Beginning on Friday, Dec. 8 we can finally state with absolute certainty that Halley’s Comet is coming.
This most famous of all comets travels around the sun in a flattened elliptical orbit that brings it near to the sun and then takes it far out to beyond the outer limits of the solar system. Ever since Feb. 9, 1986, when it arrived at perihelion — the comet’s closest approach to the sun — it then began its long journey back out into distant space. And from that time up until the present, the comet has been moving inexorably away from the sun.
But at 8 p.m. Eastern Time on Friday, Dec. 8, (0100 GMT on Dec. 9.) that will come to an end. For at that moment, Halley’s Comet will arrive at aphelion; the far end of its orbit — that location in space that places the comet at its farthest point from the sun: 3.27 billion miles (5.26 billion km) distant. The comet will be then be 472.2 million miles (759.8 million km) beyond the orbit of Neptune, the most distant known planet.
The last time Halley was at this point in its orbit was in April 1948.
Related: The Eta Aquarid meteor shower is the legacy of Halley’s Comet in May’s night sky
When Halley arrives at aphelion, its orbital velocity will have slowed to just 0.565 miles (0.909 km) per second; about 2,034 miles (3,272 km) per hour. Compare that to when the comet was speeding around the sun at perihelion on Feb. 9, 1986 at 33.77 miles (54.52 km) per second; some 121,572 miles (195,609 km) per hour.
So, after Dec. 8, the comet will again — for the first time in nearly 38 years — be approaching the sun. In concert with Kepler’s second law of motion, a celestial body moves fastest when it is at perihelion and slowest at aphelion. So, once Halley passes through aphelion, its orbital velocity will begin to — very slowly at first — increase, on its way inbound toward the sun.
In the table below, we note the times of when Halley will cross the orbits of seven planets as it moves inbound toward the sun. The mean distance of each orbit is given in astronomical units (A.U.). One astronomical unit is equal to the Earth’s mean distance from the sun of 92,955,807 miles (149,597,870 km).
| Planet orbit | Distance (AU) | Date of crossing |
|---|---|---|
| Neptune | 30.6 | May 7, 2041 |
| Uranus | 19.2 | May 1, 2053 |
| Saturn | 9.54 | Dec. 7 2058 |
| Jupiter | 5.2 | June 25, 2060 |
| Mars | 1.52 | May 16, 2061 |
| Earth | 1.0 | June 19, 2061 |
| Venus | 0.72 | July 9, 2061 |
Halley’s Comet will again arrive at perihelion on July 28,…
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