Free PDF AccessFree Resources |
||||||||||||||||
|
||||||||||||||||
|
||||||||||||||||
|
| ||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 1
Space Studies Board
Search:
Jump to
Top
NewsJump
to Science
in the
Subscribe to our FREE e-
Headlines
newsletter!
NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING INSTITUTE OF MEDICINE NATIONAL RESEARCH COUNCIL
June 18, 2004 Current Operating Status
"Assessment of Planned Scientific Content of the CRAF
Mission (1987)"
On Mary 27, 1987, Dr. Robert O. Pepin, chair of the Committee on Planetary and
Lunar Exploration, sent the following letter to Dr. Geoffrey A. Briggs, director of
NASA's Solar System Exploration Division.
On May 31, 1985, the Committee on Planetary and Lunar Exploration (COMPLEX)
of the Space Science Board communicated to you its first detailed review of the
planned scientific content of the Comet Rendezvous-Asteroid Flyby (CRAF)
mission, in the context of the committee's recommended science strategies for
comet and asteroid exploration. Our conclusions at that time were summarized as
follows: "The committee finds that science planning for the CRAF mission is
proceeding within the guidelines established by COMPLEX for in situ cometary
exploration and preliminary asteroidal reconnaissance. We feel confident that the
few points of concern expressed by the committee at this stage of the mission
strategy development will be addressed by the selected science teams. A later
assessment will deal specifically with these concerns, noted below, and with the
critical matter of measurement capabilities of the selected instrumental packages
as they relate to the measurement requirements specified in the COMPLEX
strategy report."
At its April 1987 meeting, COMPLEX carried out a second review of the CRAF
mission, based in part an presentations by M. Neugebauer/CRAF Project Scientist
on investigations and instruments selected for accommodation study, and by R.F.
Draper/CRAF Project Manager on the profile, schedule, and engineering
development for the mission.
ASSESSMENT SUMMARY
The committee finds that the scientific capabilities of the CRAF mission, as defined
by the science payload selected for accommodation study, are (1) congruent with
the prioritized scientific objectives recommended by COMPLEX for in situ comet
exploration and asteroid reconnaissance; and (2) are in generally good accord with
the specific measurement requirements established by COMPLEX to meet these
objectives. We note in particular that the principal reservation expressed by the
http://www7.nationalacademies.org/ssb/craf87.html (1 of 5) [6/18/2004 10:12:49 AM]
OCR for page 2
Space Studies Board
committee in 1985, concerning allocation of sole responsibility for acquisition of
primary data on comet nucleus composition to a penetrator experiment, has been
well addressed by inclusion in the science payload of several techniques for dust
compositional measurements that require neither penetration of nor landing on the
nucleus. Collectively, these techniques constitute a robust backup for essential
elemental composition measurements to be carried out by the selected penetrator
experiment.
We emphasize that preliminary analysis of the Halley flyby measurements has
brought into sharper focus the intrinsic physical and chemical complexity of the
cometary environment, and has mandated detailed study of a comet by a long-term
and well-instrumented rendezvous mission such as CRAF. In all respects, the
selected CRAF science payload instruments represent significant advances over
their Halley counterparts, and add essential capabilities that were not present on
the various Halley spacecraft. They promise the substantial complement of
accurate measurements needed to address the COMPLEX science objectives.
HALLEY RESULTS AND THE CRAF SCIENCE PAYLOAD
There have been two major developments since 1985 that prompt a second
assessment of the CRAF mission at this time: (1) the initial dissemination of the
scientific results of spacecraft encounters with comets Halley and Giacobini-Zinner;
and (2) selection of the CRAF science payload experiments for accommodation
study. It is therefore important for the Committee to reassess the COMPLEX
science objectives for comet exploration in the light of these results, and to ask
whether the selected payload is well suited to the accomplishment of these or
modified objectives.
