CCNet, 31 August 1999

     Dear Benny,

     The radar images of this asteroid are really exciting, and
     inspired me to write the following:

     Asteroid 1999JM8

     Peanut or potato-shaped, they wander through the void
     tumbling with a languid grace, or like sunbathers,
     turning each side in turn towards the solar warmth.
     Once in a million years, perhaps, some other wanderer
     encounters them, sends space-dirt spraying,
     changes their spin a little. Otherwise, unseen, unchanging,
     they orbit in the empty dark, until a searchlight beam
     of radar from blue Earth illumines faintly their rough face,
     shows pocks and peaks, shapes like grotesque skulls,
     that strain credulity and leave us asking "How?" and "Why?"

     Malcolm Miller

    Jacqueline Mitton <>

    Andrew Yee <>

    Michael Paine <>

    Theresa Cooper <>

    J. Kawaguchi et al., INST SPACE & ASTRONAUT SCI

    K. Sarneczky et al., JATE UNIVERSITY

    M. Grau & G. Gonzalez Casado, UNIV POLITECN CATALUNYA


    A. Rossi & M. Fulchignoni CNR, CNUCE, PISA

     V.V. Svettsov, RUSSIAN ACAD SCI

     S. Tabachnik & N.W. Evans,  UNIVERSITY OF OXFORD


From Jacqueline Mitton <>


Date: 30 August 1999
For immediaterelease

Ref. PN 99/27
Issued by: Dr Jacqueline Mitton
RAS Press Officer
Phone: Cambridge  ((0)1223) 564914
FAX: Cambridge ((0)1223) 572892

RAS Web:

* * * * * * * * * * * * * * * * * * * * * * * *


Dr David Asher (

Professor Mark Bailey (Director:

Mr John McFarland (PR Officer:

Armagh Observatory, College Hill, Armagh, BT61 9DG
Tel: 028-3752-2928, Fax: 028-3752-7174

Dr Rob McNaught (
School of Astronomy and Astrophysics, Australian National University.

* * * * * * * * * * * * * * * * * * * * * * * * *


November's Leonid meteor shower will produce good displays this year
and next, and strong storms of meteors in 2001 and 2002, according to
new research by Dr David Asher, of Armagh Observatory, and Dr Rob
McNaught of the Australian National University. Writing in the Monthly
Notices of the Royal Astronomical Society (21 August 1999 issue), they
show how the times when Earth passes through the dense streams of
matter in space that produce meteor showers can now be predicted with
remarkable accuracy.

In the early hours of 17th November last year (1998), meteor watchers
awaiting the Leonid shower were taken by surprise when a spectacular
display of bright meteors occurred 16 hours before the predicted time
for the maximum of the shower. The explanation for this phenomenon was
discovered by Dr Asher and his colleagues Professor Mark Bailey of
Armagh Observatory, and Professor Vacheslav Emel'yanenko of South Ural
University, Chelyabinsk, Russia, and was published in April (see RAS
Press Notice 99/09). They showed that the bright meteors were seen when
Earth passed through a dense arc-shaped cloud of particles shed from
Comet Tempel-Tuttle in the year 1333 and they proved for the first time
that meteoroid streams can have complex braid-like structures within
them. This work pointed the way to more precise predictions of the
timing and intensity of meteor showers, such as those Asher and
McNaught are now making for the Leonids.

The latest analysis, covering Leonid meteor storms over the past two
hundred years, shows that the peak times of the strongest storms and
sharpest outbursts are predictable to within about five minutes. The
technique involves mapping the fine `braided' structure of the dense
dust trails within the Leonid meteoroid stream. Although comet
Tempel-Tuttle, the 'parent' of the Leonid stream, passed close to the
Earth in 1998, Asher and McNaught predict strong meteor storms in both
2001 and 2002. 1999 and 2000 will be less spectacular, but good. In
1999, observers at European longitudes are favoured, and may see up to
20 meteors a minute (in ideal conditions under a clear, dark sky) at
around 2 a.m. on the morning of November 18th.

