New Horizons: A Summer's Crossing of the Asteroid Belt

The PI’s Perspective
A Summer’s Crossing of the Asteroid Belt
Alan Stern
June 1, 2006

New Horizons has entered the asteroid belt and will be traversing this
part of our solar system through August. The schematic view above shows
the scale of the asteroid belt compared to the orbits of Earth, Mars
Jupiter, each shown in blue. The sizes of the asteroids in this
illustration are vastly exaggerated - the belt is in reality almost
entirely open space. The “Trojan” asteroids orbiting ahead and behind
Jupiter are a different dynamical class of small bodies in our solar

May, like April, was a busy month for New Horizons instrument payload
commissioning. In particular, our instruments LORRI, PEPSSI, Alice and
Ralph all continued their in in-flight checkouts. In addition, the
spacecraft itself received a new suite of onboard fault protection
autonomy software resolving a number of bug fixes discovered in ground
and flight tests. We continue to see software-induced guidance computer
resets once or twice per month, but the spacecraft recovers flawlessly
from these, without any interruption to plans. The team is working on
software that will resolve the bug that causes this; we expect to have
that load tested and aboard the spacecraft around Oct. 1.

Highlights of our payload commissioning activities included door
openings for PEPSSI (May 3), Alice (May 20) and Ralph (May 29). The
Student Dust Counter (SDC) registered each of these events at the
precise time of the door openings by the noise they made on the
spacecraft. Each of these instruments also saw “first light,” i.e.,
detecting signals from stars (Ralph) or the interplanetary medium
(PEPSSI and Alice). From these tests, we appear to have a little higher
than spec sensitivity with Ralph’s color and panchromatic cameras.
Alice’s background counts are only about half what we predicted,
indicating that the Radioisotope Thermoelectric Generator (RTG) induces
a significantly lower background than we estimated before launch. This
lower background rate will significantly enhance Alice’s
ratio on faint spectral features.

From the Alice, Ralph and PEPSSI testing this month we can say that
all of the data surrounding the careful, step-by-step instrument
commissioning activities to date, our instrument payload continues to
perform as well or better than predicted from ground testing. This is a
testament to the exacting engineering that went into their development.

In other news for May, we began to finalize the suite of observations
planned for next year during our Jupiter flyby, and we continued to
track New Horizons to determine whether a fine course correction will
needed this fall. So far, no course correction appears necessary, but
the final verdict won’t be in until we have about another 90 to 100
of tracking.

Planning activities began in May for the annual 60-day checkouts we’ll
perform each year each year during Cruise 2 (“Glen’s Glide”

  • the coast from Jupiter to Pluto. From 2008 to 2011, these checkouts
    will occur in the fall. But from 2012 to 2014 our annual checkouts will
    occur in the summer, because we’ll be rehearsing the Pluto encounter
    aboard the spacecraft and we want the Earth-Sun geometry at rehearsal
    time to faithfully reproduce what will occur at the actual encounter in
    summer 2015.

In our “water cooler news story” of the month for New Horizons: In
May, we got word from Lockheed-Martin that tourists in the Bahamas
several large pieces of our Atlas V 551 launch vehicle’s nose fairing
that had washed up on shore. (Photo courtesy of Devon Ravine)

Now let’s talk about the significance of our current location: deep in
the solar system’s asteroid belt. As both the title and the first
illustration in this month’s blog indicate, we’re traversing the main
asteroid belt now. This region of the solar system consists of a
of dwarf planets, like Ceres (itself 1,000 kilometers in diameter), and
literally millions of debris bits created by collisions between
asteroids. These small bodies range up in size from mountains to
as large as 100 kilometers across. The asteroid belt also contains
innumerable boulders, rocks and dust motes created by the same
collisions. There are many good Web sites describing what is known
the asteroid belt. One I hope you’ll like can be found at:; another good site is

The first spacecraft to transit the asteroid belt was NASA’s Pioneer
which made its epic crossing in 1972 on the way to the historic first
encounter of a spacecraft with Jupiter. Later, Pioneer 11, Voyagers 1
and 2, Galileo, Cassini, NEAR and Ulysses all made the same kind of
journey to or across the main belt. Now it is our turn. Fortunately,
asteroid belt is so huge that, despite its large population of small
bodies, the chance of running into one is almost vanishingly small -
less than one in one billion. That means that if you want to actually
come close enough to an asteroid to make detailed studies of it, you
have to aim for a specific asteroid.

