ASTEROID 2004 MN4
David Morrison <dmorrison@arc.nasa.gov>
NEO News (07/21/05) Asteroids MN4 and Eros
Last December, asteroid 2004 MN4 briefly rose to Torino Scale 4, when
orbital calculations suggested a greater than 1 in 50 chance of
collision on April 13, 2029. Subsequent optical and radar
observations showed that MN4 will not collide with Earth in 2029, but
it will come very close (see "Asteroid MN4 and How to Protect the
Earth" in the news archive of the NASA Impact Hazard website,
<
http://impact.arc.nasa.gov>). Current interest (and concern) is
directed at possible future impacts, if MN4 should pass through a
"keyhole" in 2029 and find itself in a resonant orbit (such as one in
which the asteroid makes exactly 6 orbits of the Sun in 7 years,
returning to Earth's vicinity every 7 years.)
Based on current knowledge of the orbit of MN4, we cannot exclude the
possibility of it passing though a keyhole in 2029 and hitting the
Earth on a subsequent pass. However, there will be opportunities to
make improved optical and radar observations, which are likely to
confirm that it will miss the keyholes. It would also be possible to
place a transponder on the asteroid and use this to verify whether or
not it will pass through a keyhole in 2029. The issue is whether the
ground-based orbital improvements will come in time to make a mission
decision, or whether we need to plan for a transponder mission in any
case.
Why the hurry? It is because if we should need to deflect the
asteroid (which is unlikely but possible), our technology requires
that the deflection take place before the close flyby in 2029. Before
2029 we only need to deflect MN4 enough to miss the keyholes (which
are less than 1 km across). But if a deflection were required after
2029, it would have to be enough to miss the much larger target of
the Earth itself, which is far beyond present technology for
asteroids this large.
Following is a communication from former Apollo astronaut Rusty
Schweickart discussing these options. In addition, this message ends
with a story on the interior structure of asteroid Eros by Robert Roy
Britt of Space.com.
David Morrison
====================================
To David Morrison from Rusty Schweickart (July 20, 2005)
Clark Chapman and I have just concluded a meeting at JPL with NASA's
NEO folks pursuant to our letter to Administrator Mike Griffin
(printed below). This was the kickoff to NASA's process of responding
to the issues we raised in the letter. Our understanding is that NASA
will complete its analysis of the issues and respond officially to us
by the end of August.
Based on our discussion, and without pre-judging the results of
NASA's more detailed analysis, there are several key factors that
will likely shape their conclusions.
The first is the issue of the time required from decision to commit
to a mission to completion of that mission. This "mission time"
applies to both a deflection mission and a mission to emplace a
transponder on the asteroid, should either mission be required.
[Clark Chapman notes that "mission time" is used here to mean the
total duration from beginning Phase A through completion of the
mission. It thus includes development of the mission concept,
building the spacecraft, launch, flight to MN4, and sufficient
operation at the asteroid either to obtain reliable transponder
observations (in the case of a transponder mission) or time to
deflect and verify deflection].
If the mission time is as short as 6 years, then the decision of
whether or not a transponder is needed to support a deflection
decision can be delayed until 2014. This is a critical date because
not only does optical tracking data become available again by that
time, but there is also a good radar apparition in early 2014. If the
decision on a transponder mission can await this date, the
possibility that neither mission is required is much higher than if a
decision must be made now.
Our tentative conclusion that a decision must be made now is based on
the far more pessimistic assumption that the mission time for a
deflection mission might be as long as 12 years, and for a
transponder mission 7-8 years. These assumptions, on which JPL is far
more expert than we, led to the conclusion that a near-term decision
on the transponder mission is required. We await NASA's analysis
regarding the feasibility of sequential 6 year mission times.
The second key issue is the joint realization that the uncertainties
introduced by the Yarkovsky effect on this asteroid may well play a
major role in the decision making. While the Yarkovsky effect is a
relatively minor issue in the case of an asteroid headed for the
12,800-km-diameter Earth in 15 years, it is a major influence when
the immediate target of the asteroid is a 600-meter-wide keyhole!
Again, however, if the major mission decisions can be deferred until
after the 2014 radar apparition, the influence of the Yarkovsky
effect on this asteroid will be far better estimated.
As we've always emphasized, the likely need for a deflection mission
is very low, but it cannot be ignored or wished away. If a delay in
the decision time can be safely deferred to 2014, the quality of
information on which such a decision must be made will be materially
higher. We await NASA's analysis of the many factors involved this
decision process.
Rusty Schweickart
Chairman, B612 Foundation
Letter to NASA Administrator Michael Griffin:
Dear Mr. Griffin:
I am writing this letter on behalf of the B612 Foundation and others
to request your support in resolving an issue of critical importance.
The issue is best stated as a question; is it desirable, or perhaps
necessary, to launch a scientific mission to asteroid 2004MN4 in the
near future, in order to have adequately accurate knowledge of its
orbit in time to initiate a deflection mission in the unlikely event
one should be needed?
As you know, near-Earth asteroid 2004MN4 is unique because of its
unusually close pass by the Earth on April 13, 2029. Based on
current data, the asteroid (320-400 meter diameter) will pass
approximately 7,000 km inside the geostationary orbit and will be an
easily visible naked eye object to observers in Europe and Africa
early that evening. Because of this very close encounter, the orbit
of the asteroid will be substantially altered as it passes Earth,
resulting in a small probability that it will return in 2036 if it
happens to pass through the most probable of several "keyholes" which
lie within the current error ellipse.
While the probability of this event is very low (approximately 1 in
15,000 based on current data), the consequences of an impact are so
high (> $400 billion of infrastructure loss alone) that this
possibility warrants careful attention. Specifically, the size of the
keyhole which must be avoided during the 2029 encounter is so small
(~600 meters) that the best navigational data we will have, at the
time we would have to decide on a deflection mission, will be
inadequate to make that determination. This claim is based on the
anticipated optical and radar tracking opportunities between now and
2014, when we believe a deflection mission would have to be initiated
in order to assure a successful deflection.
The augmentation we propose to our ground tracking capability is the
launch and subsequent docking of a scientific mission, including a
standard radio transponder, with 2004MN4. While the improvement in
our knowledge of the asteroid's orbit would resolve the issue of a
potential impact, the scientific knowledge gained of both the surface
and interior characteristics of the asteroid would be invaluable for
future operations in and of itself. The unique circumstance presented
by this asteroid which might have to be deflected, whose size makes
it probable that it has a "rubble pile" structure, and whose orbit
will bring it within the range of distorting terrestrial tidal
forces, makes this a very attractive learning opportunity.
We sincerely urge you to direct the NASA NEO Program resources to
conduct a thorough and cooperative analysis of this issue.
Russell L. Schweickart
Chairman, Board of Directors
B612 Foundation
--
+++++++++++++++++++++++++++++++++++++++++++
NEO News (now in its tenth year of distribution) 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.nasa.gov. For
additional information, please see the website
http://impact.arc.nasa.gov. If anyone wishes to copy or redistribute
original material from these notes, fully or in part, please include
this disclaimer.
==========
