New Horizons, Not Quite to Jupiter, Makes First Pluto Sighting
November 28, 2006
The New Horizons team got a faint glimpse of the mission’s distant,
planetary target when one of the spacecraft’s telescopic cameras
Pluto for the first time.
The Long Range Reconnaissance Imager (LORRI) took the pictures during
optical navigation test on Sept. 21-24, and stored them on the
spacecraft’s data recorder until their recent transmission back to
Earth. Seen at a distance of about 4.2 billion kilometers (2.6 billion
miles) from the spacecraft, Pluto is little more than a faint point of
light among a dense field of stars. But the images prove that the
spacecraft can find and track long-range targets, a critical capability
the team will use to navigate New Horizons toward 2,500-kilometer wide
Pluto and, later, one or more 50-kilometer sized Kuiper Belt objects.
Mission scientists knew they had Pluto in their sights when LORRI
detected an unresolved “point” in Pluto’s predicted position, moving at
the planet’s expected motion across the constellation of Sagittarius
near the plane of the Milky Way galaxy. Pluto appears in all three
images of that region of space LORRI photographed on Sept. 21 and Sept.
24, confirming that it was “real” and not a cosmic ray or other object.
For further confirmation, the object moving along Pluto’s predicted
in the sky has a visual magnitude (brightness) a little brighter than
14, just what could be expected from Pluto at that time and that
distance from New Horizons.
To analyze the images for their moving target, the team actually pulled
a page out of Clyde Tombaugh’s Pluto discovery book, stroboscopically
switching between multiple images of the same area taken days apart.
Using this technique, objects such as stars appear stationary, but
moving targets, such as a planet, are easily seen jumping between
positions against the star field.
“Finding Pluto in this dense star field really was like trying to find
needle in a haystack,” says New Horizons Principal Investigator Alan
Stern, of the Southwest Research Institute. “Clyde Tombaugh would have
been proud because the LORRI team had to use the same technique that
served him so well in discovering Pluto, but because LORRI produces
digital images, they could avoid all the messy chemicals Clyde needed
develop the photographic plates!”
LORRI, designed and built by the Johns Hopkins University Applied
Physics Laboratory (APL), is crafted to obtain images at the highest
possible resolution from the longest possible distance. This latest
optical navigation test simulated the conditions under which LORRI will
be required to find a Kuiper Belt object (and potential flyby target)
New Horizons approaches Pluto.
“LORRI passed this test with flying colors, because Pluto’s signal was
clearly detected at 30 to 40 times the noise level in the images,” says
New Horizons Project Scientist Hal Weaver of APL.
“Those of us who calibrated LORRI on the ground and in flight are not
surprised to see what it can do, but we are mighty grateful that LORRI
has survived launch and its first several months in space without any
loss of performance,” says LORRI Principal Investigator Andy Cheng, of
APL. “We’ll have to wait until early 2015 for LORRI to return better
views of Pluto than have ever been seen before. In the meantime, we’re
looking forward to viewing the marvels of the Jupiter system this
January and February.”
Just beyond the Jupiter encounter, Stern says, the team will use LORRI
to begin collecting valuable data on Pluto itself.
“We won’t get useful science out of these first detections of Pluto,”
says. "But during the next several years of approach, we’ll use LORRI
study Pluto’s brightness variation with our angle to the Sun to build a
‘phase curve’ we could never get from Earth or Earth orbit. This will
allow us to derive new information about Pluto’s surface properties
while we are still far away.
The Long Range Reconnaissance Imager (LORRI) on New Horizons acquired
images of the Pluto field three days apart in late September 2006, in
order to see Pluto’s motion against a dense background of stars. LORRI
took three frames at 1-second exposures on both Sept. 21 and Sept. 24.
Because it moved along its predicted path, Pluto was detected in all
These images are displayed using false-color to represent different
intensities: the lowest intensity level is black, different shades of
red mark intermediate intensities, and the highest intensity is white.
The images appear pixilated because they were obtained in a mode that
compensates for the drift in spacecraft pointing over long exposure
times. LORRI also made these observations before operators uploaded new
flight-control software in October; the upgraded software package
includes an optical navigation capability that will make LORRI
approximately three times more sensitive still than for these Pluto