This is a first draft of what I plan to eventually post to the talk.origins newsgroup. Comments from real astronomers or anyone else are requested.
Gravity versus the Young-Earth Creationists
Young-earth creationists not only claim that the Earth is less then ten thousand years old, they almost always claim that the universe is itself under ten thousand years old as well. The reason for this is clear if one literally reads the first account of creation that starts the Book of Genesis which indicates that the Sun and Moon were created on the fourth day while grass, herbs, trees, and dry land were created on the third day. The discovery that the vast majority of astronomical objects are so distant that it would take light far longer than ten thousand years to reach the telescopes of astronomers is often pointed out as an observation that contradicts the premise that Genesis should be accepted as literal truth. How astronomers know how far astronomical objects are and how young-earth creationists deal with this objection are covered in detail in the Determining Distances to Astronomical Objects FAQ. It must be noted though that the astronomical absurdities of such creationist views are hardly limited to the problem of seeing light that should not have had time to reach us. Our solar system has many features that are strong evidence for the notion that it existed before ten thousand years ago. This essay focuses on one of them: an asteroid collision that can be conclusively dated as happening 5.8 ± 0.2 million years ago simply by tracing back the orbits of its fragments.
Asteroids or minor planets are small rocky objects orbiting the Sun many of which are in a "belt" between the planets Mars and Jupiter. As of February 12, 2004 there are 152,303 asteroids in the AstDyS database. Many asteroids in the asteroid belt form "families" as was first noted by Japanese astronomer Kiyotsugu Hirayama in 1918.
These families are groups of asteroids with orbital parameters closer to each other than would be expected by chance. Hirayama suggested that these families are the result of breakup of a larger asteroid. This was a reasonable hypothesis since with thousands of asteroids it would be reasonable to expect the collisions that would cause asteroids to breakup would occur but for decades there was no strong evidence to back this up. Starting with a 1999 Massachusetts Institute of Technology doctoral thesis by Schelte Bus this changed. One can examine the spectrum of the light from asteroids. If a Hirayama family is really is the result of an asteroid breakup then its members should have similar spectra due to similar chemical compositions. Furthermore not all Hirayama families will have the same composition. Bus showed that these predictions were true for some Hirayama families.
Ivezic et al. (2002) examined 10,592 asteroids (6,612 with well-determined orbits) using the Sloan Digital Sky Survey. That survey which is actually for objects outside of our Galaxy had to recognize the presence of asteroids moving in front of objects that it intended to observe so they would not contaminate the data. Ivezic et al. (2002) showed that just mentioned predictions were true and thus provided strong evidence that the members of a Hirayama family have a common origin. Indeed each of the three largest Hirayama families (Eos, Koronis, and Themis families each named after their largest member) represented different category of the four major types of asteroids. The Vesta family represents the fourth type. This family is named after (4) Vesta, which is the third largest and brightest asteroid. (The "4" in "(4) Vesta" indicates that fourth asteroid cataloged.) The studied showed that asteroids judged to be members of a family based on orbital considerations have similar composition based their observed light. The study furthermore concludes that 90% of the main-belt asteroids (i.e. between Mars and Jupiter) are members of a family.
Now that a family of asteroids has a common origin is something which can be put to good use by astronomers. But unfortunately if these families are hundreds of millions of years old, as astronomers believe them to be then enough time has passed that the family to been affected by additional asteroid collisions, by the effects that radiation has on orbits, and by chaotic orbital changes.
Nesvorny et al. (2002) set out to remedy this situation. They used a computer to search the known orbits of asteroids for groups of asteroids that might have been the result of a recent event. "Recent" meaning as an astronomer understands the term but not as a creationist would. They found that 39 asteroids that they looked at formed a "compact feature." For those familiar with statistics the clustering was statistically significant to a level greater than 99%. Calculations showed that the cluster cannot be older then a few tens of millions of years old since it would have dispersed in a greater time. Thus they concluded that the cluster "must be young--so young, in fact, that it may be the first essentially unaltered disruption structure ever identified in the main belt." They named it the Karin family after its largest member, (832) Karin.
