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>X-Lotus-FromDomain: CWB
>Date: Wed, 18 Aug 1999 15:30:15 CST6CDT,4,1,0,7200,10,-1,0,10800,3600
>X-OldDate: Tue, 17 Aug 1999 15:47:53 -0500
>Sender: carlsagan-owner <carlsagan-owner@craigerware.avalon.net>
>Reply-To: "Carl Sagan List" <carlsagan@cwe.cx>
>From: "Greg Loeb" <Greg_Loeb@cwb.ca>
>To: carlsagan@craigerware.avalon.net
>Subject: Sagan: A Memo to the Administrator: NEOs
>
>Someone asked me for this recently, so I thought I would also post it to the
>list for those who haven't yet seen it..
>
>=========================
>
>A Memo to the Administrator
>
>In 1995, the Administrator of NASA, Daniel Goldin, asked Carl Sagan to
provide a
>summary of reasons for sending spacecraft to the near-Earth objects
(comets and
>asteroids that come very close to the Earth). Such missions are
expensive, they
>must compete with other worthy projects within NASA and without, and
>?
>especially because the catastrophes from space are so easily lampooned or
>dismissed (what is sometimes called the ?giggle factor?) ? it might be a good
>idea to stack up the reasons and see if they make sense.
>
> o A population of some 2,000 small worlds more than 1 kilometer (0.6
mile)
>in diameter orbits the Sun very near Earth. Some have arisen from the main
>asteroid belt; others are dead or dying comets deriving ultimately from the
>outer solar system. Most NEOs are gravitationally perturbed into and out of
>near-Earth orbits on timescales of 10 million to 100 million years.
>
> o NEOs, therefore, represent a convenient opportunity to study small
worlds
>at much less cost than missions beyond the orbit of Mars.
>
> o NEOs represent fairly pristine samples of the early planetesimals that
>built the planets ? with clues to conditions in the solar nebula from
which the
>solar system was formed. Some NEOs have undergone more physical and chemical
>processing than others.
>
> o The dumbbell shape of some NEOs (e.g., Toutatis) raises the intriguing
>possibility that we are seeing here remnants of the accretion of
planetesimals,
>and therefore the processes that led to the building of Earth and of the
>planets.
>
> o Some NEOs are carbonaceous. Organic molecules of cometary and/or
>asteroids origin played an important role in the origin of life on Earth
around
>4 billion years ago. Comets entering Earth?s atmosphere today have their
>organic molecules largely destroyed or modified thermally. Microscopic
>particles of cometary debris collected in the stratosphere represent so
little
>cumulative mass that it is very difficult to determine their organic
chemistry.
>The study of carbonaceous NEOs may, therefore, provide important clues to the
>origin of life.
>
> o While many meteorites have been collected on Earth, we are unable to
>relate these meteorites to the physics, chemistry, and history of any
specific
>solar system asteroids or comets. We do not even clearly understand how
>meteorites are connected to the different asteroidal and cometary
populations.
>It is possible that a weakly coherent class of NEOs exists that never
survives
>entry into Earth?s atmosphere and is unknown to us.
>
> o The dinosaurs and 75 percent of the species then on Earth seem to have
>been rendered extinct by the impact of a 10-kilometer-diameter (6-mile)
NEO some
>65 million years ago. The Tunguska event of 1908 in Siberia, the recently
>declassified United States Air Force data on small objects entering Earth?s
>atmosphere, and particularly the impact of some two dozen fragments of Comet
>Shoemaker-Levy 9 with Jupiter all underscore the probability of a
>civilization-threatening collision in the next century is nearly one in a
>thousand.
>
> o A program to inventory all large NEOs is needed to determine not only
>orbital elements but also something of the physical and chemical
properties of
>the inventoried NEOs. Ground truth on the NEO is needed to calibrate remote
>observations. Transponders may have to be emplaced on potentially dangerous
>NEOs.
>
> o The population of NEOs probably ranges from highly coherent objects to
>weakly cohesive fluffballs. If the time ever comes when it is necessary to
>deflect or disintegrate a NEO on an Earth-impact trajectory, it will be
>essential to have experience with the various populations of NEOs. For
example,
>the coupling constant in the use of standoff chemical or nuclear weapons
may be
>highly variable from NEO to NEO.
>
> o The dangers presented by NEOs over the next century probably provide
>another coherent justification for the development of long-duration human
>spaceflight capability. Some NEOs are easier to get to than the Moon. Some
>round-trip missions to interesting NEOs with as much as a 30-day stay time on
>the NEO surface take less than a year. In general, missions to NEOs are
>intermediate in difficulty and risk between lunar and Martian missions.
>
> o As for Mars, robotic missions and human missions to NEOs provide an
>opportunity for the development of rover, telepresence, return-sample, and
>virtual-reality space technologies and provide a justification for
space-station
>and other means of investigating long-term human survival in space.
Likewise,
>observations and missions to NEOs are intrinsically international because
>everyone on Earth is equally at risk from the impact of a large NEO. (The
>language of the House Science and Space Committee?s proposed legislation
for a
>program like Spaceguard mandates it be done internationally.)
>
> o NEOs represent a new and unknown environment for exploration ? in many
>respects, more tantalizing than the partially explored moon.
>