Suzy B Software 2/Suzy B Software CD-ROM 2 (1994).iso/nasa/plut9309/plut9309.txt

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                         PLUTO FAST FLYBY FACT SHEET
                                March 1993

Solar System Exploration Division

* MISSION SUMMARY *

Pluto, the smallest planet in our solar system, has remained enigmatic
since its discovery by astronomer Clyde Tombaugh in 1930.  Pluto is the
only planet not yet viewed close-up by spacecraft, and given its great
distance and tiny size, study of the planet continues to challenge and
extend the skills of planetary astronomers.  Most of what we know about
Pluto we have learned since the late 1970s.  Such basic characteristics
as the planet's radius and mass were virtually unknown before the
discovery of Pluto's moon Charon in 1978.  Since then, observations and
inferences about Pluto-Charon, now considered a "double-planet" system,
have progressed steadily to a point where many of the key questions
about the system must await the close-up observation of a space flight
mission.

For example, there is a strong variation in brightness, or albedo, as
Pluto rotates, but we do not know if what we are seeing is a system of
varied terrains, or areas of different composition, or both.  We need a
much closer look to understand these features and the chemical,
geological and perhaps orbital history they represent.  We know there is
a dynamic, largely nitrogen and methane atmosphere around Pluto that
waxes and wanes with the planet's elliptical orbit around the sun, but
we need to understand how the atmosphere arises, persists, is again
deposited on the surface, and how some of it escapes into space.
Telescopic studies indicate that Pluto and Charon are very different
bodies, Pluto being more rocky, Charon more icy.  How and when the two
bodies in a double-planet system could have evolved so differently is a
question that awaits data from close-up observation.

More fundamentally, beyond our basic interest in Pluto and Charon, is
the likelihood that these bodies hold important keys to our
understanding of the giant planets and comets and their role in the
formation of the solar system.  From the Voyager missions to the outer
planets and their moons, we have a basic inventory of the
characteristics of the icy and rocky bodies of the outer solar system.
We have learned much about such planet-like bodies as the moons Triton
and Titan, and are beginning to understand Pluto as a third member of
this triad of small outer "planets." Data about Pluto and Charon,
gathered using ground-based and Earth-orbiting observatories like the
Hubble Space Telescope, continually improve our understanding of these
bodies and have helped define the important questions about
Pluto-Charon.  To address these questions, NASA is now studying a
robotic reconnaissance mission to Pluto-Charon called PLUTO FAST FLYBY.

Pluto Fast Flyby will be unique in its approach.  In order to minimize
cost, while containing the risks associated with lower cost, Pluto Fast
Flyby is being conceived as a pair of very small spacecraft, using,
where possible, lightweight advanced-technology hardware components.
The baseline Pluto Fast Flyby mission, based on a 1996 new start
authorization, calls for launch of the two ~110-150 kg spacecraft in
1999-2000 toward encounters with Pluto and Charon around 2006-8.  Pluto
began receding from the Sun in 1989, and its thin atmosphere is
condensing out into surface frost as it cools. Therefore, minimizing
flight time and launching at an early opportunity is important for the
mission's atmospheric and surface science objectives (see below).
There is a direct relationship between spacecraft weight and flight
time, so spacecraft design tradeoff analyses are particularly critical
for this mission.

* PLUTO FAST FLYBY SCIENCE OBJECTIVES *

o   Characterize Pluto's and Charon's global geology and geomorphology.

o   Map the surface composition of both sides of each body.

o   Characterize Pluto's neutral atmosphere, measuring its composition,
    thermal structure, and aerosol content.

* CANDIDATE EXPERIMENTS *

o   Visible Imaging System: a charge-coupled device (CCD) imaging
    camera to map surface features and geomorphology of Pluto and
    Charon, and to search for small satellites.

o   Infrared Mapping Spectrometer (perhaps sharing foreoptics with the
    CCD camera) to study the surface composition of Pluto and Charon.

o   Ultraviolet Spectrometer to measure atmospheric composition.

o   Radio Science Uplink Occultation Experiment to profile temperature
    and pressure of the atmosphere from the surface through the
    ionosphere.

* MISSION CHARACTERISTICS *

TRAJECTORY:              Two spacecraft, on direct trajectories (i.e.,
                         no gravity-assists)
LAUNCH VEHICLES:         Titan IV/Centaur or Proton; both would entail
                         kick stages
LAUNCH DATES:            1999-2000, assuming a 1996 new start
CRUISE:                  6.5-8.5 years, depending on mass
CRUISE SCIENCE:          None planned, but asteroid flyby, other
                         imaging, H/He detection, and radio science
                         are possible
ARRIVAL AT PLUTO-CHARON: 2006-2008, depending on mass and assuming a
                         1996 new start
FLYBYS:                  PFF-1 @ 10,000 km; PFF-2 TBD based on PFF-1
                         results; both flybys @12-18 km/sec
DATA RETURN:             Onboard storage capability of at least 400Mb
                         per spacecraft; science data downlink at 25-
                         40 bps to 34-meter ground stations


* BASELINE SPACECRAFT CHARACTERISTICS *
TYPE:                    Highly miniaturized descendant of the present
                         class of outer solar system platforms:
                         aluminum hexagonal bus, no deployables
MASS:                    Less than 150 kg; goal is 110-120 kg
                         (7 kg total instrument allocation)
Power:                   RTG source providing 65 watts at Pluto
Communication:           X-Band transponder; 1.47 meter high-gain antenna
Propulsion:              Pressure-fed hydrazine monopropellant design
                         delivering 350 m/s delta-V
Attitude Control:        Widefield star sensor and three solid-state
                         rate sensors
Pointing Knowledge:      Will exceed 1.5 mrad; stability of 10 urad
                         over 1 sec
Slewing Ability:         90 in 3 minutes via cold nitrogen gas
                         thrusters

                         THE PLUTO FAST FLYBY CHALLENGE:
                        A BIG MISSION FOR SMALL SPACECRAFT

Recent interplanetary spacecraft like Galileo and the upcoming Cassini
have been designed relatively large and heavy in order to bring a
maximum exploration payload (including probes) through gravity assists
and the intense radiation of Jupiter.  A large mission of this type to
Pluto had been under consideration since the Voyager 2 encounter with
the frigid Neptunian moon Triton in 1989.  The encounter revealed to a
surprised science community that Triton had polar ice caps, evidence of
seasonal changes, active volcanism, and an atmosphere.  The implications
for Pluto and Charon were recognized immediately, and spurred plans for
a Cassini-class mission.  But recent emphasis at NASA on smaller,
cheaper, and faster missions pointed toward the possibility of a much
smaller mission to Pluto-Charon.  The key for such a mission is to
deliver a scientifically useful payload to the distant system at minimum
cost, and to do so before Pluto's atmosphere collapses (in about 2020).

The Pluto Fast Flyby baseline emerged from careful consideration of a
complex web of tradeoff analyses regarding trajectory, weight, risk, and
durability, within an envelope of low-cost and scientific goals. The
overall scientific goals for a mission to Pluto and Charon were
articulated and prioritized by NASA's Outer Planet Science Working Group
(OPSWG) and endorsed by the Solar System Exploration Subcommittee of the
NASA Advisory Council.  The goals adopted for Pluto Fast Flyby are the
three first-priority goals of the OPSWG:  study of the geology and
morphology, mapping of the surface composition, and neutral atmosphere.