Nesvorny et al. (2002) wished to deduce the actual age of the Karin family. The method is simply to trace back the orbits of the family's members. For this analysis they used 13 of the 39 asteroids. This subset was chosen on the basis of how well known the asteroid orbits were known so that only asteroids with well-determined orbits were used for reasons that should be obvious. Basically they used a software package that used Newton's theory of gravity to determine what the orbits of the asteroids were in the past. Since the inner planets (Mercury, Venus, Earth and Mars) have so little gravitational influence they were neglected, but the four gas giants (Jupiter, Saturn, Uranus, and Neptune) had their gravitational influence accounted for. They also neglected the effects of radiation.
The results showed that about 5.8 ± 0.2 million years ago the orbits converged to almost the exact same orbit. The researchers estimated the odds of 13 unrelated orbit randomly meeting in a period of four and a half billion years is less then one in a million. Thus it is safe to rule out coincidence and thus it would seem certain that these asteroids had the same orbit and thus were the result of a breakup that occurred 5.8 million years ago. The researchers further tested their ideas by redoing the analysis including the gravity of the inner planets and found it made no difference in the results. They also ran the calculations 50 million years into the future and 50 million years into the past and found no other alignment of orbits.
The point of all this should be obvious: The Solar System must be at least 5.8 million years old. Actually it must be even older than that since the Karin family is the only known grouping of asteroids that is young enough that such calculations can be done for the reasons already discussed above. Indeed the Karin family is itself part of much larger Koronis family of asteroids which itself is the remains of a more ancient breakup.
How can a young-earth creationist deal with this? They can't say that this was an event that was before the Earth was formed since a literal reading of the Bible clearly states that the Sun -- the object these asteroids are orbiting -- was created after the Earth was. Their usual methods of denying that it took light of astronomical objects millions or billions of years to reach us simply don't apply to the Solar System with the exception of God creating a universe that falsely appears to be old. They can't deny the theory of gravity since it so precisely and correctly predicts where objects in our Solar System will be and since it used so successfully to precisely navigate space probes. For radiometric dating and for light travel time creationists sometimes argue that constants of physics are not constant but greatly vary in time. But there is only one physical constant relevant to this analysis: the gravitational constant that governs how strong gravity is. But this is not an option since that would change Earth's orbit, affect the stability of the Sun, and make objects on Earth weigh many times greater. There is no way out for creationists other than ignoring the evidence or admitting that they were wrong about the universe being young.
Meanwhile researchers continue to use the discovery of the Karin family. Michel et al. (2003) did one such follow up study. They modeled asteroid collisions in a computer in an attempt to simulate the asteroid collision that created the Karin family. They discovered that the collision of monolithic asteroids (i.e. solid bodies) could not result in the formation of a cluster with the properties of the observed Karin family. However if the colliding asteroids were pre-fragmented, they could generate results that resembled the Karin family. Thus they concluded that the parent body of the Karin family was pre-fragmented. This should be unsurprising since the Karin family is part of the older Koronis family. Furthermore the largest members of the Karin family are according to the modeling the results of gravitational reaccumulation of the smaller objects created by the collision. Michel et al. (2001) had previously simulated that formation of the Eunomia and Koronis asteroid families and had concluded that their larger members were gravitationally reaccumulated fragments and that it was likely true for all the large asteroid families.
Ivezic, Zeljko et al. 2002. "Color confirmation of asteroid families." The paper was accepted for publication in The Astronomical Journal, Volume 124, Issue 5, pp. 2943-2948 NASA ADS link
Michel, Patrick et al. 2001. "Collisions and Gravitational Reaccumulation: Forming Asteroid Families and Satellites." Science, Volume 294, pp. 1696-1700.
Michel, Patrick et al. 2003. "Disruption of fragmented parent bodies of the origin of asteroid families." Nature, Volume 421, pp. 608-611.
Nesvorny, David et al. 2002. "The recent breakup of an asteroid in the main-belt region." Nature, Volume 417, pp. 720-722.
Schilling, Govert 2002. "All in the asteroid family." accessed February 12, 2004. Originally published in the "News Notes" section of the February 2002 issue of Sky and Telescope.
Links fixed for length by The Bad Astronomer plus three edits for grammatical and spelling corrections by Harlequin.