To summarize briefly, the comet science objectives stated in the report "Strategy
for the Exploration of Primitive Solar System Bodies—Asteroids, Comets, and
Meteorites: 1980-1990," focused on the composition and physical state of the
nucleus, processes that govern the development and behavior of the cometary
atmosphere, and interaction with the solar wind, in that order of priority. The
information returned by the encounter spacecraft has confirmed important aspects
of the general picture of the volatile-rich nucleus and coma deduced from
traditional ground-based and Earth-orbital remote observations. Yet it has left
unanswered many significant questions, and raised fundamental new questions,
concerning coma composition and the structures that arise from solar wind
interaction. The new data have provided only rudimentary (size, shape, albedo)
information on the physical state of the nucleus (e.g., there is currently a factor of
at least two uncertainty in the density of the nucleus). Dust measurements on the
Halley flybys provided the first in situ information on the composition of cometary
solids. Although considerable diversity was seen among submicron particles, the
mean composition deduced from the analysis of many particles is consistent with
chondritic abundances. The only compositional anomalies in the mean dust are
higher abundances of carbon and nitrogen than are seen in chondrites.
http://www7.nationalacademies.org/ssb/craf87.html (2 of 5) [6/18/2004 10:12:49 AM]
OCR for page 3
Space Studies Board
Due to intrinsic limitations of the flyby type of mission, uncertainties in the
measurements are large, and the data clearly fall short of the analytic goals
specified in the 1980 COMPLEX strategy report. Halley measurements did not
cover a sufficiently broad range of elements and compounds, and were not of
adequate accuracy to permit comparison of the comet with compositional patterns
seen in chondrites. Isotopic measurements were likewise too inaccurate to see
effects at the levels displayed by bulk meteorites. The diversity and relative
abundances of organic compounds were not determined. The Halley results do
indicate that comets are primitive objects, and underscore the importance of
implementing rendezvous missions where more sensitive measurements can be
made. Overall, in terms of the primary goal of the investigation of comets stated by
COMPLEX in 1980, "to determine their composition and structure and to deduce
their history in order to increase our knowledge of the chemical and isotopic
composition and physical state of the primitive solar nebulan…" results from the
comet Halley encounters, at least so far, are ambiguous.
COMPLEX thus concludes that the scientific results from the recent comet
encounters (for which we must applaud our colleagues who conducted those
missions) only reaffirm the goals originally stated in the strategy report and
preserve the order of priority of the science objectives. Moreover, the extreme
variability exhibited by Halley, determined from both in situ and remote
observations, confirms the need for a rendezvous mission. At this writing, it seems
likely that fast flyby comet missions will not contribute to the stated COMPLEX
exploration objectives at the level inherent in the CRAF type of mission.
The encounter results also provide a yardstick for assessing the in situ
measurement capabilities of the experiments selected for the CRAF payload.
Instruments for analysis of solids are much more powerful than those on the Halley
encounter missions, and they appear capable of accomplishing the first-order
priorities outlined by COMPLEX. Gamma-ray spectrometry, x-ray fluorescence,
and gas chromatography will yield abundances of a significant set of major and
minor elements (and some trace elements) of cosmochemical importance. The
target precisions of 0.5% for major elements set by COMPLEX are probably not
generally attainable, but accuracies of a few percent are practical for most
elements and should be adequate for meaningful bulk characterization. Bulk
analyses of both collected coma dust and in-place solids on the cometary crust are
critical elements in the CRAF plan because of the possibility of chemical
fractionation between the cometary surface and the active gas-producing regions;
inactive regions observed on the Halley surface could be thick lag deposits of
devolatilized material. The compositional diversity seen in micron and submicron
Halley dust suggests that individual particle measurements from the SEMPA and
COMA instruments on CRAF will provide critical data for comparison of cometary
matter with chondritic and interstellar grain models. If all comets—and other
primitive bodies—have approximately chondritic non-volatile bulk composition, then
the diversity at the individual grain level will be the only significant means for
comparing materials formed by different processes in different environments.
