The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/a96/disk12/kuipbelt.txt

Text File | 1996-01-19 | 4KB | 74 lines

                            SCIENCE BACKGROUND

                     THE SEARCH FOR THE KUIPER BELT

In 1950, Dutch astronomer Jan Oort hypothesized that comets came from a
vast shell of icy bodies about 50,000 times farther from the Sun than
Earth is.  A year later astronomer Gerard Kuiper suggested that some
comet-like debris from the formation of the solar system should also be
just beyond Neptune.  In fact, he argued, it would be unusual not to
find such a continuum of particles since this would imply the
primordial solar system has a discrete "edge."

This notion was reinforced by the realization that there is a separate
population of comets, called the Jupiter family, that behave strikingly
different than those coming from the far reaches of the Oort cloud.
Besides orbiting the Sun in less than 20 years (as opposed to 200
million years for an Oort member), the comets are unique because their
orbits lie near the plane of the Earth's orbit around the Sun.  In
addition, all these comets go around the Sun in the same direction as
the planets.

Kuiper's hypothesis was reinforced in the early 1980s when computer
simulations of the solar system's formation predicted that a disk of
debris should naturally form around the edge of the solar system.
According to this scenario, planets would have agglomerated quickly in
the inner region of the Sun's primordial circumstellar disk, and
gravitationally swept up residual debris.  However, beyond Neptune, the
last of the gas giants, there should be a debris-field of icy objects
that never coalesced to form planets.

The Kuiper belt remained theory until the 1992 detection of a 150-mile
wide body, called 1992QB1 at the distance of the suspected belt.
Several similar-sized objects were discovered quickly confirming the
Kuiper belt was real. The planet Pluto, discovered in 1930, is
considered the largest member of this Kuiper belt region.  Also,
Neptune's satellites, Triton and Nereid, and Saturn's satellite, Phoebe
are in unusual orbits and may be captured Kuiper belt objects.


Observational Techniques

To isolate and subtract the effects of cosmic ray strikes on the WFPC
2's electronic detectors, which could mimic the faint signature of a
comet, thirty-four images were taken of the same piece of sky.  The
cosmic ray hits change from picture to picture, but real objects remain
constant.  However, pinpointing comets was even trickier because they
drift slowly along their orbit about the Sun.  Although the orbital
periods of these objects are 200 years or longer, the HST has
sufficient spatial resolution to see them move in just a few minutes.
This means the comets change position from picture to picture, just as
cosmic ray strikes would.  However, cosmic ray strikes are randomly
placed events while the motions of the comets are well defined.

To distinguish between the comets and cosmic ray effects, the 34 images
were then digitally shifted and stacked to the predicted offset to
account for the expected drift rate of comets.  It's like having a
fixed camera on a tripod take a rapid series of snapshots of someone
walking in front of the lens.  The resulting snapshots could be stacked
so that the person appeared stationary.

The researchers tested the reliability of this approach by shifting the
stacked pictures in the opposite direction of the expected comets'
motion.  Ideally, no comets should have appeared, but random alignments
added up to 24 anomalous detections.

When the team stack-shifted the pictures in the direction of the
predicted comet motion, they came up with 53 objects.  Assuming that 24
of these are, statistically, anomalous too, leaves a remainder of 29
objects considered "real."

The shift-stack technique was further tested by dividing the images
into two groups and running an automated search algorithm to look for
objects that showed up in the same position on sets of exposures.