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of space exploration—30 AU, or
2.7 billion miles (4.4 billion km) from
Earth. To achieve this, the spacecraft
was given the fastest launch ever,
blasting off with an escape velocity
of 36,373 mph (58,536 km/h). A year
after launch, the spacecraft reached
Jupiter. In addition to making some
observations of the Jovian system,
New Horizons used Jupiter’s gravity
to gain a 20 percent speed boost.
This cut the flight time to Pluto
from over 12 years to 9.5 years.
Instruments on board
The precision of the trajectory from
Jupiter was crucial to the success of
New Horizons. If it was just slightly
off, the craft would miss Pluto
altogether. The main observation
window was about 12 hours long,
after which New Horizons would
be leaving Pluto behind. It takes
radio signals from Pluto 4.5 hours
to reach Earth, plus the same again
to send a return message. Therefore,
it would take at least nine hours to
make even tiny course corrections,
by which time the primary mission
would be almost over.
New Horizons carried seven
instruments. They included two
imaging spectrometers, built to
work together and named after the
characters in the 1950s US sitcom
The Honeymooners. Ralph was the
visible and infrared spectrometer
used to make maps of Pluto’s
surface, while Alice was sensitive
to ultraviolet and was tasked with
studying Pluto’s thin atmosphere.
REX (Radio science EXperiment)
would take the temperature of
Pluto and its moons; LORRI (LongSTUDYING PLUTO
Range Reconnaissance Imager), a
telescopic camera, would produce
the highest resolution pictures
of the Pluto system; SWAP (Solar
Wind Around Pluto) would, as the
name suggests, observe Pluto’s
interaction with the solar wind,
while PEPSSI (Pluto Energetic
Particle Spectrometer Science
Investigation) detected the plasma
given off by Pluto. This would help
in understanding the way the
dwarf planet’s atmosphere is
formed by sublimation (the change
of a solid directly into a gas) from
the icy surface during Pluto’s
“summer” as it nears the sun
(and then freezes again in winter).
Finally the SDC (Student Dust
Counter) was an instrument
operated by university students
throughout the mission. This
experiment was renamed VBSDC
for Venetia Burney, the British girl
who had proposed the name Pluto.Destination reached
New Horizons began its approach in
January 2015. One of the first things
it did was to make an accurate
measurement of Pluto’s size. This
had always been a tricky problem to
solve. When it was first discovered,PEPSSILORRISWAP
VBSDCRalphAliceREXNew Horizons’ scientific instruments were
switched off to conserve power for most of its
10-year journey, but they were powered up for
one month a year so that checks could be made.It used to be that Pluto was a
misfit. Now it turns out that
Earth is the misfit. Most
planets in the solar system
look like Pluto, and not like
the terrestrial planets.
Alan Stern