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(meteorobs) Excerpts from "NEO News (5/27/01) Deep Impact..."
An interesting issue of David Morrison's regular e-newsletter. Much
off-topic discussion of NEOs is mixed in, but I could find no way to
remove that without losing the gist of the more on-topic material.
Lew Gramer
------- Forwarded Message
Date: Sun, 27 May 2001 18:46:43 -0700
To: david.morrison@arc.nasadot gov
From: David Morrison <dmorrison@arc.nasadot gov>
Subject: NEO News (5/27/01) Deep Impact & 2 letters
NEO News (5/27/01) Deep Impact & 2 letters
Dear Friends & Students of NEOs:
This edition includes a press release announcing that the Deep Impact
comet mission has passed Preliminary Design Review. Also printed are
two letters commenting on earlier stories. Andrew Glickson discusses
the end-Triassic mass extinction, and Al Harris and Rick Binzel
discuss the relationship between the Spaceguard metric, which is
based on finding NEAs with absolute magnitude brighter than H=18, and
the underlying goal to survey NEAs larger than 1 km diameter.
For those who do not wish to read their detailed essay, I provide the
following summary of the conclusions from Harris & Binzel. They write
that the missing parameter in transforming between H magnitude and
diameter is the albedo or reflectivity of the object. The canonical
translation of H=18 to 1 km diameter assumes an average albedo of
about 0.1 for all discovered objects. In this essay, they examine how
well this 0.1 average albedo assumption is likely to fit the actual
discovered population. The relevance of this work is that we must be
aware of the corresponding uncertainties in where the goalpost
resides for a survey goal (such as the Spaceguard goal) specified as
completeness with regard to diameter, rather than completeness with
regard to observed H magnitude.
Harris & Binzel conclude that the often assumed population mix of
roughly equal fractions of light and dark NEAs leads to completion
models that lie within a couple tenths of a magnitude, or
equivalently a couple percent of completion, of a simple equivalence
of H = 18.0 equal to D = 1 km for the entire population. The biggest
current uncertainty is in the population model, not the
implementation of it. We barely know the relative distributions of
the different compositional types that make up the NEO population,
and the number of well-determined albedos of NEAs is very small. For
the moment, the largest uncertainty in relating survey efforts to the
"Spaceguard Goal" is in the population fractions and albedos of the
various constituents. Only by further physical observations to
characterize the population of NEAs in terms of taxonomic class and,
most importantly albedo, will we be able to reliably say how we are
doing toward achieving the Spaceguard Goal. Nailing down the 1 km
goalpost for the Spaceguard survey requires knowing the albedo
distribution of the population for securing the H magnitude value
where the
goal of 90% completeness must occur.
Note that since many of us will be attending several professional
meetings dealing with NEOs during coming weeks, it is unlikely that
there will be any editions of NEO News during the month of June.
David Morrison
=================================================
MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
FOR IMMEDIATE RELEASE May 24, 2001
NASA GIVES GO-AHEAD TO BUILD 'DEEP IMPACT' SPACECRAFT
The Deep Impact mission, the first mission to ever
attempt to impact a comet nucleus in order to answer basic
questions about the nature of comets, has successfully
completed its preliminary design phase and has been approved
by NASA to begin full-scale development for a launch in
January 2004.
The Deep Impact team of scientists, engineers and mission
designers, from the University of Maryland, NASA's Jet
Propulsion Laboratory and Ball Aerospace and Technologies
Corporation, Boulder, Colo., have been working for more than
18 months designing the mission, the dual spacecraft and three
science instruments. The encounter with Comet Tempel 1 on
July 4, 2005 will reveal clues to the origin of comets and the
composition and structure of perhaps the most mysterious
objects in our solar system.
Now the Deep Impact team is completing the final design
details and will begin building the mission's two spacecraft:
a flyby spacecraft and a 350-kilogram (771-pound) impactor
spacecraft. They will be launched together in early 2004 and
travel to Comet Tempel 1's orbit where they will separate and
operate independently. The flyby spacecraft will release the
impactor into the comet's path, then watch from a safe
distance as the impactor guides itself to collide with the
comet, making a football field-sized crater in the comet's
nucleus.
"This is a major milestone for us," said Dr. Michael
A'Hearn, the prinicipal investigator and director of the Deep
Impact mission, from the University of Maryland, College Park,
Md. "We have now shown NASA that we have a viable design for
the spacecraft and the mission to carry out a truly rare,
large-scale experiment on another body of the solar system."
"The Deep Impact mission follows in the tradition of
other Discovery missions like Mars Pathfinder and the Near-
Earth Asteroid Rendezvous by doing first of a kind science on
a low-cost, highly focused project," said Brian Muirhead, the
manager of the Deep Impact mission, of NASA's Jet Propulsion
Laboratory, Pasadena, California. "The project team is fully
prepared to implement this technically challenging and
scientifically unique mission."
As the gases and ice inside the comet are exposed and
expelled outward by the impact, the flyby spacecraft will take
pictures and measure the composition of the outflowing gas.
