PLEASE NOTE:


*

From: Benny J Peiser B.J.PEISER@livjm.ac.uk
To: cambridge-conference@livjm.ac.uk
Priority: NORMAL

CC DIGEST, 12 March 1998
------------------------

(1) ONE-MILE-WIDE ASTEROID TO PASS CLOSER THAN THE MOON IN 2028
Brian Marsden brian%cfaps1.span@vms.noao.edu

(2) NEAR EARTH ASTEROID RENDEZVOUS: MISSION OVERVIEW
A.F. Cheng, JOHNS HOPKINS UNIVERSITY

(3) MULTI-SPECTRAL IMAGER ON NEAR RENDEZVOUS
S.E. Hawkins et al., JOHNS HOPKINS UNIVERSITY

(4) INFRARED SPECTROMETER FOR NEAR RENDEZVOUS
J.W. Warren et al., JOHNS HOPKINS UNIVERSITY

(5) X-RAY/GAMMA-RAY SPECTROMETER ON NEAR RENDEZVOUS
J.O. Goldsten et al., JOHNS HOPKINS UNIVERSITY

(6) RADAR DETECTION OF A NUMBER OF NEAR-EARTH ASTEROIDS
L.A.M. Benner et al., CALTECH, JET PROP LAB

(7) ARE MAIN-BELT ASTEROIDS THE ONLY SOURCE FOR EARTH-APPROACHING
ASTEROIDS?
D.L. Rabinowitz, CALTECH, JET PROP LAB

(8) THE NEAR-EARTH FOLLOW-UP PROGRAM
P. Pravec et al., ACADEMY OF SCIENCE OF THE CZECH REPUBLIC

===============================
(1) ONE-MILE-WIDE ASTEROID TO PASS CLOSER THAN THE MOON IN 2028

From: Brian Marsden brian%cfaps1.span@vms.noao.edu

Recent orbit computations on an asteroid discovered last December
indicate it virtually certain that it will pass within the moon's
distance of the earth a little more than 30 years from now. The chance
of an actual collision is small, but one is not entirely out of the
question.

The asteroid, known as 1997 XF11, was discovered by Jim Scotti in
the course of the Spacewatch program at the University of Arizona.
This program utilizes modern electronic technology on a 36-inch
telescope at Kitt Peak that was built 77 years ago.

After the discovery observations on December 6, observations made by
two Japanese amateur astronomers during the following two weeks showed
that the minimum distance between the orbits of 1997 XF11 and the earth
was very small. Given also that the object was quite large as
earth-approaching asteroids go, perhaps one mile across, it was added
to the list of "potentially hazardous objects" (PHAs) that need to be
monitored, lest they are destined to come dangerously close to the
earth over the course of the next several centuries. There are
currently 108 PHAs.

As astronomers continued to gather data on 1997 XF11, it slowly
began to become apparent that there would be a particularly close
approach to the earth in October 2028. A computation from observations
spanning 60 days suggested that the miss distance would be 500 thousand
miles. This distance may seem large in human terms, but it was less
than had previously been predicted in advance for any other known
asteroid during the foreseeable future.

Observations made on March 3 and 4 by Peter Shelus with a 30-inch
telescope at the McDonald Observatory in western Texas extended the
observed arc of 1997 XF11 to 88 days. This time, the orbit computation
indicated a miss distance of only 30 thousand miles from the center
of the earth; the earth's radius is about 4 thousand miles. The time
of encounter would be around 1:30 p.m. Eastern Daylight Time on
Thursday, October 26, 2028. That evening the object should be visible
with the naked eye. In Europe, where it would be dark by that time, the
object should be a splendid sight as it moves from northwest to
southeast across the sky over a couple of hours.

There is still some uncertainty to the computation. On the one hand,
it is possible that 1997 XF11 will come scarcely closer than the moon.
On the other hand, the object could come significantly closer than
30 thousand miles. Further observations are necessary in order to
refine the figures. It is also possible that prediscovery observations
of 1997 XF11 can be located on archival photographs. Particularly
favorable opportunities for recording the object would have occurred in
1990, 1983, 1976, 1971 and 1957. Ephemerides for these times are
available.

