Sky and Telescope - July 2015 - 24

New Horizons at Pluto

NASA / JPL / SPACE SCIENCE INST. / E. LAKDAWALLA

Rhea (150%) as Pluto

Prometheus as Nix
Pandora as Hydra
Helene as Kerberos
Telesto as Styx
Dione (108%) as Charon

1,000 km

DIVERSE TARGETS Assorted moons of Saturn serve as proxies to show the range of sizes of Pluto and its ﬁve moons. Nix is
only about 100 km across, Styx no more than 25 km. Only Pluto,
Charon, and Nix will be seen in detail during July's ﬂyby.

ognizable geology. This transition happened once Dawn's
camera recorded 200 pixels across Ceres's disk. For Pluto,
this same threshold won't occur until July 12th, just two
days before New Horizons' closest approach.
It's impossible to know when Pluto's true face will
become evident. Its high-contrast surface should provide
interesting views even from far away, but bright and dark
patches could make it diﬃcult to tell impact craters from
surface coatings. At some point during the approach,
Pluto will transition from a world seen only fuzzily to
one that has mappable, nameable features.
Mission scientists hope this transition happens by
July 7th, a week before the encounter. At that point the
disk will only appear about 50 pixels across. Pluto's slow
rotation means July 11th is the last day the spacecraft will
see some regions in sunlight.
To mitigate the possible loss of New Horizons and all
that high-priority science data, the last two days of the
approach include four "fail-safe" radio transmissions.
The spacecraft will sacriﬁce time that it might have used
for observations close to Pluto in order to send home a
tiny, carefully selected sampling of data from each of

its science instruments. Back on Earth, anxious mission scientists will receive just a few images that resolve
Pluto and its moons as geological worlds before the craft
crosses the system's potentially hazardous orbital plane.

The Long Silence
The 48 hours around closest approach are so precious
that the observing plan for that period has been set in
stone since 2009. New Horizons has even rehearsed
every turn and instrument operation of the sequence,
gathering data on empty space in a dress rehearsal of
July's command performance.
Any radio communications near this window would
come at a cost of priceless time for science. Therefore,
New Horizons will be mostly out of contact with Earth
during the ﬂyby's most critical period, called Near
Encounter Phase. It begins with the spacecraft 1.2 million km from Pluto. Atmospheric scientists believe this
is a bit beyond the maximum distance at which PEPSSI
and SWAP might detect the shock front along which the
solar wind collides with Pluto's escaping atmosphere.
Once inside this boundary, PEPSSI and SWAP will
directly sample gases and ions that originate from the
dwarf planet's atmosphere.
The spacecraft will twist and scan, pointing at Pluto,
Charon, Nix, and Hydra in turn. It will map the reﬂected
and thermal radiation coming from their surfaces
while searching for undiscovered moonlets and rings.
Its spectrometers and particle instruments will also
sweep the space into which Pluto's atmosphere might
be escaping. The plan is internally redundant so that, if
any one instrument fails, New Horizons' highest-priority
scientiﬁc goals can still be achieved with data from other
instruments.
New Horizons will brieﬂy turn toward Earth one last
time before closest approach, transmitting a ﬁ nal image
of Pluto, its globe still ﬁtting easily inside LORRI's ﬁeld
of view. This transmission will end at 3:15 UT on Tuesday, July 14th. Then we'll have to endure 22 long hours of
silence as the spacecraft gyrates through its busy obser-

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NASA / ESA / M. BUIE

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July 2015 sky & telescope

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PLUTO'S CHANGING FACE
These maps, derived from sets of
Hubble images, show that Pluto's
surface is a mottled patchwork of white,
dark-orange, and charcoal-black terrain.
Researchers believe the coloration
results from a combination of low-temperature ices and tar-like organic compounds created when solar ultraviolet
light breaks up methane molecules.
1,400 mi
2,300 km

Sky and Telescope - July 2015

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