planets and pulled and trapped in the inner solar system, developing its current orbit and periodicity. Halley likely originated in the Oort Cloud.

A question I asked myself, if this comet was seen in 240BC (possibly recorded in 467BC) how old could it be? It has been speculated that this comet could have been in its present orbit for 16.000 to 200.000 years and a projected lifetime of 10,000,000 years, (yes 10 Million years). Well that’s the positive thinking, others speculate that the Comet might vaporise, split in two or be thrown out of the solar system, in a shorter time period. As if to prove the point research has shown the comet has lost mass by 80 to 90 percent over the last 2,000 / 3,000 revolutions.

Visits by space craft have confirmed the comet is a nobly, peanut shaped “dirty snowball” ( A counter description as it now believed that Halley is mainly composed of non-volatile materials and hence should be called a “snowy dirt ball”,) . by structure. comprised of a large number of small pieces held loosely together, called by some a rubble pile. The leftovers from the dawn of the solar system around 4.6billion years ago. The surface of the comet is black dusty, no doubt once up to 90% bigger it may have been whiter but the constant erosion over time, the ice content has reduced vaporised away leaving the solid behind. ( I imagine its akin to that pile of snow you create when clearing the paths and drive to your house, weeks after the snow has gone, you are still left with a dirty pile as the ice melts away and road dirt and other solids stay behind.)

Every approach to the sun causes gassing off as component parts of low boiling point begin to sublimate ( changing direct from solid to gas) from its surface and even nucleus. Examples are water, carbon monoxide , carbon dioxide. As the gas jets out it takes with it dust particles, and probably larger items of rubble. This causes the comet to develop a coma or atmosphere up to 63,000 miles across. Evaporation of the dirty ice releases dust particles which travel with the ejected gas away from the nucleus. Between them they fluoresce and scatter the solar light making the coma visible. A fraction of gas is ionised the solar ultra violet radiation, ions of the coma are pulled away by the solar winds blown away from the sun forming the comets massive tail, Dust and loose rubble no longer fixed to the comet trail behind. Gas and dust form separate tails, the gas tail follows the solar wind and is influenced by the magnetic line

This tail leads back to my meteor shower observations. The Earth passes through the Halley dust trail twice a year, the first is Eta Aquarids, the second Orionids. As the dust is travelling in the opposite direction to the Earth, entry speeds into our atmosphere are high.

These are my interpretation and a limited and personal view of Halley’s Comet.

Birmingham Astronomical Society. Author John Young