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(meteorobs) Excerpt from "CCNet 23/2001 - 8 February 2001"
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 23/2001 - 8 February 2001
Date: Thu, 8 Feb 2001 13:10:22 -0000
CCNet 23/2001 - 8 February 2001
-------------------------------
(1) UA SCIENCE TEAM READIES FOR NEAR LANDING
Ron Baalke <baalke@jpl.nasadot gov>
[...]
(3) DEBRIS COLLISION ADVOIDANCE PREDICTIONS
Andrew Yee <ayee@nova.astro.utorontodot ca>
[...]
========================================================================
(1) UA SCIENCE TEAM READIES FOR NEAR LANDING
>From Ron Baalke <baalke@jpl.nasadot gov>
http://uanews.opi.arizonadot edu/cgi-bin/WebObjects/UANews.woa/wa/SRStoryDetail
s?ArticleID=3134
UA SCIENCE TEAM READIES FOR NEAR LANDING
(From Agnieszka Przychodzen, UA Lunar and Planetary Lab)
CONTACT: William V. Boynton, 520-621-6941, wboynton@lpl.arizonadot edu
(EDITORS - Boynton is available for interviews at the UA through Friday,
Feb. 9)
NASA's Near Earth Asteroid Rendezvous (NEAR) mission, the first to orbit an
asteroid, is coming to an end.
February 7, 2001
With the spacecraft almost out of fuel, on Feb. 12 mission engineers will
attempt the first-ever, controlled descent to the surface of an asteroid.
NEAR has been orbiting asteroid 433 Eros since Feb. 14, 2000 and is now more
than 196 million miles (316 million kilometers) from Earth
The main goal of the controlled descent is to gather close-up pictures of
the surface of Eros, particularly the saddle area, a six-mile (10-km) wide
depression peppered with huge boulders and cut with grooves.
"The landing will take a few hours. We'll see the images coming in real time
as the NEAR spacecraft is approaching closer and closer to the asteroid's
surface," William V. Boynton said. Boynton, professor of planetary sciences
in the UA Lunar and Planetary Laboratory, leads the UA group involved in the
NEAR mission.
Boyton and the UA team will watch the descent at the Applied Physics
Laboratory (APL) in Laurel, Md., which built the spacecraft and managed the
mission.
NEAR Shoemaker's 4-hour descent is scheduled to start at 10:31 a.m. EST. A
series of thruster firings will decelerate the spacecraft from about 20 mph
to 5 mph.
"It is not really a landing in the sense of a spacecraft being alive once it
touches down. NEAR has no legs to steady it, so it's just going to fall
over. The antenna will no longer point to the Earth so we'll not be able to
communicate with it," Boynton said.
NEAR's camera will be taking a photo every minute. The last clear images,
shot from about 1,650 feet (500 meters), could details on the surface as
small as 4 inches (10 centimeters) across.
"If the instruments survive the touch down we will not be able to see
whatever the camera will be looking at. NEAR will snap the last image just
before it reaches the surface," Boynton added.
During its one-year orbiting mission, the NEAR Shoemaker spacecraft provided
among other data X-ray, gamma ray, and infrared readings on the composition
and spectral properties of the asteroid. Initial results from the X-ray
Gamma Ray Spectrometer suggested that Eros might be similar in elemental
composition to primitive meteorites called chondrites.
"Previously Eros was thought to be a very standard meteorite, but now it
looks that it might be like a meteorite we've never seen before. Its
composition might be different than that of typical meteorites. There might
be some very rare meteorites that resemble Eros, but right now we're not
sure. We need to wait for another few months for complete analyses results
to find out, says Boynton, who is the principal investigator for NEAR's
X-ray Gamma Ray Spectrometer.
"One thing that has been learned from this mission is how to operate the
spacecraft up close with an irregularly shaped body that has very little
gravity. Because of its strange shape, the gravity on Eros is not constant
like it is with a spherical body, such as Earth or Mars. When one of the
asteroid's lobes comes by, the pull of gravity is greater. It is very
complicated to take all of these effects into account," Boynton explained.