It is hoped that continuing observations will be made during the next
few months. The object is starting to move into the dusk and to fade
week by week. Nevertheless, it should be quite accessible for a while
with large telescopes, which in addition to helping establish whether a
collision in 2028 is possible, could usefully provide more definite
information about the object's size.

Further observations of 1997 XF11 should be possible with
moderate-sized telescopes equipped with electronic sensors early in the
year 2000. A better opportunity will occur in late 2002, when the
object should be detectable with quite modest telescopes. On that
occasion the closest approach will be on Halloween, but the miss
distance will be a safe 6 million miles.

Brian G. Marsden
1998 March 11

============================
(2) NEAR EARTH ASTEROID RENDEZVOUS: MISSION OVERVIEW

A.F. Cheng: Near Earth Asteroid Rendezvous: Mission overview, SPACE
SCIENCE REVIEWS, 1997, Vol.82, No.1-2, pp.3-29

JOHNS HOPKINS UNIVERSITY,APPLIED PHYSICS LAB, JOHNS HOPKINS RD, LAUREL,
MD, 20723

The Near Earth Asteroid Rendezvous (NEAR) mission launched successfully
on February 17, 1996 aboard a Delta II-7925. NEAR will be the first
mission to orbit an asteroid and will make the first comprehensive
scientific measurements of an asteroid's surface composition, geology,
physical properties, and internal structure. It will orbit the
unusually large near-Earth asteroid 433 Eros for about one year, at a
minimum altitude of about 15 km from the surface. NEAR will also make
the first reconnaissance of a C-type asteroid during its flyby of the
unusual main belt asteroid 253 Mathilde. The NEAR instrument payload
is: a multispectral imager (MSI), a near infrared spectrometer (NIS),
an X-ray/gamma ray spectrometer (XRS/GRS), a magnetometer (MAG), and a
laser rangefinder (NLR),while a radio science investigation (RS) uses
the coherent X-band transponder. NEAR will improve our understanding of
planetary formation processes in the early solar system and clarify the
relationships between asteroids and meteorites. The Mathilde flyby will
occur on June 27, 1997, and the Eros rendezvous will take place during
February 1999 through February 2000. Copyright 1998, Institute for
Scientific Information Inc.
========================
(3) MULTI-SPECTRAL IMAGER ON NEAR RENDEZVOUS

S.E. Hawkins*), E.H. Darlington, S.L. Murchie, K. Peacock, T.J.
Harris, C.B. Hersman, M.J. Elko, D.T. Prendergast, B.W. Ballard,
R.E. Gold, J. Veverka, M.S. Robinson: Multi-Spectral Imager on the Near
Earth Asteroid Rendezvous mission, SPACE SCIENCE REVIEWS, 1997, Vol.82,
No.1-2, pp.31-100

*) JOHNS HOPKINS UNIVERSITY, APPLIED PHYSICS LAB, JOHNS HOPKINS RD,
LAUREL,MD,20723

A multispectral imager has been developed for a rendezvous mission with
the near-Earth asteroid, 433 Eros. The Multi-Spectral Imager (MSI) on
the Near-Earth Asteroid Rendezvous (NEAR) spacecraft uses a
five-element refractive optical telescope, has a held of view of 2.93 x
2.25 degrees, a focal length of 167.35 mm, and has a spatial resolution
of 16.1 x 9.5 m at a range of 100 km. The spectral sensitivity of the
instrument spans visible to near infrared wavelengths, and was designed
to provide insight into the nature and fundamental properties of
asteroids and comets. Seven narrow band spectral filters were chosen to
provide multicolor imaging and to make comparative studies with
previous observations of S asteroids and measurements of the
characteristic absorption in Fe minerals near 1 mu m. An eighth filter
with a much wider spectral passband will be used for optical navigation
and for imaging faint objects, down to visual magnitude of +10.5. The
camera has a fixed 1 Hz frame rate and the signal intensities are
digitized to 12 bits. The detector, a Thomson-CSF TH7866A
Charge-Coupled Device, permits electronic shuttering which effectively
varies the dynamic range over an additional three orders of magnitude.
Communication with the NEAR spacecraft occurs via a MIL-STD-1553 bus
interface, and a high speed serial interface permits rapid transmission
of images to the spacecraft solid state recorder. Onboard image
processing consists of a multi-tiered data compression scheme. The
instrument was extensively tested and calibrated prior to launch; some
inflight calibrations have already been completed. This paper presents
a detailed overview of the Multi-Spectral Imager and its objectives,
design, construction, testing and calibration. Copyright 1998,
Institute for Scientific Information Inc.