Meteors, popularly known as 'shooting stars', can be seen on any night,
given a sufficiently clear, dark sky. They are produced by the impact
on the Earth's atmosphere of small dust grains released from comets.
Most meteors arrive in 'showers' at fixed times of the year, when the
Earth passes close to the orbit of the parent comet. But occasionally -
just a few times a century - a phenomenon known as a meteor storm
occurs. During a storm, meteors appear at astonishing rates, sometimes
several per second. The most famous example, the incredible Leonid
display of 1833, is credited with starting the serious scientific study
of meteors.

Good news for meteor observers can be a concern for satellite
operators. A satellite can be disabled by the impact of even a small
dust grain. While the hazard from man-made space debris is well known,
the danger from meteoroids has been more difficult to assess. Prior
knowledge of the detailed structure of the Leonid stream is potentially
of immense value. Satellite operators could use this information to
take appropriate avoiding action and minimise the risk. With this new
work, McNaught and Asher have defined the structure of the Leonid dust
trails more accurately than ever before.


What are the Leonid meteoroid stream and the Leonid meteor shower?

The Leonid meteor display is associated with the Earth's passage
through the Leonid stream. This stream consists of the debris of
Tempel-Tuttle, a comet that orbits the Sun about every 33 years.

When do the most intense outbursts occur?

Although the Earth goes through the Leonid stream every November, in
most years the Leonid meteor shower is unspectacular. However, there is
fine structure within the stream, and meteor storms occur when the
Earth runs through the highest density regions. The new technique for
mapping out the structure involves precise calculations of the effect
of the gravity of the planets on the dense dust trails, covering many
revolutions of the dust grains about the Sun over periods of a century
or two.

Why are some longitudes favoured?

The meteors in any given shower come from a particular direction in
space. You need to be on the hemisphere facing that direction to see
the meteors. It also has to be night-time, except for incredibly bright
fireballs. In the case of the Leonids, an approximate rule is to
observe after midnight. Background Leonids (a few meteors per hour)
appear for a few days, and so all parts of the Earth have a chance to
catch them. But some outbursts are of high intensity for less than an
hour, and you have to be at a longitude where the time is between
midnight and dawn. The next few years will provide various excellent
Leonid opportunities, of which 2001 from East Asian longitudes will be
best, especially as the moon will be absent from the sky. Most
immediately, 1999 should produce a good display, although rates will
not match the most spectacular ones: the Zenithal Hourly Rate (defined
for an individual observer in near-ideal observing conditions) is
estimated to peak at 1,200 per hour at 02:08 GMT on November 18th.

Can damage to satellites occur?

Very high speed impacts of tiny dust grains on satellites can cause
plasma to be generated, which can lead to electrical failure. There is
evidence that the Olympus communications satellite was disabled owing
to the impact of a meteoroid from the Perseid stream in 1993.

History of this work

The famous Leonid storms of 1833, 1866 and 1966 were known to relate to
the roughly 33 year period of the comet. But it was only when McNaught
examined the details of those and other Leonid outbursts of the past
two hundred years that the full predictive power of the 'dust trail'
technique became apparent. Whereas theories that considered the comet
alone, rather than the dust trail structure in the stream, would
sometimes match observed timings of storms within hours (but
occasionally fail completely), the dust trail theory allows an accuracy
that many astronomers never suspected possible. Further refinements to
the theory, including a topographic correction, have reduced the
uncertainty to around five minutes.

A few months after developing the technique, McNaught and Asher
extended their work to permit estimates of meteor rates (in addition to
predicting storm timings), and applied it to forthcoming encounters of
the Earth with Leonid dust trails. There is no doubt that 2001 and 2002
will provide opportunities to witness exceptional Leonid meteor storms.

The fact that something out of the ordinary is expected in both 2001
and 2002 had in fact been published more than a decade ago, by two
researchers, Kondrat'eva and Reznikov, in Kazan, Russia. The English
translation of their paper did not come to the notice of many western


From Andrew Yee <>


Friday, 27 August 1999, 5 pm PST

Water From the Dawn of the Solar System
By Govert Schilling

Scientists have found tiny droplets of water, dating from the dawn of
the solar system, in two meteorites that fell to Earth last year. It's
the first time liquid water has been found in extraterrestrial samples.
A study about one of the meteorites is published in today's Science, p.