The first such asteroid flyby was made by Galileo in October 1991.
Galileo also made a second asteroid encounter in 1993. Other
most notably the NEAR (Near Earth Asteroid Rendezvous) mission, have
also made close main belt asteroid flybys, yielding important
and geophysical insights into these bodies. Galileo also made the first
discovery of an asteroid satellite in its 1991 flyby of asteroid Gaspra
-since then, ground-based observers have found dozens of asteroid
satellites. In addition to main belt asteroid flybys, NASA’s NEAR and
the Japanese Hayabusa mission have both made orbital rendezvous and
landings on asteroids closer to Earth. And next year, NASA plans to
launch the Dawn ( Discovery mission to orbit
two of the largest asteroids - Vesta and Ceres. Dawn will arrive in
orbit about Vesta in 2012, and will reach Ceres, the largest asteroid,
in August 2015, just a month or so after New Horizons encounters Pluto.

A long time ago, we considered the possibility of targeting a close
asteroid flyby with New Horizons during our crossing of the main belt.
As the mission PI, I rejected this early on for two reasons. First,
an encounter would take about half of our Kuiper Belt fuel to
accomplish. Secondly, even for this amount of fuel, the only asteroids
we could hope to reach would be tiny - just a few kilometers across.
While such an encounter would certainly be scientifically useful, it
couldn’t be justified for the amount of Kuiper Belt fuel it would cost
us. And when all is said and done, our job is to reconnoiter bodies in
the Kuiper Belt.

Although we specifically decided not to target any asteroid, after
launch we did conduct a thorough search for chance encounters along our
trajectory. Just the statistics of such chance encounters indicated
we might expect to pass perhaps 1 million to 3 million kilometers from
small asteroid by chance as we transited the main belt. We found
such opportunities back in February.

But, as it turns out, we got more than what we expected: in early May
also discovered that we’d pass within approximately 104,000 kilometers
of the little-known asteroid 2002 JF56 on June 13! This little
mountain-sized body ( is only 3 to 5
kilometers across; virtually nothing is known about it - not even its
compositional type or its rotational period.

Although we cannot resolve something as small as 2002 JF56 from this
distance with Ralph (LORRI, which has higher resolution cannot open its
door until late August to guard against accidental Sun pointings), the
June 13 “encounter” with 2002 JF56 is still going to be useful to New

The primary use of this distant flyby will be to test Ralph’s optical
navigation and moving target tracking capabilities. We will also get a
handle on the asteroid’s light curve, composition, phase curve, and
perhaps even refine its diameter if all goes as planned. But this event
is really a flight test, so we aren’t guaranteeing anything but a best
effort. If it works, you’ll see images that just barely resolve the
asteroid into perhaps 1 or 2 pixels and perhaps a spectrum of this chip
off some larger body. More importantly, of course, we will gain some
valuable experience that will yield benefits at both the Jupiter and
Pluto flybys, so we’re excited to give this a try.

Stay tuned, we’ll report on the results at mid-month on this Web site.

Flight activities for June will center on SWAP instrument testing,
instrument calibrations, and beam mapping observations for our high
antenna and REX (radio science) instrument. By July 4, we’ll be 3
Astronomical Units (AU) from the Sun. Although the sunlight there is
still 100 times as strong as it is on the brightest day at Pluto, it’ll
be about 10 times dimmer than at Earth’s orbit. Less than six months
into a 114-month journey to Pluto, New Horizons is beginning to reach
the cooler thermal conditions it was designed to thrive in!

Well, that’s all I have for you for now. So until next time, keep

-Alan Stern

On June 13, New Horizons will pass approximately 104,000 kilometers
a small main belt asteroid called 2002 JF56. This fortuitous encounter
is too far away for detailed imagery, but it will allow us to test our
moving target tracking capabilities. The positions of 2002 JF56 and New
Horizons are illustrated here for June 1.