=======================
(4) INFRARED SPECTROMETER FOR NEAR RENDEZVOUS

J.W. Warren*), K. Peacock, E. Darlington, S.L. Murchie, S.F. Oden,
J.R. Hayes, J.F. Bell, S.J. Krein, A. Mastandrea: Near Infrared
Spectrometer for the Near Earth Asteroid Rendezvous mission, SPACE
SCIENCE REVIEWS, 1997, Vol.82, No.1-2, pp.101-167

*) JOHNS HOPKINS UNIVERSITY, APPLIED PHYSICS LAB, JOHNS HOPKINS RD,
LAUREL, MD, 20723

The Near-Infrared Spectrometer (NIS) instrument on the Near-Earth
Asteroid Rendezvous (NEAR) spacecraft is designed to map spectral
properties of the mission target, the S-type asteroid 433 Eros, at
near-infrared wavelengths diagnostic of the composition of minerals
forming S asteroids. NIS is a grating spectrometer, in which light is
directed by a dichroic beam-splitter onto a 32-element Ge detector
(center wavelengths, 816-1486 nm) and a 32-element InGaAs detector
(center wavelengths, 1371-2708 nm). Each detector reports a 32-channel
spectrum at 12-bit quantization. The field-of-view is selectable using
slits with dimensions calibrated at 0.37 degrees x 0.76 degrees (narrow
slit) and 0.74 degrees x 0.76 degrees (wide slit). A shutter can be
closed for dark current measurements. For the Ge detector, there is an
option to command a 10x boost in gain. A scan mirror rotates the field-
of-view over a 140 degrees range, and a diffuse gold radiance
calibration target is viewable at the sunward edge of the field
of regard. Spectra are measured once per second, and up to 16 can be
summed onboard. Hyperspectral image cubes are built up by a combination
of down-track spacecraft motion and cross-track scanning of the mirror.
Instrument software allows execution of data acquisition macros, which
include selection of the slit width, number of spectra to sum, gain,
mirror scanning, and an option to interleave dark spectra with the
shutter closed among asteroid observations. The instrument was
extensively characterized by on-ground calibration, and a comprehensive
program of in-flight calibration was begun shortly after launch. NIS
observations of Eros will largely be coordinated with multicolor
imaging from the Multispectral Imager (MSI). NIS will begin observing
Eros during approach to the asteroid, and the instrument will map Eros
at successively higher spatial resolutions as NEAR's orbit around Eros
is lowered incrementally to 25 km altitude. Ultimate products of the
investigation will include composition maps of the entire illuminated
surface of Eros at spatial resolutions as high as similar to 300 m.
Copyright 1998, Institute for Scientific Information Inc.

========================================
(5) X-RAY/GAMMA-RAY SPECTOMETER ON NEAR RENDEZVOUS

J.O. Goldsten*), R.L. Mcnutt, R.E. Gold, S.A. Gary, E. Fiore, S.E.
Schneider, J.R. Hayes, J.I. Trombka, S.R. Floyd, W.N. Boynton, S.
Bailey, J. Bruckner, S.W. Squyres, L.G. Evans, P.E. Clark, R. Starr: The
X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid
Rendezvous mission, SPACE SCIENCE REVIEWS, 1997, Vol.82, No.1-2,
pp..169-216

