by Jonathan Sarfati (This article has been excerted from Creation Ministries International . The original article can be found here.)
When comets pass close to the sun, some of the ice evaporates, and forms a coma typically 10,000–100,000 km (rarely up to one million km) wide. Also, the solar wind (charged particles radiating from the sun) pushes a tail of ions (electrically charged atoms) directly away from the sun. Solar radiation pushes away dust particles to generate a second tail that curves gently away from the sun and backwards.
The coma and tails have a very low density—even the best vacuums produced in laboratories are denser. The Earth passed through a tail of Halley’s comet in 1910, and it was hardly noticeable. But comets reflect the sun’s light very strongly, which can make them very spectacular when they are close to both the sun and Earth. The appearance like a hairy star is responsible for the term ‘comet’, from the Greek word κομητης comētēs (long-haired) from κομα (coma) = hair.
This means that the comet is slowly being destroyed every time it comes close to the sun. In fact, many comets have been observed to become much dimmer in later passes. Even Halley’s comet was brighter in the past. Also, comets are in danger of being captured by planets, like Comet Shoemaker–Levy crashing into Jupiter in 1994, or else being ejected from the solar system. A direct hit on Earth is unlikely, but could be disastrous because of the comet’s huge kinetic (motion) energy. The problem for evolutionists is that given the observed rate of loss and maximum periods, comets could not have been orbiting the sun for the alleged billions of years.
Two Groups of Comets
Comets are divided into two groups: short-period (<200 years) comets, such as Halley’s (76 years); and long-period (>200 years) comets. But the comets from the two groups seem essentially the same in size and composition. Short-period ones normally orbit in the same direction as the planets (prograde) and in almost the same plane (ecliptic); long-period comets can orbit in almost any plane and in either direction. One exception is Halley’s, which has retrograde motion and a highly inclined orbit. Some astronomers suggest that it was once a long-period, and strong gravity from a planet dramatically shrunk its orbit, and thus the period. So long-period and Halley-type comets are grouped together and called ‘nearly isotropic comets’ (NICs).
The highest period of a stable orbit would be about four million years if the maximum possible aphelion (furthest distance of an orbiting satellite from the sun) were 50,000 AU.9 This is 20% of the distance to the nearest star, so there’s a fair chance other stars could release the comet from the sun’s grip.10
However, even with this long orbit, such a comet would still have made 1,200 trips around the sun if the solar system were 4.6 billion years old. However, it would have been extinguished long before. The problem is even worse with short-period comets.
Comets lose so much mass every time they shine that they could not be billions of years old. Evolutionists propose various sources to replenish the comet supply, but there is no real observational evidence, and numerous unsolved theoretical difficulties. Therefore comets make much more sense under a Biblical timescale.