Commonly Asked Questions:
 
Q:	How do RTG's work ?
A:	The Cassini spacecraft derives its electrical power from
radioisotope thermoelectric generators (RTGs), which are lightweight,
compact spacecraft power systems that are extraordinarily reliable. RTGs
are not nuclear reactors and have no moving parts. They use neither
fission nor fusion processes to produce energy. Instead, they provide
power through the natural radioactive decay of plutonium (mostly Pu-238, a
non-weapons-grade isotope). The heat generated by this natural process is
changed into electricity by solid-state thermoelectric converters.
	RTGs enable spacecraft to operate at significant distances from
the Sun or in other areas where solar power systems would not be feasible.
They remain unmatched for power output, reliability and durability by any
other power source for missions to the outer solar system.
	The United States has an outstanding record of safety in using
RTGs on 23 missions over the past 30 years. While RTGs have never caused a
spacecraft failure on any of these missions, they have been onboard three
missions that experienced malfunctions for other reasons. In all cases,
the RTGs performed as designed.
	An RTG consists of two parts: a source of heat and a system for
converting the heat to electricity. The heat source contains a
radioisotope, such as plutonium-238, which becomes physically hot from its
own radioactive decay. This heat is then converted to electricity by a
thermoelectric converter which uses the Seebeck effect, a basic principle
of thermoelectricity discovered in 1822. An electromotive force, or
voltage, is produced from the diffusion of electrons across the joining of
two different materials (like metals or semiconductors) that then form a
circuit when the ends of the converter are at different temperatures.
	Each RTG contains 18 separate heat source modules, and each module
encases four plutonium-238 pellets. The modules are designed to survive a
range of postulated accidents: launch vehicle explosion or fire, reentry
into the atmosphere followed by land or water impact, and post-impact
situations. An outer covering of graphite provides protection against the
structural, thermal, and eroding environments of a potential reentry.
Additional graphite components provide impact protection, while iridium
cladding of the actual fuel cells provides post-impact containment. The
fuel is in the form of plutonium-238 dioxide, a ceramic material which is
resistant to fracturing. Plutonium is dangerous only if very small
particles become lodged in the lungs.
 
Q:	What if the RTG's landed in the Ocean ?
A:	Depending on where it lands it would be recovered or left there.
If left there it will create a "heat pocket" since heat will be given off.
Salt water and the ocean pressure will not effect the RTG. An outer
covering of graphite provides protection against the structural, thermal,
and eroding environments of a potential reentry. Additional graphite
component provides impact protection, while iridium cladding of the actual
fuel cells provides post-impact containment. The fuel is in the form of
plutonium-238 dioxide, a ceramic material which is resistant to
fracturing.
 
Q:	How Many Accidents occurred with RTG's ?
A:  	Three.  In 1964 there was an accidental re-entry of the  SNAP-9A
RTG of the Navy's Transit 5BN-2 						
satellite which burned up in the atmosphere (which was how early RTG's
were designed). Subsequently RTG's were redesigned to contain their
plutonium in reentry and launch accidents and have done so in the 1968
Nimbus B-1 mission and also the Apollo 13 mission of which itŐs RTG
remains in one of the deepest parts of the Pacific Ocean.
 
(over)
 
Q:	Are they as dangerous as nuclear power plants ?
A:	Potential RTG accidents are sometimes mistakenly equated with
accidents at nuclear power plants. It is completely inaccurate to
associate an RTG accident with Chernobyl or any other past radiation
accident involving fission.  RTGs do not use either a fusion or fission
process and could never explode like a nuclear bomb under any accident
scenario. Neither could an accident involving an RTG create the acute
radiation sickness similar to that associated with nuclear explosions. 
Also the main threat with nuclear reactor accidents is not plutonium, but
radioactive fission products such as Iodine-131 and Cesium-137.
	Even in the highly unlikely release of plutonium dioxide from
Cassini's RTGs in the event of an accident, independently reviewed
analysis shows that the radiation hazard to the average exposed individual
would be minuscule, about 1/15,000 of the lifetime exposure a person
receives from natural radiation sources.
 
Q:	Why couldnŐt Cassini use solar panels, fuel cells or batteries ?
A:	(1)  The behavior of solar cells at vast distances from the Sun is
not well understood and would add significant risk to the success of a
solar-powered mission to Saturn. Saturn is located approximately 1.42
billion kilometers (882 million miles) from the Sun, nearly twice as far
from the Sun as Jupiter, the next closest planet. The size of solar arrays
that would be needed, about the size of two tennis courts, would not only
be difficult to deploy reliably, but would make turns and other critical
maneuvers extraordinarily difficult to perform. This would severely
inhibit Cassini's ability to achieve its science objectives. The large
arrays would seriously interfere with the fields of view of many of the
science experiments and navigation sensors, further limiting the Cassini
mission's ability to achieve the science objectives
	(2)  Studies conducted by NASA's Jet Propulsion Laboratory (JPL)
have concluded that neither fuel cells nor spacecraft batteries
demonstrate the operational life needed for planetary missions, whose
duration can exceed 10 years from launch. In addition, the large mass of
batteries that would be needed to power a mission such as Cassini exceeds
current launch vehicle lift capabilities.
 
Q: 	Have RTG's been used on other space related missions ?
A:	YES.   Cassini, Voyager, Galileo, Ulysses, Pioneer 10 & 11 ,
Apollo 12 Science Experiments,  Mars Viking 1 Lander, Mars Soujourner,
along with numerous military  Earth orbiting spacecraft.  And even on two
Shuttle missions an radioisotope thermoelectric generator cooling and
gaseous nitrogen purge system was installed in Discovery OV 103 and
Atlantis OV-104 to support those payloads with RTGs or gaseous nitrogen
purging requirements 
 
Q:	Who made the RTG's ?
A: 	The Dept. of Energy (DOE) was responsible for producing ALL of the
RTG (Radioisotope Thermoelectric Generators) for NASA.  RTGŐs  are
necessary to power the spacecraft especially since no battery can last
that long, nor can it recharge since it will be so far from our Sun. Such
an alternate power supply source must be devised.
 
Q:	What is the success rate of the Titan IV rockets ?
A:	As of February 1997, there have been 20 Titan IV launches, of
which 19 have been successful. The only failure was in August 1993 which
was due to a flaw in one of the solid rocket motors, which has been fixed.
 
Show your support to President Clinton
1.	Call the White House Comment Office between 9:00 a.m. and 5:00
p.m. EST at  (202) 456-1111 to record your brief comment to the effect
that you are PRO-CASSINI  and that you support the launch. 
				and/or
2.	Send an e-mail message to  President Clinton
president@whitehouse.gov
 
A lot more information about the Cassini mission, itŐs scientific value,
and RTG safety can be found on the web site listed at the top of the first
page