[meteorite-list] Deep Impact Mission Update - November 2005

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Dec 1 18:47:35 2005
Message-ID: <200512012346.jB1Nk1903795_at_zagami.jpl.nasa.gov>

http://deepimpact.jpl.nasa.gov/mission/update-200511.html

Deep Impact
Mission Update - November 2005
Lucy McFadden

In the past month, the science team has continued with its data
analysis. Many people don't realize the detailed computations that are
required to convert a picture made up of raw data numbers (DN) returned
from space, into an image containing numbers of physical meaning. This
process is called calibration.

It is carried out by observing stars of known light output, or radiance,
and scaling the value of that output to the known quantity of radiation
produced by the star. This is like synchronizing your watch, so that
everyone has the same time reference. In this case the reference is to a
scale of energy output. We have updated our calibration numbers using
the most recent data taken just before and after impact. We have made
improvements in subtraction of the background signal that exists in
every electronic detector so that we can analyze the signal from the
comet and not the noise from the camera's detector. With the known value
of the star expressed in units of energy, we then determine the energy
released from the comet. Ken Klaasen and other team members have been
working hard on this.

Another effort that has taken significant time has been processing
images to find evidence of the crater formed by the impact. Our experts
in deconvolution and image processing have taken on the task of image
enhancement to find the crater. It is apparent that the crater cannot be
seen with certainty through all the dust that was ejected from the
impact. We measure the width of the shadow cast by the ejecta plume to
set a maximum value for the crater diameter. The crater cannot be larger
than the width of the shadow cast.

In analyzing the infrared spectrum, one asks does this spectrum
represent the nucleus or ejecta? The team is working with engineers to
get the best understanding of where the IR spectrometer aboard the flyby
spacecraft is pointing. In some cases, the IR spectrum was very
different from one second to the next. Was this due to a change in the
pointing of the spacecraft, or a change in the nature of the material
observed? To be sure, we had to verify the spacecraft pointing using
telemetry. We needed engineering verification of what the IR
spectrometer was "seeing" so that we didn't interpret a feature as
something on the nucleus when in fact, the spectrometer may have been
pointing at the expanding ejecta cloud. Engineers have confirmed our
understanding of the spectrometer's pointing. The MRI and HRI cameras
allow us to associate the spectra with position on the nucleus, or off
the nucleus, as the case may be - see figure 1.

[Figure 1: Follow the dotted line]
Images of impact taken with MRI. The blue dotted line is the position of
the spectrometer's slit. At each point along the slit, a spectrum exists
that contains information about the constituents in the spectrometer's
field of view. Click for larger view.
Received on Thu 01 Dec 2005 06:46:01 PM PST