The images and data will be transmitted to Earth as quickly as
possible. Many observatories on Earth should be able to see
the comet dramatically brighten just after the impact on July
4, 2005.
Scientists refer to comets as time capsules that hold
clues about the formation and evolution of the solar system.
Comets are composed of ice and dust, the primitive debris from
the solar system's earliest and coldest formation period, 4.5
billion years ago. They would also like to learn much more
about a comet's composition, structure and how its interior is
different from its surface. The controlled cratering
experiment of the Deep Impact mission will provide answers to
these questions.
Comet Tempel 1 was discovered in 1867. Orbiting the Sun
every 5.5 years, it has made many passages through the inner
solar system. This makes it a good target to study
evolutionary change in the mantle, or outer crust, of a comet.
"Ball Aerospace is pleased and proud to be involved with
JPL and the University of Maryland in working on this first of
a kind deep space mission," said Ball's John Marriott, deputy
project manager.
Principal investigator A'Hearn oversees Deep Impact's
scientific investigations. Project manager Brian Muirhead, of
NASA's Jet Propulsion Laboratory manages and will operate the
Deep Impact mission for NASA's Office of Space Science,
Washington D.C. JPL is managed by the California Institute of
Technology, Pasadena, Calif., for NASA. John Marriott of Ball
Aerospace and Technology Corporation manages the spacecraft
development in Boulder, Colo.
=====================================================
[...]
==============================================
LETTER: NAILING DOWN THE SPACEGUARD SURVEY GOALPOST:
UNDERSTANDING THE RELATIONSHIP BETWEEN H MAGNITUDE AND DIAMETER
As well described by David Morrison ("Origin and Meaning of the NASA
Spaceguard Goal", NEO News 5/12/01), there are good reasons from the hazard
point of view for setting 1 km diameter as the goal for the Spaceguard
Survey. Of course, we are discovering and vigorously cataloging numerous
objects below this size,and prudence dictates that we should indeed
continue to achieve completeness well below 1 km. The "Spaceguard Goal" is
a metric for tracking the overall progress of our survey efforts.
When objects are discovered, we determine only their H magnitude, not their
diameter. The missing parameter in transforming between H magnitude and
diameter is the albedo of the object. The canonical translation of H=18 to
1 km diameter assumes an average albedo of about 0.1 for all
discovered objects.
In this essay, we examine how well this 0.1 average albedo assumption is
likely to fit the actual discovered population. The relevance of this work
is that we must be aware of the corresponding uncertainties in where the
goal post resides for a survey goal (such as the Spaceguard goal) specified
as completeness with regard to diameter, rather than completeness with
regard to observed H magnitude.
In establishing an equivalent, or average, relation between absolute
magnitude and diameter, one must consider whether you mean a mean
albedo with respect to what may hit you, or a mean albedo with respect to
what you see in the sky. The mean albedo of the objects that strike the
Earth, for any given size, is the weighted mean according to the
relative fraction of the population (at that size) that has a given albedo.
The mean albedo of what is discovered in the sky, when considered at a
specific limiting magnitude, will be biased toward higher albedo objects.
This is because for any magnitude limited survey, for any two equally sized
objects the one with the higher albedo appears brighter and is more easily
discovered.
Consider the following example (for the moment, the first four columns):
Population Fraction:
Of a Given Of a Given H magnitude Completeness
Class Albedo Diameter H Magnitude @ 1 km Dia [1] [2]
C-like 0.06 0.45 0.16 18.67 0.794 0.831
S-like 0.18 0.45 0.62 17.48 0.946 0.957
M 0.12 0.05 0.04 17.92 0.909 0.928
E,V 0.40 0.05 0.18 16.61 0.975 0.978
- ------------------------------------------------------------------------
<albedo> - 0.112 0.172 0.112 - 0.093
H(D=1 km) - 18.00 17.51 18.00
<Completeness> 0.877 0.900
H(C=0.90) 18.00 18.20
The column "Given Diameter" is an assumed population fraction similar to
what one often sees claimed for the makeup of NEAs, or for that matter
main-belt asteroids, roughly equal numbers of S and C types with small
components (~5%) of metalic objects, very high albedo E-class,
"Vestoids", and miscellaneous others. As noted above, higher albedo
objects are more easily discovered. In addition to this, as you go to
smaller sizes the number of objects increases. Thus, aa fixed value for the
H magnitude, smaller objects having high albedos tend to be more
abundant than larger objects having low albedos.
The column labeled "Of a Given H Magnitude" gives the relative
fractions of discovered objects in each of the four albedo categories for
an assumed population index of -2.4, i.e. N(>D) = A*D^-2.4. Within this
last column can be seen that the population fraction with respect to H is
dominated by higher albedo "S-like" objects relative to lower albedo
"C-like" objects, even though the total population postulated to exist (in
this model) is equal between these two categories. Even more noticable is
that the ultra-high albedo objects (labeled here as the "E,V" group)
contributes equally to the fraction in the last column as do the C-types,
even though the "E,V" group has only about 10% of the number (0.05 versus
0.45) of objects within our total model population.
