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NASA Downgrades Asteroid-Earth Collision Risk
coondoggie writes "NASA scientists have recalculated the path of a large asteroid known as Apophis and now say it has only a very slim chance of banging into Earth.. The Apophis asteroid is approximately the size of two-and-a-half football fields, and updated computational techniques and newly available data indicate the probability of an Earth encounter on April 13, 2036 for Apophis has dropped from one-in-45,000 to about four-in-a million, NASA stated."
Isn't four in a million the same as one in 250,000 ?
This kind of uncertainty is much easier to derive with fewer question marks than deriving the risk of catastrophic failure in a complex machine.
Basically what this implies is that taking new measurements, we have an improved estimate of the position of the asteroid at the current time, and the risk of impact is taken by projecting those into the future using well known and tested dynamic estimation methods. Current uncertainty is easily defined as a 6x6 covariance matrix (for the 6 state variables), and this matrix can be determined using a good least-squares estimation method and published measurement numbers.
In other words I give these numbers a lot more credence than risk numbers on the space shuttle. Theres a lot more science and lot fewer assumptions.
Also I would be careful comparing practices in the huge human-spaceflight program, centered at JSC and KSC with those of smaller planetery exploration programs from places like JPL and Ames. They have amazingly different cultures and practices -- NASA is in no way a monolithic entity.