Though the mission has been successful, there are still some mysteries to be
solved.
"One interesting thing we still don't understand is why there are some
places which have a lot of very large boulders. They are unexpected. We
don't see them on the moon. The saddle area is the region where we see
grooves looking like cracks around Eros, and we are not sure what caused
them," he said.
Eros almost certainly used to be a piece of a now extinct, bigger object. At
one time in the past, Eros got knocked off due to a large impact. The shock
from that event might have left the cracks.
"We don't know when it happened. The space where Eros resides is crowded
enough to have such collisions today. In order to answer this question we
would have to land on it, pick up a sample, and bring it back to Earth for
analysis." Boynton said.
What would he recommend for future asteroid missions?
"If we had the opportunity to do this again, I'd want to land on the surface
with instruments designed to measure the surface composition and bring the
samples back," Boynton said.
The UA NEAR team has began archiving the spectra data collected by
NEAR-Shoemaker and will make them available to other scientists for
analysis. The task will take about 12 months.
========================================================================
(3) DEBRIS COLLISION ADVOIDANCE PREDICTIONS
>From Andrew Yee <ayee@nova.astro.utorontodot ca>
[From January 2001 issue of ORBITAL DEBRIS QUARTERLY NEWS, NASA JSC
http://www.orbitaldebris.jsc.nasadot gov/newsletter/v6i1/v6i1-3.html]
Flight Readiness Review Report
By M. Matney
Before every Shuttle mission, the Orbital Debris Program Office performs a
Flight Readiness Review (FRR) for the mission. Primarily, this consists of a
detailed analysis of the Shuttle sensitive surfaces with the Bumper code to
determine the debris and meteoroid risk to the vehicle and mission. Each
mission has a target risk level that can be altered by the flight geometry
during the mission. This risk calculation is based on the standard debris
and meteoroid models and does not take into account short time-scale
variations in the collision risk. For this reason several analyses are
performed to estimate any enhancement to the baseline risk for a particular
mission.
The first source of possible enhancement is that an annual meteoroid shower
could peak during the mission that might temporarily increase the net
meteoroid flux over the short two-week Shuttle mission. Currently, we use a
model of shower activity based on ground observations to compute a simple
meteoroid flux enhancement factor to be added to the Bumper results. Because
meteor showers typically last only a few days, it may be possible to shift
the launch time of a mission to avoid the strongest outbursts of meteoroid
activity such as a Leonid meteor storm.
The second source of possible enhancement is that the Shuttle might fly
through a dense region of debris from a recent on-orbit breakup event. This
could potentially add an enhanced flux onto the time-averaged ORDEM flux
used by Bumper. The SBRAM code is used each Shuttle mission to compare all
recent breakups to the future Shuttle orbit to look for potential debris
cloud enhancements.
During each Shuttle mission, US Space Command performs collision avoidance
predictions for all catalogued objects in Earth orbit. The purpose is to
give the Shuttle a warning in case an object is predicted to enter a
collision warning "box". Currently, this "shoe box" is 10 km long in
down-track direction, and 4 km wide in radial and cross-track directions.
NASA is assessing a new "pizza box" that is 14 km wide in down-range and
cross-track directions, and 2 km wide in the radial direction. Future
collision avoidance calculations should include more sophisticated estimates
of the actual estimated position uncertainties computed by Space Command.
The FRRs are performed some weeks before the actual mission -- too early to
compute actual collision probabilities. However, the flight directors like a
"heads-up" on the expected number of collision warnings they may expect for
the mission. For typical Shuttle missions, this number is less than one, so
that the prediction becomes the probability that a collision warning will be
issued during this mission. This probability is computed using the latest
catalog at the time of the FRR using simple estimates of the collision flux
based on average flux models. We are working on improving our ability to
make these estimates by using more orbit plane prediction information.
We are always improving the FRR process, and are also assessing how we can
provide similar information on a regular basis to the International Space
Station program in the future.
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