Sky and Telescope - July 2015 - 26

NASA / JHU-APL / SWRI

New Horizons at Pluto

LORRI OpNav Campaign 2

2015-01-25 2:01:00 UT
Distance to Pluto: 202,976,400 km

2015-01-27 3:21:00 UT
Distance to Pluto: 200,526,000 km

VIEW FROM AFAR New Horizons' LORRI telephoto camera
recorded Pluto and dimmer Charon on January 25th and 27th
from a distance of more than 200 million km. Flight planners use
images like these to reﬁne the objects' positions and to tweak
the spacecraft's trajectory as needed.

measuring atmospheric composition from its eﬀect on
the solar spectrum. In between the Pluto and Charon
occultations, the spacecraft will spin, allowing PEPSSI
and SWAP to sensitively map the ions and neutral atoms
streaming down Pluto's magnetotail (if it has one).
Nearly 9 hours after the ﬂyby, New Horizons will take
the briefest pause from its frenzied observations to radio
a few precious packets of engineering data about its
health. This "phone home" will take 4 hours 23 minutes to reach its apprehensive observers back on Earth,
arriving - we hope! - at 1:09 UT on July 15th, conﬁrming that the spacecraft survived the encounter and is
continuing to operate normally.
But it'll be no time for New Horizons to rest. Following the ﬂyby, as it rapidly recedes from Pluto, New
Horizons will see Pluto and all its moons with the Sun
positioned nearly behind them. This will yield pretty
crescents in photos but also provide hints about the

structure of the objects' surfaces from the way they scatter the high-angle light toward the spacecraft's cameras.
New Horizons will also attempt an imaging feat: looking down on the nightside of Pluto, it will try to record
regions not seen at high resolution in daylight, by using
sunlight that has reﬂected ﬁrst oﬀ Charon and then onto
Pluto's darkened landscape.
The Near Encounter Phase ends a day after the ﬂyby
and transitions to the Departure Phase. New Horizons
will use its cameras to study the system for only one full
Pluto day (6.4 Earth days) following the close approach,
seeing Pluto and all its moons as skinny crescents.
Facing toward the Sun, New Horizons will continue its
search for the dusty rings that might be generated from
micrometeorite impacts onto the small moons. Meanwhile, PEPSSI, SWAP, and SDC will gather data for at
least a month after the ﬂyby, corresponding to one rotation of the distant Sun.

What Will We See - and When?
All these observations will ﬁ ll the spacecraft's onboard
memory to bursting. In all, it will take about 18 months
to transmit the full, uncompressed data set back to
Earth. Naturally, the mission scientists (and we!) want
to get results as soon as possible. So through July 20th,
New Horizons will return a carefully selected 1% of all
the near-encounter science data, including 14 LORRI and
two Ralph images of Pluto, Charon, Nix, and Hydra.
After that, the image pipeline will go dry for about
two months. New Horizons will continue to transmit
data in real time from PEPSSI, SWAP, and SDC. But
before sending back any more images, the spacecraft
will ﬁrst transmit housekeeping data on all those
frames, building a "library catalog" back on Earth that is

The Long Road to Pluto
When New Horizons whizzes past Pluto this July, it will

NASA / JPL / J. FOUST

have been 9½ years since the
spacecraft left Cape Canaveral,
Florida. Yet that span pales
in comparison to the 16 years
it took to get a Pluto mission
on the launch pad in the ﬁrst
place. Starting in 1990, ﬁve
Pluto vehicles were proposed
and cancelled before New
Horizons got a green light from
NASA in late 2001. Building a

spacecraft is hard - but often
the political hurdles are harder.
The ﬁrst three Pluto proposals varied wildly in cost and
complexity. "Pluto 350" was
named after its mass in kilograms. That gave way to a twoton behemoth based on the
Saturn-bound Cassini spacecraft. The third concept, "Pluto
Fast Flyby" (PFF), called for
a pair of lightweight probes
launched a year apart.

The Pluto Fast Flyby team poses with a mockup of their
proposed spacecraft in 1992.

July 2015 sky & telescope

Sky and Telescope - July 2015

Table of Contents for the Digital Edition of Sky and Telescope - July 2015