Instruments on CRAF will yield first measurements of the compositions and
http://www7.nationalacademies.org/ssb/craf87.html (3 of 5) [6/18/2004 10:12:49 AM]
OCR for page 4
Space Studies Board
abundances of organic materials in a comet. These are essential data for
comparison of cometary and meteoritic organic species, and of chemical
evolutionary processes in the two environments. The CHON particles found in
Halley are strong presumptive evidence for the presence of organic compounds,
but no definitive detection or characterization of complex molecules was possible in
the flyby mission mode. Gas chromatography in the PENL (DSC/EGA) and CIDEX
experiments on CRAF will be diagnostic for relatively simple compounds in the
comet nucleus and in coma dust particles. Composition of parent organic
molecules in coma gas is an important objective of the NIMS measurements.
Another example is isotopic mass discrimination, such as the resolution of 13C
from 12CH. High mass resolution will be possible for both dust and gas
components of the coma. Large isotopic effects recently seen in meteoritic carrier
components and in separated grains from interplanetary dust particles suggest that
valuable isotopic information on particles will be returned from CRAF, although the
ultimate precision of the spacecraft instruments is not yet well determined.
The penetrator experiment. The penetrator warrants separate comment in view of
COMPLEX's earlier reservations about the technological readiness of this
technique to carry by itself the responsibility for acquiring high priority data on
nuclear composition, without backup by other techniques in the event of
deployment failure. Two characteristics of the selected CRAF science payload
address these concerns. First, a major strength of the payload is the inclusion of
four instruments, three on the main spacecraft, for compositional measurements an
solids. All of the instruments (PENL, CIDEX, SEMPA, and COMA) have unique
capabilities that are important for characterizing complex solid materials within the
nucleus, or emitted from it over a range of comet activity. However, in that they all
measure composition, a level of redundancy is provided for critical data acquired
by techniques that have never been employed in spacecraft or cometary
environments—including, for example, SEMPA as well as PENL. Second, the
penetrator experiment selected for accommodation study exceeds the baseline
penetrator concept—primarily elemental compositional analysis—in capability. The
DSC/EGA augmentation is designed to determine the bulk phases (structures) of
the water ice, and the molecular composition and modes of trapping of volatile
species within it, and is thus responsive in a unique manner to the prime scientific
goal of determining the conditions under which cometary ices formed. While the
ambient temperature regime at penetrator depth may have erased some of this
information, such in situ analysis of ice structures and volatile contents presents
one of the better opportunities, short of pristine, thermally insulated sample return,
for diagnosing cometary formation conditions.
COMPLEX continues to caution that substantial development' work during the
accommodation study is required to demonstrate the viability and reliability of the
penetrator concept, and of the selected augmented penetrator experiment.
Outstanding issues include assurance of penetration and retention within the
nucleus for a wide range of possible surface conditions, and investigation of the
effects of penetrator passage and sample collection on the ice-volatile sample and
http://www7.nationalacademies.org/ssb/craf87.html (4 of 5) [6/18/2004 10:12:49 AM]
OCR for page 5
Space Studies Board
on the measured thermal diffusivity of the penetrated medium. We are confident
that these and other issues are being addressed by the experiment team. While
the penetrator remains an unproven means of obtaining data, the data are of
extraordinary interest and do not—in the case of DSC/EGA analyses, for
example—appear accessible by any other means. Developments of penetrator
technology and experimental sophistication appear to have been substantial over
the past two years—as they have been for SEMPA, to note a parallel instrumental
case—and there are thus excellent reasons for encouraging the continuing
development of both instruments for inclusion in the final CRAF science payload.
Please feel free to contact me with any questions you may have regarding this
assessment, or for further discussion.
Last update 5/18/00 at 3:31 pm
Site managed by Anne Simmons, Space Studies Board
Site managed by the SSB Web Group.
To comment on this Web page or report an error, please send feedback to the Space Studies Board.
Subscribe to e-newsletters | Feedback | Back to Top
Copyright © 2004. National Academy of Sciences. All rights reserved. 500 Fifth St. N.W., Washington, D.C. 20001.
Terms of Use and Privacy Statement
http://www7.nationalacademies.org/ssb/craf87.html (5 of 5) [6/18/2004 10:12:49 AM]