*) JOHNS HOPKINS UNIVERSITY, APPLIED PHYSICS LAB, JOHNS HOPKINS RD,

LAUREL, MD, 20723

An X-ray/gamma-ray spectrometer has been developed as part of a
rendezvous mission with the near-Earth asteroid, 433 Eros, in an effort
to answer fundamental questions about the nature and origin of
asteroids and comets. During about 10 months of orbital operations
commencing in early 1999, the X-ray/Gamma-ray Spectrometer will develop
global maps of the elemental composition of the surface of Eros. The
instrument remotely senses characteristic X-ray and gamma-ray emissions
to determine composition. Solar excited X-ray fluorescence in the 1 to
10 keV range will be used to measure the surface abundances of Mg, Al,
Si, Ca, Ti, and Fe with spatial resolutions down to 2 km. Gamma-ray
emissions in the 0.1 to 10 MeV range will be used to measure cosmic-ray
excited elements O, Si, Fe, H and naturally radioactive elements K, Th,
U to surface depths on the order of 10 cm. The X-ray spectrometer
consists of three gas filled proportional counters with a collimated
field of view of 5 degrees and an energy resolution of 850 eV @ 5.9
keV. Two sunward looking X-ray detectors monitor the incident solar
flux, one of which is the first flight of a new, miniature solid-state
detector which achieves 600 eV resolution @ 5.9 keV. The gamma-ray
spectrometer consists of a NaI(Tl) scintillator situated within a
Bismuth Germanate (BGO) cup, which provides both active and passive
shielding to confine the held of view and eliminate the need for a
massive and costly boom. New coincidence techniques enable recovery of
single and double escape events in the central detector. The NaI(Tl)
and BGO detectors achieve energy resolutions of 8.7% and 14%,
respectively @ 0.662 MeV. A data processing unit based on an RTX2010
microprocessor provides the spacecraft interface and produces
256-channel spectra for X-ray detectors and 1024-channel spectra for
the raw, coincident, and anti-coincident gamma-ray modes. This paper
presents a detailed overview of the X-ray/Gamma-ray Spectrometer and
describes the science objectives, measurement objectives, instrument
design, and shows some results from early in-flight data. Copyright
1998, Institute for Scientific Information Inc.

========================
(6) RADAR DETECTION OF A NUMBER OF NEAR-EARTH ASTEROIDS

L.A.M. Benner*), S.J. Ostro, J.D. Giorgini, R.F. Jurgens, D.L.
Mitchell, R. Rose, K.D. Rosema, M.A. Slade, R. Winkler, D.K. Yeomans,
D.B. Campbell, J.F. Chandler, I.I. Shapiro: Radar detection of
near-earth asteroids 2062 Aten, 2101 Adonis, 3103 Eger, 4544 Xanthus,
and 1992 QN, ICARUS, 1997, Vol.130, No.2, pp.296-312

*) CALTECH, JET PROP LAB, 4800 OAK GROVE DR, PASADENA, CA, 91109

We describe Doppler-only radar observations of near-Earth asteroids
2062 Aten, 2101 Adonis, 3103 Eger, 4544 Xanthus, and 1992 QN that were
obtained at Arecibo and Goldstone between 1984 and 1996. Estimates of
the echo spectral bandwidths, radar cross sections, and circular
polarization ratios of these objects constrain their pole-on breadths,
radar albedos, surface roughnesses, taxonomic classes, rotation
periods, and pole directions. Aten's bandwidth is consistent with its
radiometrically determined diameter of 0.9 km. Adonis has a rotation
period P less than or equal to 11 h and an effective diameter (the
diameter of a sphere with the same projected area as the asteroid)
between 0.3 and 0.8 km. The radar properties of Adonis suggest it is
not a member of taxonomic classes C or M. The effective diameter of
Xanthus is between 0.4 and 2.2 km with a rotation period P less than or
equal to 20 h. Echoes from 1992 QN constrain the asteroid's pole-on
breadth to be greater than or equal to 0.6 km and probably exclude it
from the C and M taxonomic classes. The strongest Eger echoes are
asymmetric with bandwidths that set lower bounds of 1.5 and 2.3 km on
the minimum and maximum breadths of the asteroid's pole-on silhouette.
If Eger is modeled as a 1.5 x 2.3 km biaxial ellipsoid, then its
effective diameter for an equatorial view is 1.5 km end-on and 1.9 km
broadside or pole-on, implying a geometric albedo smaller than
published values but still consistent with a classification as an
E-type object, The near-unity circular polarization ratios of Adonis,
Eger, and 1992 QN are among the highest values measured for any
asteroid extreme near-surface roughness at centimeter to meter scales.
(C) 1997 Academic Press.

