【英文】宇宙诞生于几千年前证据之：短命彗星
2011-05-24 04:14阅读：
Short Period Comets
by Jonathan Sarfati (This article has been excerted from
Creation Ministries International . The original article
can be found here.)
http://bit.ly/iTYYem
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.
Conclusion
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.
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