Meteorites are fragments of rocky asteroids, and the mineral
composition of some meteorites had convinced scientists that their
"parent bodies" must have contained liquid water in the past.
Consequently, "we have been looking for water in meteorites for a
generation," says mineralogist Michael Zolensky of NASA's Johnson Space
Center in Houston.

In their paper, Zolensky and his colleagues present an analysis of
Monahans, a meteorite that fell near a Texan town on 22 March 1998. The
team also studied a meteorite called Zag, which landed in the Moroccan
part of the Sahara desert 5 months later. Although it hasn't been fully
analyzed yet, Zolensky says Zag's composition closely resembles that of
Monahans. Both contain millimeter-sized, purple salt crystals with
small inclusions of briny water.

Radioactive dating of the crystals -- which contain ordinary table salt
(NaCl) and sylvite (KCl) -- show that they are over 4.5 billion years
old, which means the water must have been trapped around the time the
solar system was formed. But it's unclear if the water was indigenous
to the parent asteroid, or if it was deposited there by a comet or a
rock slamming into it. Determining the brine's isotopic composition
would answer that question, but the inclusions are too small to measure
that composition with current instruments. However, says Zolensky, a
very sensitive mass spectrometer currently being developed at Cambridge
University in the U.K. should be able to do the trick next year.

The discovery, described as "astonishing" in an accompanying
perspective (Science, 27 August, p. 1364) by Robert Clayton of the
University of Chicago, leaves several mysteries unsolved. For one, both
meteorites are ordinary chondrites -- the most common type of meteorite
-- whose parent bodies were believed to be relatively dry. Also, if
there's water in an asteroid, says Zolensky, you would expect to find
it in the interior, where it's safe from evaporation, erosion, and
cosmic radiation; but Monahans and Zag are both from the asteroid's
surface, as evidenced by contamination with atoms from the solar wind.
The largest riddle, however, is why the salt crystals, which are formed
when water evaporates after reacting with the rock, are so big. "You
need a lot of evaporating water to produce these amounts of [salt],"
says Zolensky.

(c) 1999 The American Association for the Advancement of Science

[Extracted from INSCiGHT, Academic Press.]


From Michael Paine <>

Dear Benny,

The 1999 Space Frontier Conference, to be held on September 23-26, 1999
in Los Angeles has several items on NEOs:

The Watch, Planetary Defense
    Dr. Tom Gehrels - University of Arizona
    Dr. Alan Hale - co-discoverer of the Hale-Bopp comet
    Dr. Eleanor Helin - Jet Propulsion Laboratories
    Dr. John Lewis - University of Arizona

The Moon, Asteroids, Mars
    Jim Benson - Space Dev
    Dr. Kenneth Cox - NASA-JSC
    Al Globus - AsterAnts
    Dr. Wendell Mendell - NASA-JSC
    Dr. Madhu Tongavayla - University of Southern California


Michael Paine


From Theresa Cooper <>

Benny, would you post the attached notice of a convention to be held in
Cardiff in October. Its free and all are welcome!!

Many thanks,
Theresa Cooper
(for South Wales Astronomical Societies)


The 2nd  South Wales Astronomical Societies (Swansea, Port Talbot,
Bridgend, Cardiff and Usk) Convention will take place on Saturday
October 16th at the Department of Physics and Astronomy, University of
Wales College Cardiff, 5 The Parade, Cardiff. The speakers and their
lectures  will include:

Professor Mike Edmunds
University of Wales College Cardiff
The Chemical Evolution of the Universe

Dr Victor Clube
Oxford University
The Mindless Millenium

Professor Chandra Wickramasinghe
University of Wales College Cardiff
Extra Terrestrial Life, New Frontiers

Dr Iain Steele
John Moores University, Liverpool
Robotic Telescopes

Trade stands* will also be present, as well as the Techniquest
inflatable planetarium. There will also be an opportunity to look
around this world renowned physics and astronomy department.