==============================
(7) ARE MAIN-BELT ASTEROIDS THE ONLY SOURCE FOR EARTH-APPROACHING
ASTEROIDS?

D.L. Rabinowitz: Are main-belt asteroids a sufficient source for the
earth-approaching asteroids? Part II. Predicted vs observed size
distributions, ICARUS, 1997, Vol.130, No.2, pp.287-295

CALTECH, JET PROP LAB, 4800 OAK GROVE DR, MS 183-501,PASADENA,CA,91109

This paper predicts the size distribution of the Earth-approaching
asteroids with diameter d = 10 m to 10 km, assuming they originate as
the fragments of main-belt asteroids with a cumulative size
distribution proportional to d(-25) and that they have self-similar
fragmentation properties. The resulting distribution is dominated by
'fast-track' bodies originating from parent asteroids with orbits close
to the 3:1 mean-motion resonance with Jupiter. Because the dynamical
lifetimes of these Earth approachers are shorter than their collisional
lifetimes, their size distribution is nearly proportional to d(-3.0),
the production distribution in the main belt. This prediction, however,
is at odds with the Spacewatch observations. The observed distribution
is relatively flat for d > similar to 100 m, and relatively steep for d
< similar to 100 m, so that the number of Earth approachers with d
similar to 10 m to 0.3 km is overestimated. If these populations are
predominantly of main-belt origin, then the size distribution in the
main belt is not a simple power law. A nonuniform size distribution
with wave-like oscillations, possibly caused by a cutoff at small
sizes, would lead to Earth approachers with a size distribution in
better agreement with the observations. If such wave-like oscillations
are realistic, then the main belt is sufficient to supply the observed
number of Earth approachers throughout the observed size range. (C)
1997 Academic Press.

=========================
(8) THE NEAR-EARTH FOLLOW-UP PROGRAM

P. Pravec*), M. Wolf, L. Sarounova, S. Mottola, A. Erikson, G. Hahn,
A.W. Harris, J.W. Young: The near-earth objects follow-up program - II.
Results for 8 asteroids from 1982 to 1995, ICARUS, 1997, Vol.130, No.2,
pp.275-286

ACADEMY OF SCIENCE OF THE CZECH REPUBLIC, INST ASTRON, CZ-25165
ONDREJOV,CZECH REPUBLIC

We present the results of photometric observations of the near-Earth
asteroids (1943) Anteros, (2102) Tantalus, (2212) Hephaistos, (3199)
Nefertiti, (5751) Zao = 1992 AC, (6322) 1991 CQ, (7474) 1992 TC, and
1989 VA made between 1982 and 1995, Synodic rotation periods in the
range from 2.39 to 5.54 hr were derived for five of them, and we were
able to place lower limits on periods of (2212) and (5751)-both > 20
hr, Only the period of the low amplitude case of (1943) was not
constrained, The most interesting results were obtained for the
following objects: (2102), a fast rotator (period 2.39 hr) in an
extremely inclined orbit (i approximate to 64 degrees); (2212), a low
amplitude slow rotator considered as an inactive cometary nucleus
candidate; (3199), which showed similar lightcurves at quite different
positions of the phase angle bisector, constraining its rotational
pole; and 1989 VA, an unusual Aten asteroid with a rotation period of
2.51 hr and a relatively large amplitude, Overall, these results
continue the pattern that NEO spins exhibit a bimodal distribution of
spin rates. (C) 1997 Academic Press.

--------------------------------
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