1. Overview

The ultimate goal of IceHunters is to discover Kuiper Belt Objects with just the right orbit and just the right characteristics to make them eligible for a visit from the New Horizons mission. At this time, the space probe has enough fuel in reserve to allow up to two different objects to be visited.

Trying to find one or two objects that the mission can visit is a bit like looking for a needle in a haystack that also contains a bunch of pins. Astronomers working with the mission team are taking thousands of images, each of which contains tens of thousands of stars, along with many asteroids and Kuiper Belt objects on orbits that cannot be reached by the spacecraft. All these objects must be sifted through to find the desired KBOs. While all these other objects are making it hard to find the final destination of New Horizons, they do represent potential bonus science for you and for the planetary science community.

Beyond simply identifying the targets that the New Horizons mission may visit, we will also be using your data - your identification objects in the images - to generate catalogues of previously undiscovered Kuiper Belt Objects, asteroids, and variable stars.

2. How We Make These Images

The images you will be examining were mostly taken by the 8-meter Subaru telescope on Mauna Kea, or the 6.5-meter Magellan telescope in Chile.

One of the hardest parts of this project is sorting the Kuiper Belt Objects from the background stars, which are very densely packed in this part of the sky, looking towards the galactic center. The way we do this is to make what are called difference images. We use pairs of images taken a few hours or days apart that are subtracted from one another, after our best attempt to match the sharpness of the two images. In an ideal situation, the appearance of the stars on the two images will match perfectly after this processing, and all the non-moving objects that stay the same brightness (e.g. stars) will politely disappear. All that should be left after image subtraction are things that aren't constant, like our moving KBOs and asteroids, and variable stars.

However, this process is not perfect, particularly when conditions aren’t ideal, and often we can’t make the appearance of the stars on the two images match perfectly before subtracting them. In these cases we are left with blotchy halos where the stars used to be, usually containing a mixture of bright and dark blotches in a pattern that repeats for all stars of similar brightness. These blotches need to be ignored when searching for our KBOs.

3. Kuiper Belt Objects

In 1992, Dave Jewitt and Jane Luu at the University of Hawaii discovered a small object, designated 1992QB1, orbiting the Sun beyond Neptune at a distance of about 40 Astronomical Units (AU). Since then, more than 1,000 similar objects have been discovered beyond Neptune's orbit, and scientists estimate there are about 500,000 objects bigger than 20 miles across waiting to be discovered in that vast region.

We call this swarm of bodies the Kuiper Belt, in honor of Dutch-American astronomer Gerard Kuiper, who speculated about the existence of small bodies beyond Neptune in the 1950s. There is controversy about who deserves credit for the idea; some people call this the Edgeworth/Kuiper Belt, sharing the honor with Irish scientist Kenneth Edgeworth who published a similar idea in the 1940s. The inhabitants of this realm are called Kuiper Belt Objects (KBOs), Edgeworth/Kuiper Belt Objects (EKOs), or simply Trans-Neptunian Objects (TNOs). It is very likely that most of the short-period comets found in the inner solar system come from the Kuiper Belt, being held there in cold storage until random perturbations nudge them inward.

While we have a good sense of the distribution of objects in the Kuiper Belt both in terms of size and place, a complete mapping of these objects has not been done. There are likely hundreds of thousands of objects waiting to be discovered and catalogued. With this project, we are going to work to identify all the Kuiper Belt Objects bright enough for our telescopes to see in a roughly 2-square degree field in front of the constellation Sagittarius. Objects that are in this part of the sky in 2011 have the potential to pass close to the path that New Horizons will follow after 2015, perhaps allowing the spacecraft to visit them. It is unknown just how many objects we'll find, perhaps dozens, but every object we find is something new we've learned about the universe.

When you find a small, round, point of light in an image which is smaller and sharper than the residual star images, and does not contain dark blotches, you are probably seeing one of three things: a Kuiper Belt Object, a variable star (more below), or a synthetic object added to the images by the science team (used to test the accuracy of this project's results). All the Kuiper Belt Objects will be collected into a catalogue and published with the names of the Zooniverse members who made the discoveries and the scientists who confirm the discoveries. Want to see your name associated with a KBO? Start hunting through the images, and make sure your publishable name is set correctly on your Zooniverse account page (must be logged in to access).

4. Asteroids

One of the banes and boons of astronomy is the asteroid population. These far flung hunks of rock are primarily concentrated in the asteroid belt, much closer to the Sun than the Kuiper Belt, but can be found in small numbers almost everywhere in the solar system. Many an astronomer has worked hard to take a long-exposure of a favorite galaxy, star cluster, or other extended object, only to see the final image wrecked by a long bright streak of an interrupting rock. While annoying to astroimagers, these asteroids are still scientifically interesting because they represent left over materials from the formation of the solar system. From the perspective of Earth defense, it is also important to map asteroids so that we can track all the things that have the potential (no matter how small) to potentially hit the planet earth (we don't expect to find dangerous asteroids, but the potential always exists).

In this project, asteroids appear as small streaks in the images, because they are closer to us than the KBOs, and thus move fast enough across the sky to be smeared out in these long-exposure images. The length of the streak reflects how fast an asteroid appears to be moving across the sky. Long streaks are likely nearby objects, while shorter streaks (that may look like cigars of light), are likely more distant objects. While some of the asteroids you'll see are may already be known and catalogued, your data may help scientists refine their orbits. And you are also likely to find asteroids that aren't known! These objects will be reported to the Minor Planets Center with the names of the discovers (that's you) and the scientists who confirm the objects.

5. Variable Stars

Of course stars that change brightness between the times when the two images are taken will not disappear when the two images are subtracted, but will leave a bright or dark image that is indistinguishable from a KBO. So any particular object you click on might be a variable star. But by looking at multiple sets of images we can differentiate between variable stars and KBOs or other moving objects (since we can see the KBOs move, and the stars stay put).

While not useful for this project, some of these variable stars will be useful to other scientists. Just as we are creating catalogues of KBOs and asteroids, we will also generate a catalogue of variable stars and we will report your discoveries to the American Association of Variable Star Observers.