For more information please contact Theresa Cooper on 01446-700722 or
email Updated information will also be
placed on the Web site

* Trade stands to date are SCS Astro, Venturescope, Earth and Sky,
Springer Books, Nigel Wakefield, Techniquest, University of Glamorgan,
The Webb Society, BAA, Astronomy Now, FAS, Andromeda Books


J. Kawaguchi*), T. Hashimoto, T. Misu, S. Sawai: An autonomous optical
guidance and navigation around asteroids?. ACTA ASTRONAUTICA, 1999,
Vol.44, No.5-6, pp.267-280


An impending demand for exploring the small bodies such as the comets
and the asteroids envisioned the Japanese MUSES-C mission to the near
Earth asteroid Nereus, An autonomous optical guidance and navigation
strategy around the asteroid is discussed in this paper. Four major new
schemes are dealt with hers: They are (1) Aligned intercept guidance,
(2) Strategic building of the flight phases, (3) Image processing of
line-of-sight shift information instead of characteristic point
tracking, and (4) Stability and accuracy analysis associated with the
guidance and navigation strategies developed here. Some comprehensive
numerical illustrations are also given to support them. 1999 Elsevier
Science Ltd. All rights reserved.


K. Sarneczky*), G. Szabo, L.L. Kiss: CCD observations of 11 faint
No.2, pp.363-368


We present new CCD observations of 11 poorly studied, faint and
moderately faint asteroids. Six of them (1089, 1452, 2415, 9262, 1998
FM5, 1989 UR) have never previously been observed photometrically. thus
our lightcurves are the first ones in the literature. The achieved
accuracy ranges between 0.01 - 0.03 mag depending mainly on the
brightness of the target objects. The obtained sinodic periods and
amplitudes: 1089 - > 4.(h), 0.025 mag; 1452 - 17.(h)2 +/- 0.(h)1,
greater than or equal to 0.34 mag; 2415 - > 2.(h)5, 0.15 mag; 9262 - >
6.(h)3, greater than or equal to 0.08 mag; 1989 UR - > 4(h), greater
than or equal to 0.15 mag 1998 FM5 - > 2.(h)8, greater than or equal to
0.61 mag. Additionally lightcurves are presented for asteroids observed
earlier in only one or two oppositions (792, 1508, 1604, 1865). The
resulting periods and amplitudes: 792 - 9.(h)19 +/- 0.(h)01, 0.76 +/-
0.02 mag; 1508 - 9.(h)15 +/- 0.(h)03, 0.52 +/- 0.01 mag; 1604-6.(h)15
+/- 0.(h)02, greater than or equal to 0.17 +/- 0.01 mag; 1865 - 6.(h)87
+/- 0.(h)03, 2.3 +/- 0.1 mag. We have conducted shape fitting with a
triaxial ellipsoid and determined spin vector and sens of rotation for
1727 combining our new observations with previously published
lightcurves. The results are: lambda(p) = 126/306 +/- 10 degrees,
beta(p) = 56 +/- 15 degrees, a/b = 1.9 +/- 0.1, b/c = 1.6 +/- 0.1.
Copyright 1999, Institute for Scientific Information Inc.


M. Grau*), G. Gonzalez Casado: Dynamical behavior of asteroids near
resonance: The 4:1 gap and the 7:2 group. CELESTIAL MECHANICS &
DYNAMICAL ASTRONOMY, 1998, Vol.72, No.3, pp.169-186


A comparative study of the evolution of the Sun-Jupiter-Asteroid system
near the 4:1 and 7:2 resonances is performed by means of two techniques
that proceed differently from the Hamiltonian corresponding to the
planar restricted elliptic three-body problem. One technique is based
on the classical Schubart averaging while the other is based on a
mapping method in which the perturbing part of the Hamiltonian is
expanded and the resulting terms are ordered according to a weight
function that depends on the powers of eccentricities and the
coefficients of the terms. For the mapping method the effect of Saturn
on the asteroidal evolution is introduced and the degree of chaos is
estimated by means of the Lyapunov time. Both methods are shown to lead
to similar results and can be considered a suitable tool for describing
the evolution of asteroids in the Kirkwood gap and the group
corresponding to the 4:1 and 7:2 Jovian resonances, respectively.
Copyright 1999, Institute for Scientific Information Inc.