On the line immediately below the dashes is listed the geometric mean
albedo of this assumed population, weighted according to the fractions with
respect to size, and with respect to H. The last line gives the value of H
that corresponds to D = 1 km for that value of albedo. If your interest is
in "what may hit you", the diameter weighting is more representative, and
is extremely close to the equivalence of H = 18.0 to 1 km diameter.
However, if one asks what is the mean albedo of a sample of discovered
objects (remembering that absolute magnitude H is the primary measure of
whether an object is discovered or not), the mean albedo is much higher,
0.17 in this model. Thus when a new object is discovered and you are asked
"how big is it?", the best probabilistic guess would be to assume the
higher albedo (that is, H = 17.5 corresponding to D = 1 km).
How does this relate to the Spaceguard Goal of 90% completion of all
objects larger than 1 km in diameter? In the next column of the table we
list the H magnitude of a 1 km diameter NEA for the assumed albedo of each
class. Thus a 1 km diameter low albedo C-class object has H = 18.67, while
the same diameter high albedo (E,V) asteroid has H = 16.61. For each of
the H magnitudes we can estimate the expected fraction completion of a
survey at that value of H, and then weight those completions by the assumed
fractions in the total population to obtain an estimate of the average
completion for the total population. In the last two columns we do this
for two assumed completion levels, using the survey completion model of
Harris (Planet. Space Sci. 46, 283-290, 1998, see fig. 4). The first one
is normalized to 90% completion at H = 18.00, and the second is normalized
to 90% completion at H = 18.20. These are the values listed on the very
last row below these two columns. Just above those entries are listed the
average completeness for the entire population. We can see from these
numbers that the usual equivalence of 1 km diameter equivalent to H = 18.0
is not quite right. For the normalization where 90% completion at H = 18.0
is achieved for a single albedo (0.112), for our assumed model of albedo
populations the completion is only 87.7%. To achieve 90% completion of the
model population requires 90% completion to H = 18.2, as enumerated in the
last column. The very last number to note is the <albedo> given in the
last column, just below the dashed line, 0.093. This is the albedo which
relates H = 18.2 equivalent to D = 1 km. This can be thought of as an
effective albedo relating D to H such that a survey to 90% completion in
terms of H is also 90% complete in terms of the related value of D.
One final item of note is that the above completion figures relate to
differential completion, that is, completion at a given H or D value. The
"Spaceguard Goal" is usually assumed to be an integral completion, that is,
90% completion of objects larger than 1 km. Since completion is greater
for larger objects, the integral completion is higher than the differential
values given above. In fact, in the range of completion from ~0.85 to
0.90, the integral completion for the assumed population index of -2.4 is
about 5% higher than the differential value. Thus the integral completion
represented by the normalization used in the next-to-last column is
probably already more than 90%, and in fact one might move in the other
direction to a normalization of H(C=0.90) = 17.80 to yield an integral
completion of 90% for the assumed population.
In conclusion, it appears that the often assumed population mix of roughly
equal fractions of light and dark objects leads to completion models that
lie within a couple tenths of a magnitude, or equivalently a couple percent
of completion, of a simple equivalence of H = 18.0 equal to D = 1 km for
the entire population. The biggest current uncertainty is in the
population model, not the implimentation of it. We barely know the
relative distributions of the different compositional types that make up
the NEO population, and the number of well-determined albedos of NEAs is
very small and such as they are seem to be systematically much higher than
measured main-belt albedos. Is this a real difference, or something due to
observational circumstances (typically larger phase angles) or model errors
(faster rotation, lack of regolith). We don't know yet, and only further
observations will tell. For the moment, the largest uncertainty in
relating survey efforts to the "Spaceguard Goal" is in the population
fractions and albedos of the various constituents. Only by further
physical observations to characterize the population of NEAs in terms of
taxonomic class and, most importantly albedo, will we be able to reliably
say how we are doing toward achieving the Spaceguard Goal. Nailing down
the 1 km goalpost for the Spaceguard survey requires knowing the albedo
distribution of the population for securing the H magnitude value where the
goal of 90% completeness must occur.
Alan Harris (JPL) and Richard Binzel (MIT)
--
+++++++++++++++++++++++++++++++++++++++++++
David Morrison, NASA Ames Research Center
Tel 650 604 5094; Fax 650 604 1165
david.morrison@arc.nasadot gov or dmorrison@arc.nasadot gov
website: http://space.arc.nasadot gov
website: http://astrobiology.arc.nasadot gov
website: http://impact.arc.nasadot gov
+++++++++++++++++++++++++++++++++++++++++++
NEO News is an informal compilation of news and opinion dealing with
Near Earth Objects (NEOs) and their impacts. These opinions are the
responsibility of the individual authors and do not represent the
positions of NASA, the International Astronomical Union, or any other
organization. To subscribe (or unsubscribe) contact:
dmorrison@arc.nasadot gov
For additional information, please see the website:
http://impact.arc.nasadot gov.
If anyone wishes to copy or redistribute original material from these
notes, fully or in part, please include this disclaimer.
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