World's most famous comet is on its way back towards Earth

 The world's most famous comet is on its way back towards Earth Starting today

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 which brings it near to the sun and then takes it far out to beyond the outer limits of the solar system. Ever since 9 Feb, 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. From that time until the present, the comet has been moving away from the sun. 

But at 8 p.m. Eastern Time on Friday, 8 Dec, (0100 GMT on 9 Dec ) which will come to an end. For this moment, Halley's Comet will arrive at aphelion; the far end of its orbit — the location in space which 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. When Halley arrives at aphelion, its orbital velocity will get slow to just 0.565 miles (0.909 km) per second; about 2,034 miles (3,272 km) per hour. Compare to when the comet was speeding around the sun at perihelion on 9 Feb, 1986 at 33.77 miles (54.52 km) per second; some 121,572 miles (195,609 km) per hour.  So, after 8 Dec, 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 increase, on its way inbound toward the sun.