M. Grau*), M. Noguera: Periodic orbit families near the 4:1 jovian
No.3, pp.201-218


An enlarged averaged Hamiltonian is introduced to compute several
families of periodic orbits of the planar elliptic 3-body problem, in
the Sun-Jupiter-Asteroid system, near the 4:1 resonance. Four resonant
critical point families are found and their stability is studied. The
families of symmetric periodic orbits of the elliptic problem appear
near the corresponding fixed points computed in this model. There is a
good agreement for moderate eccentricity of the asteroid for three of
these families, whereas the remaining family cannot be considered as
a family of periodic orbits of the real model. Copyright 1999,
Institute for Scientific Information Inc.


A. Rossi*), M. Fulchignoni: Study of the environment around the Rosetta
candidate target asteroids. PLANETARY AND SPACE SCIENCE, 1999, Vol.47,
No.6-7, pp.873-881


The ROSETTA spacecraft will fly-by a few asteroids during its course to
the final cometary target. The candidate asteroids presently are 3840
Mimistrobel (S-type), 2703 Rodari (S-type) and 140 Siwa (C-type).
With the limited data presently available on these bodies we calculated
some approximate quantities which may be useful to select the fly-by
trajectories of the ROSETTA probe. In particular we derived the zones
in which particles could stably orbit by analyzing Hill's problem of
three hierarchical masses-the sun, the asteroid and the orbiting
particle. Then, following the approach of Hamilton and Burns, the
effects of solar radiation pressure and of the ellipticity of the
orbits were also taken into account. In this way for each asteroid we
could calculate not only a classical quantity like the radius of the
Hill sphere, but also the critical starting orbital distance (as a
function of orbital inclination) within which most orbits remain bound
to the asteroid, and outside which most escape as a consequence of
perturbations. Moreover we determined the orbital stability zone,
defined as the union of all the numerically integrated orbits showing
long-term stability, for each of the target asteroids. The particular
shape of these zones would suggest to have the spacecraft's close
approach out of the orbital plane of the asteroids. To further
investigate this problem and, in particular, to take into account the
irregular shape of the asteroids, we developed a model using a
polyhedral representation of the central rotating body, following a
theory developed by Werner and Scheeres. This model is described here
and the first orbital integration results are presented. (C) 1999
Elsevier Science Ltd. All rights reserved.


V.V. Svettsov: Explosions of meteoroids and estimating their parameters
from light emission. COMBUSTION EXPLOSION AND SHOCK WAVES, 1998,
Vol.34, No.4, pp.474-484


High-power visible light pulses detected by sensors mounted on
geostationary satellites are analyzed. The distinctive features
of meteoroid explosions which produce these flashes in the atmosphere
are studied. A method is described for determining the parameters of
bodies in space from the known radiation power and altitude of the
explosion. Numerical calculations show that these light pulses were
produced by falling stony and, once, ferrous bodies with dimensions on
the order of a meter. The bursts produced by bodies from outer space in
the atmosphere are compared with the light pulses from spherically
symmetric instantaneous explosions with similar energies. Copyright
1999, Institute for Scientific Information Inc.


