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New Horizons Crosses The Asteroid Belt

Artist's impression of New Horizons travelling near Pluto.
by Alan Stern
Southwest Research Institute
Boulder CO (SPX) Jun 02, 2006
New Horizons has entered the main asteroid belt and will be traversing this part of our solar system through August. 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 needed bug fixes discovered in ground and flight testing.

We continue to see software-induced guidance computer resets once or twice per month on average, but the spacecraft recovers flawlessly from these, without any interruption to plans. New software for this computer is in the works and will resolve the bug that causes this; we expect to have it 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 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.

We also found that Alice's background counts are only about half of what we predicted, indicating the RTG induces a significantly lower background than we estimated before launch. This lower background rate will substantially enhance Alice's signal-to-noise ratio on faint spectral features.

From the Alice, Ralph, and PEPSSI testing this month, we can continue to say, from all of the data surrounding the careful, step-by-step instrument-commissioning activities to date, that our instrument payload continues to look like it's performing 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 Jupiter observations planned for next year during our Jupiter flyby, and we continued to track New Horizons to determine whether a fine course correction will be needed this fall. So far, none appears necessary, but the final verdict won't be in until we have about another 90 to 100 days of tracking.

Planning activities began in May for the 60 day checkouts we'll perform each year during Cruise 2, also known as Glen's Glide: the coast from Jupiter to Pluto.

From 2008 to 2011, these checkouts will occur in the fall of each year, but in 2012, 2013 and 2014 the checkouts will occur in the summer. The summertime checkouts will occur in 2012 and 2014 because we'll be rehearsing the Pluto encounter aboard the spacecraft during these checkouts, and we want the Earth-Sun geometry at rehearsal time to reproduce faithfully what will occur at the encounter, in the summer of 2015. The 2013 checkout provides a backup opportunity for an additional rehearsal if one becomes necessary.

I'll now turn to the "water cooler news story" of the month for New Horizons: In early May, we got word from Lockheed-Martin that tourists in the Bahamas found several large pieces of our Atlas V 551 launch vehicle's nose fairing that had washed up on shore.

Now, turn to the significance of our current location: deep in the solar system's main asteroid belt. This region comprises a handful of dwarf planets, such as Ceres - itself 1,000 kilometers (620 miles) in diameter - and literally millions of debris bits created by collisions between asteroids.

These small bodies range in size from mountains to objects as large as 100 kilometers (62 miles) across. The asteroid belt also contains innumerable boulders, rocks and dust motes created by the same collisions.

The first spacecraft to transit the asteroid belt was NASA's Pioneer 10, 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 have all made the same kind of journey across the main belt. Now it is our turn.

Fortunately, the asteroid belt is so huge that, despite its large population of small bodies, the chance of running into one is almost vanishingly small - far less than one in a billion. That means if you want to come close enough to an asteroid to make detailed studies of it, you have to aim for one.

The first such asteroid flyby was made by Galileo in October 1991, and Galileo made a second asteroid encounter in 1994.

Other spacecraft, most notably the NEAR (Near Earth Asteroid Rendezvous) mission, also have made close main-belt-asteroid flybys, yielding important geological and geophysical insights into these bodies.

Galileo made the first discovery of an asteroid satellite in its 1991 flyby of 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 both have made orbital rendezvous and landings on asteroids closer to Earth.

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 main belt traverse. As the mission's principal investigator, I rejected this early on for two reasons.

First, such an encounter would take about half of our Kuiper Belt fuel to accomplish. Second, even for this amount of fuel, the only asteroids we could hope to reach would be tiny - just a few kilometers across.

Though such an encounter certainly would be scientifically useful, it couldn't be justified for the amount of fuel it would cost us - after all, our job is to reconnoiter bodies in the Kuiper belt with that fuel, not the asteroid belt.

As a result, we specifically decided not to target any asteroid, but after launch we did conduct a thorough search for chance encounters along our trajectory. Just the statistics of such chance encounters indicated that we might expect to pass perhaps 1 million to 3 million kilometers (620,000 to 1.8 million miles) from a small asteroid by chance as we transited the main belt. We found several such opportunities back in February.

As it turned out, we got more than what we expected: In early May we also discovered we'd pass within just 104,000 kilometers (63,000 miles) of the little-known asteroid 2002 JF56 on June 13. This little mountain-sized body is only 3 kilometers to 5 kilometers (1.9 miles to 3.1 miles) across, and virtually nothing is known about it - not even its compositional type or rotational period.

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), but the June 13 encounter with 2002 JF56 is still going to be useful to New Horizons.

The primary use of this distant flyby will be to test Ralph's optical navigation and moving-target tracking capabilities. We also will be able to get a handle on the asteroid's light curve, composition, phase curve, and perhaps even refine its diameter, if all goes as planned.

The 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 one or two pixels and perhaps a spectrum of this chip off some larger body.

More important, 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 our Web site.

Other flight activities for June will center on SWAP instrument testing, Ralph instrument calibrations and beam-mapping observations for our high gain antenna and REX (radio science) instrument.

By July Fourth, we'll be 3 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!

That's all I have for now. So, until next time, keep exploring.

Related Links
New Horizons at NASA
New Horizons at SwRI
Asteroid Information
Asteroid Belt Information
Asteroid 2002 JF56

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