Pluto encounter planning
Dr. Young, who is based at the Southwest Research Institute in Boulder, Colorado, describes some key considerations when planning for
New Horizons’ Pluto encounter and beyond. Describing herself as a
‘Pluto person, rather than a spacecraft person’ and ‘pretty well-rounded when it comes to all aspects of Pluto science’, she was also heavily
involved in planning the mission as the Pluto Encounter Planning Team
Leader. Invited by Principal Investigator Dr. Alan Stern at the mission
proposal stage to plot the quality of the data needed, which required
her to plan for how the data was to be collected and how well the different instruments might work, she says she ‘was in a good position to
be his right hand.’ She says that her early work on stellar occultation
on Professor James Elliot’s team at MIT,
a field involving meticulous advance
planning, and which led to her passion
for Pluto, also stood her in good stead.
‘It was the discovery that Pluto had an
atmosphere while I was working with Jim
Elliot in 1988 that changed my life!’
Obviously, Pluto’s atmosphere was a
key target for New Horizons, as was its
interaction with the solar wind and the
complex relationship with Pluto’s surface, especially regarding the behaviour of volatiles like nitrogen and methane. And then, of course, there was
Pluto’s large moon, Charon, discovered
Figure 2: Size of Pluto and Charon compared with America. Credit: NASA
by United States Naval Observatory astronomer James Christy in 1978. At just over half Pluto’s size and about
one-eighth its mass (see Fig. 3 for statistics on Pluto and its moons),
Charon is large enough to be considered as part of a binary system
with Pluto, especially as the two orbit a barycentre (common centre of
gravity) outside Pluto itself. Figure 2 shows the sizes of Pluto in Charon
compared to that of the USA. Charon’s contrasting brightness with that
of Pluto and Earth-based observations of its surface chemistry had indicated that it could be an equally intriguing world.
Diameter
(km)
Mean distance
from the system’s
centre± of mass (km)
Orbital
period
(days)
Mass (1019kg)
(Earth = 5.97x1024kg)
Pluto
2,374
2,035
6.38723
1,305 (0.002 Earth’s)
Charon (P1)
1,212
17,536
6.38723
158.7
Styx (P5)
16x9x8
42,410
20.16
?
Nix (P2)
50x35x33
48,690
24.85
0.005 ± 0.004
Kerberos (P4)
19x10x9
57,750
32.17
?
Hydra (P3)
65x45x25
64,720
38.20
0.005 ± 0.004
Figure 3: Pluto and its family of moons – Selected statistics. Credit: NASA
Earth-based observations of Pluto’s mysterious, mottled surface,
barely resolved by the Hubble Space Telescope, also informed the
choice of hemisphere for the encounter. Figure 4 shows (a) the best
low-resolution mapping of Pluto from Earth compared to (b) the global
map of Pluto based on imaging by the New Horizons spacecraft.
Says Leslie Young, ‘We knew from early low resolution mapping from
the Earth, before we even considered the mission, that Pluto is one of
the most contrasty bodies in the solar system. We knew that where you
have rapid transitions from very dark to very bright, there’s going to be
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