S. Tabachnik*), N.W. Evans: Cartography for Martian Trojans.
ASTROPHYSICAL JOURNAL, 1999, Vol.517, No.1 Pt2, pp.L63-L66


The last few months have seen the discovery of a second Martian Trojan
(1998 VF31) as well as two further possible candidates (1998 QH56 and
1998 SD4). Together with the previously discovered Martian satellite
5261 Eureka, these are the only known possible solar system Trojan
asteroids not associated with Jupiter. Here maps of the locations of
the stable Trojan trajectories of Mars are presented. These are
constructed by integrating an ensemble of in-plane and inclined orbits
in the vicinity of the Martian Lagrangian points for between 25 and 60
million years. The survivors occupy a band of inclinations between 15
degrees and 40 degrees and longitudes between 240 degrees and 330
degrees at the L5 Lagrangian point.. Around the L4 point, stable Trojans
inhabit two bands of inclinations (15 degrees < i < 30 degrees and 32
degrees < i < 40 degrees) with longitudes restricted between 25 degrees
and 120 degrees. Both 5261 Eureka and 1998 VF31 lie deep within one of
the stable zones, which suggests that they may be of primordial origin.
Around Mars, the number of such undiscovered primordial objects with
sizes greater than 1 km may be as high as similar to 50. The two
candidates 1998 QH56 and 1998 SD4 are not presently on Trojan orbits
and will enter the sphere of influence of Mars within half a million
years. Copyright 1999, Institute for Scientific Information Inc.

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Migrating Planets

Did the solar system always look the way it does now? New evidence
indicates that the outer planets may have migrated to their
present orbits.

by Renu Malhotra

In the familiar visual renditions of the solar system, each planet
moves around the sun in its own well-defined orbit, maintaining a
respectful distance from its neighbors. The planets have
maintained this celestial merry-go-round since astronomers began
recording their motions, and mathematical models show that this
very stable orbital configuration has existed for almost the
entire 4.5-billion-year history of the solar system. It is
tempting, then, to assume that the planets were "born" in the
orbits that we now observe.

Certainly it is the simplest hypothesis. Modern-day astronomers
have generally presumed that the observed distances of the planets
from the sun indicate their birthplaces in the solar nebula, the
primordial disk of dust and gas that gave rise to the solar
system. The orbital radii of the planets have been used to infer
the mass distribution within the solar nebula. With this basic
information, theorists have derived constraints on the nature and
timescales of planetary formation. Consequently, much of our
understanding of the early history of the solar system is based on
the assumption that the planets formed in their current orbits.

It is widely accepted, however, that many of the smaller bodies in
the solar system--asteroids, comets and the planets' moons--have
altered their orbits over the past 4.5 billion years, some more
dramatically than others. The demise of Comet Shoemaker-Levy 9
when it collided with Jupiter in 1994 was striking evidence of the
dynamic nature of some objects in the solar system. Still smaller
objects--micron-and millimeter-size interplanetary particles
shaken loose from comets and asteroids--undergo a more gradual
orbital evolution, gently spiraling in toward the sun and raining
down on the planets in their path.

Furthermore, the orbits of many planetary satellites have changed
significantly since their formation. For example, Earth's moon is
believed to have formed within 30,000 kilometers (18,600 miles) of
Earth--but it now orbits at a distance of 384,000 kilometers. The
moon has receded by nearly 100,000 kilometers in just the past
billion years because of tidal forces (small gravitational
torques) exerted by our planet. Also, many satellites of the outer
planets orbit in lockstep with one another: for instance, the
orbital period of Ganymede, Jupiter's largest moon, is twice that
of Europa, which in turn has a period twice that of Io. This
precise synchronization is believed to be the result of a gradual
evolution of the satellites' orbits by means of tidal forces
exerted by the planet they are circling.

Until recently, little provoked the idea that the orbital
configuration of the planets has altered significantly since their
formation. But some remarkable developments during the past five
years indicate that the planets may indeed have migrated from
their original orbits. The discovery of the Kuiper belt has shown
that our solar system does not end at Pluto. Approximately 100,000
icy "minor planets" (ranging between 100 and 1,000 kilometers in
diameter) and an even greater number of smaller bodies occupy a
region extending from Neptune's orbit--about 4.5 billion
kilometers from the sun--to at least twice that distance. The
distribution of these objects exhibits prominent nonrandom
features that cannot be readily explained by the current model of
the solar system. Theoretical models for the origin of these
peculiarities suggest the intriguing possibility that the Kuiper
belt bears traces of the orbital history of the gas-giant
planets--specifically, evidence of a slow spreading of these
planets' orbits subsequent to their formation.

