Re: SPACE: Cassini Mission Consequences

Richard Plourde (
Mon, 22 Sep 1997 13:00:16 -0400

At 06:23 PM 9/21/97 -0500, Eliezer S. Yudkowsky wrote:

>As a simple example: How can the chances of the probe
malfunctioning be a
>million to one? Do you really believe NASA on this one?

I don't think that the million-to-one estimate refers to a generic
malfunction of the probe. I think that it refers to the specific
malfunction of a course error during a specific time window of the
mission. That estimate, for example, explicitly does *not*
include launch failure, where re-entry friction does not represent
a factor.

A course error during that specific window-of-vulnerability would
represent a malfunction, while only a very small set of possible
course errors would result in a high-speed re-entry. From space,
the earth is a very small target. The velocities involved exceed
earth escape velocity, and so "decaying orbit" considerations
don't enter into the calculations.

I don't know the algorithm or the data used to calculate the
million-to-one estimate. The "roundness" of the number (a million
and not 500,000 or 2 million) leads me to suspect that the
algorithm involved inequalities, and represents a "not to exceed"
value rather than an exact value. When considered from the
perspective of the entire mission profile, it sounds pessimistic,
and not optimistic, to me.


Richard Plourde ..

"The word is not the thing, the map is not the territory"