STATUS REPORT
Date Released: Tuesday, August 9, 2005
Source: Ames Research Center
NASA NEO News: Deflection Scenarios for Apophis
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Following is an unusually long and technical edition of NEO News. The subject is the deflection options for Apophis (MN4) as described in a new analysis by Donald Gennery, who has kindly made this draft available to NEO News. Future editions will revert to the usual format.
David Morrison
WHAT SHOULD BE DONE ABOUT ASTEROID APOPHIS (2004 MN4)?
Donald B. Gennery
dgennery@earthlink.net
August 7, 2005
1. Introduction
In a recent paper [1] and letter [2], Rusty Schweickart made some recommendations on dealing with the threat of a possible impact in 2036, and he called on further analysis to be done. This is my input to that analysis. Comments are welcome.
The most important thing that I propose is that deflection by the impact of a spacecraft is practical in this case. Such a mission could be done fairly quickly at a reasonable cost.
The asteroid under discussion, with the provisional designation 2004 MN4, has now been assigned the number 99942 and the name Apophis. (Apophis was the Greek name of the Egyptian god Apep, "the destroyer.") Therefore, I use this name below.
2. Background Review
Apophis will make a very close pass by Earth (roughly 37,000 km) on April 13, 2029. The deflection of its trajectory by Earth's gravity at that time will greatly magnify the uncertainty in its orbit, making predictions of a possible future collision with Earth difficult at this time. There are several dates that (as of July 31) have a slight chance of impact. Especially, April 13, 2036, has a probability of impact equal to 0.00012, with lesser probabilities for April 14, 2035, and April 13, 2037 [3]. Since the diameter of Apophis is 320 m, it could cause destruction over a large local area. Apophis will make fairly close passes by Earth (roughly 0.1 AU) in 2013 and 2021 that will allow accurate measurements of its orbit, and easier trajectories to it are available around those times.
Because of the above facts, Schweickart called for immediate consideration of a plan to start work very soon on a mission to Apophis that would place a radio transponder on the asteroid, so that the knowledge of its orbit can be improved enough to make a decision by 2014 as to whether or not to start work on a mission to deflect Apophis. He said that any later start date than 2014 on a deflection mission might not allow enough time to deflect Apophis before the close pass in 2029, after which deflection will become much more difficult, especially for a possible impact only about 7 years thereafter. He considered the possibility that 6 years might be enough for the deflection mission, but he considered it more likely that a deflection mission might require as long as 12 years and a transponder mission 7-8 years.
In deciding how much deflection might be needed, there are three components to consider. One is the width of the "keyhole" through which the center of mass of Apophis would have to pass in 2029 in order to hit Earth in 2036. According to Schweickart, this is only 641 m. Therefore, to move out of the keyhole might take as much as half of this, or 0.32 km. Another, much larger, component is the uncertainty in the orbit due to measurement errors. At present, as extrapolated to 2029, this has a standard deviation (sigma) of 1800 km. Using a 5-sigma tolerance for safety thus could require a deflection of 9000 km. However, this large uncertainty results from data having only a short time span. As more measurements are taken around 2013 and 2021 this value will greatly decrease, probably to much less than 100 km. The third component is the fact that the orbit is changing because of the Yarkovsky effect, as Schweickart pointed out in his July letter.
The Yarkovsky effect is the phenomenon in which the orbital energy of an object changes due to a nonradial force caused by the fact that the absorption and reradiation of energy from the Sun are in different directions, depending on the rotation of the object. This causes the object to either accelerate or decelerate in its orbit, depending on whether energy is being subtracted or added. If the rotation, shape, and thermal properties of the object are known, the direction and magnitude of this effect can be calculated. However, at present these are largely unknown for Apophis, so extrapolating from the present to 2029 could produce an uncertainty from this cause of a few thousand kilometers. Future measurements will reduce this uncertainty also; some possibilities are mentioned in Section 4.