What is more, the recent discovery of several Jupiter-size
companions orbiting nearby sunlike stars in peculiarly small
orbits has also focused attention on planetary migration. It is
difficult to understand the formation of these putative planets at
such small distances from their parent stars. Hypotheses for their
origin have proposed that they accreted at more comfortable
distances from their parent stars--similar to the distance between
Jupiter and the sun--and then migrated to their present positions.

Pluto: Outcast or Smoking Gun?

Until just a few years ago, the only planetary objects known
beyond Neptune were Pluto and its satellite, Charon. Pluto has
long been a misfit in the prevailing theories of the solar
system's origin: it is thousands of times less massive than the
four gas-giant outer planets, and its orbit is very different from
the well-separated, nearly circular and co-planar orbits of the
eight other major planets. Pluto's is eccentric: during one
complete revolution, the planet's distance from the sun varies
from 29.7 to 49.5 astronomical units (one astronomical unit, or
AU, is the distance between Earth and the sun, about 150 million
kilometers). Pluto also travels 8 AU above and 13 AU below the
mean plane of the other planets' orbits. For approximately two
decades in its orbital period of 248 years, Pluto is closer to the
sun than Neptune is.

In the decades since Pluto's discovery in 1930, the planet's
enigma has deepened. Astronomers have found that most
Neptune-crossing orbits are unstable--a body in such an orbit
will either collide with Neptune or be ejected from the outer
solar system in a relatively short time, typically less than 1
percent of the age of the solar system. But the particular
Neptune-crossing orbit in which Pluto travels is protected from
close approaches to the gas giant by a phenomenon called resonance
libration. Pluto makes two revolutions around the sun during the
time that Neptune makes three; Pluto's orbit is therefore said to
be in 3:2 resonance with Neptune's. The relative motions of the
two planets ensure that when Pluto crosses Neptune's orbit, it is
far away from the larger planet. In fact, the distance between
Pluto and Neptune never drops below 17 AU.

In addition, Pluto's perihelion--its closest approach to the
sun--always occurs high above the plane of Neptune's orbit, thus
maintaining Pluto's long-term orbital stability. Computer
simulations of the orbital motions of the outer planets, including
the effects of their mutual perturbations, indicate that the
relationship between the orbits of Pluto and Neptune is billions
of years old and will persist for billions of years into the
future. Pluto is engaged in an elegant cosmic dance with Neptune,
dodging collisions with the gas giant over the entire age of the
solar system.

How did Pluto come to have such a peculiar orbit? In the past,
this question has stimulated several speculative and ad hoc
explanations, typically involving planetary encounters. Recently,
however, significant advances have been made in understanding the
complex dynamics of orbital resonances and in identifying their
Jekyll-and-Hyde role in producing both chaos and exceptional
stability in the solar system. Drawing on this body of knowledge,
I proposed in 1993 that Pluto was born somewhat beyond Neptune and
initially traveled in a nearly circular, low-inclination orbit
similar to those of the other planets but that it was transported
to its current orbit by resonant gravitational interactions with
Neptune. A key feature of this theory is that it abandons the
assumption that the gas-giant planets formed at their present
distances from the sun.

Instead it proposes an epoch of planetary orbital migration early
in the history of the solar system, with Pluto's unusual orbit as
evidence of that migration. The story begins at a stage when the
process of planetary formation was almost but not quite complete.
The gas giants--Jupiter, Saturn, Uranus and Neptune--had nearly
finished coalescing from the solar nebula, but a residual
population of small planetesimals--rocky and icy bodies, most no
larger than a few tens of kilometers in diameter--remained in
their midst. The relatively slower subsequent evolution of the
solar system consisted of the scattering or accretion of the
planetesimals by the major planets. Because the planetary
scattering ejected most of the planetesimal debris to distant or
unbound orbits--essentially throwing the bodies out of the solar
system--there was a net loss of orbital energy and angular
momentum from the giant planets' orbits. But because of their
different masses and distances from the sun, this loss was not
evenly shared by the four giant planets.

