Message-Id: <v01540b11b059dc6e722d@[192.174.2.173]>
Date: Thu, 2 Oct 1997 15:21:28 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: Re: Cassini probe
Hi Pinholers
Here are NASA'a answers to the questions on Cassini's radioisotope thermal
generator. at
http://www-b.jpl.nasa.gov/cassini/MoreInfo/faq.html#faq31
I can't find any large errors in thier statements.
Paul D
Why can't Cassini use solar-power?
It was determined that 12 science instruments are needed to
investigate Saturn, its rings, moons and
magnetosphere over a 4-year period in order to meet the Cassini
science objectives that were set by the NASA
Solar System Exploration Committee. This results in a spacecraft and
instrument power demand of between
600-700 watts of power in outer space. This power must be produced
reliably for over 12 years at a distance
that is 9 times further from the sun than the earth, and still be
small and light enough to be launched from the
earth and reach Saturn.
NASA's Jet Propulsion Laboratory conducted an in-depth analysis of
the available electrical power systems,
including many different solar, battery, and long life fuel cell
power sources and hybrid systems to identify the
most appropriate power source for the Cassini mission. A Cassini
spacecraft equipped with the highest
efficiency solar cells available (including the new high-efficiency
cells under development by ESA) would
make the spacecraft too massive for launching to Saturn. The
resulting solar arrays would be over the size of
two tennis courts. RTGs are the only feasible power system for the
Cassini mission.
If the Cassini spacecraft had been: on the Titan IV that failed during launch on
August 2, 1993, at Vandenberg; or on the Space Shuttle Challenger when it
failed,
what would have happened to the RTGs?
Neither of these launch accidents would have been expected to result
in a release of fuel from the RTGs had
the Cassini spacecraft been onboard. Years of extensive safety
testing and analyses have demonstrated that
RTGs are extremely rugged and resistant to a release of the
plutonium dioxide fuel, even in severe accident
environments. An event somewhat similar to the August 1993 Titan IV
accident occurred in October 1968. An
Atlas rocket carrying an RTG was destroyed shortly after launch from
Vandenberg. The plutonium-containing
portions of the RTG fell into the ocean intact. All of the plutonium
was recovered and used in a subsequent
mission
What is the probability of an accident between launch and leaving Earth
orbit that
might release plutonium?
While it is estimated that the probability of a Cassini launch
failure is about 1 in 20, most failures would not
result in a release of plutonium. Though more detailed assessments
are underway, initial estimates are that
about 1 in 25 Titan IV/Centaur failures could to result in releases
of small quantities of plutonium dioxide to
the environment. It is possible that there could be small releases
of plutonium dioxide particles from some
RTG components, but if the components strike water there would be no
release. None of the releases are
expected to result in any cancer fatalities in the exposed population.
How can the probability of an Earth swingby reentry accident be so low?
The Cassini mission is being designed to ensure than an inadvertent
swingby accident does not occur. Mission
rules state that the chance of such an accident occurring must be
less than one in one million. JPL has
conducted an in-depth analysis, which incorporated human error and
historical JPL spacecraft data, to
determine the probability of an inadvertent reentry. This analysis
determined that the probability of an
inadvertent Earth reentry is less than one in one million. This
result may be surprising to some people (at first)
since it is difficult to prove that failures of any system,
particularly spacecraft, can be that small. The result is
driven by two factors.
First, for most of the Cassini trajectory it is very hard to hit the
Earth. In fact, until about 50 days before Earth
swingby, the probability of hitting the Earth is much less than one
in one million regardless of the spacecraft
failure (this is because of the vastness of space, the smallness of
the Earth as a target, and the randomness of a
spacecraft failure or micrometeoroid hit leading to a velocity change).
Second, JPL has "biased" the trajectory for Earth swingby. This
scheme further limits the time and events that
could cause inadvertent reentry by eliminating all failures except
those that give the spacecraft the proper
velocity magnitude and direction to impact the Earth. The spacecraft
is biased 5,000 kilometers or more away
from the 800 km swingby altitude for all but the last week prior to
the swingby. The navigation accuracy of
NASA spacecraft is better than 20 km. The biasing strategy effects,
coupled with redundant spacecraft system
design, built-in fault detection and correction systems, and
controlled operation (via sending commands to the
spacecraft), particularly during the limited time when failures
could cause impact, lead to the exceedingly small
probability of Earth impact.