In particular, consider the orbital evolution of the outermost
giant planet, Neptune, as it scattered the swarm of planetesimals
in its vicinity. At first, the mean specific orbital energy of the
planetesimals (the orbital energy per unit of mass) was equal to
that of Neptune itself, so Neptune did not gain or lose energy
from its gravitational interactions with the bodies. At later
times, however, the planetesimal swarm near Neptune was depleted
of the lower-energy objects, which had moved into the
gravitational reach of the other giant planets. Most of these
planetesimals were eventually ejected from the solar system by
Jupiter, the heavyweight of the planets.

Thus, as time went on, the specific orbital energy of the
planetesimals that Neptune encountered grew larger than that of
Neptune itself. During subsequent scatterings, Neptune gained
orbital energy and migrated outward. Saturn and Uranus also gained
orbital energy and spiraled outward. In contrast, Jupiter lost
orbital energy; its loss balanced the gains of the other planets
and planetesimals, hence conserving the total energy of the
system. But because Jupiter is so massive and had so much orbital
energy and angular momentum to begin with, its orbit decayed only

The possibility of such subtle adjustments of the giant planets'
orbits was first described in a little-noticed paper published in
1984 by Julio A. Fernandez and Wing-Huen Ip, a Uruguayan and
Taiwanese astronomer duo working at the Max Planck Institute in
Germany. Their work remained a curiosity and escaped any comment
among planet formation theorists, possibly because no supporting
observations or theoretical consequences had been identified.

In 1993 I theorized that as Neptune's orbit slowly expanded, the
orbits that would be resonant with Neptune's also expanded. In
fact, these resonant orbits would have swept by Pluto, assuming
that the planet was originally in a nearly circular,
low-inclination orbit beyond Neptune. I calculated that any such
objects would have had a high probability of being "captured" and
pushed outward along the resonant orbits as Neptune migrated. As
these bodies moved outward, their orbital eccentricities and
inclinations would have been driven to larger values by the
resonant gravitational torque from Neptune. (This effect is
analogous to the pumping-up of the amplitude of a playground swing
by means of small periodic pushes at the swing's natural
frequency.) The final maximum eccentricity would therefore provide
a direct measure of the magnitude of Neptune's migration.
According to this theory, Pluto's orbital eccentricity of 0.25
suggests that Neptune has migrated outward by at least 5 AU.
Later, with the help of computer simulations, I revised this to 8
AU and also estimated that the timescale of migration had to be a
few tens of millions of years to account for the inclination of
Pluto's orbit.

Of course, if Pluto were the only object beyond Neptune, this
explanation of its orbit, though compelling in many of its
details, would have remained unverifiable. The theory makes
specific predictions, however, about the orbital distribution of
bodies in the Kuiper belt, which is the remnant of the primordial
disk of planetesimals beyond Neptune [see "The Kuiper Belt," by
Jane X. Luu and David C. Jewitt]. Provided that the largest bodies
in the primordial Kuiper belt were sufficiently small that their
perturbations on the other objects in the belt would be
negligible, the dynamical mechanism of resonance sweeping would
work not only on Pluto but on all the trans-Neptunian objects,
perturbing them from their original orbits. As a result, prominent
concentrations of objects in eccentric orbits would be found at
Neptune's two strongest resonances, the 3:2 and the 2:1. Such
orbits are ellipses with semimajor axes of 39.5 AU and 47.8 AU,
respectively. (The length of the semimajor axis is equal to the
object's average distance from the sun.)

More modest concentrations of trans-Neptunian bodies would be
found at other resonances, such as the 5:3. The population of
objects closer to Neptune than the 3:2 resonant orbit would be
severely depleted because of the thorough resonance sweeping of
that region and because perturbations caused by Neptune would
destabilize the orbits of any bodies that remained. On the other
hand, planetesimals that accreted beyond 50 AU from the sun would
be expected to be largely unperturbed and still orbiting in their
primordial distribution.

Fortunately, recent observations of Kuiper belt objects, or KBOs,
have provided a means of testing this theory. More than 174 KBOs
have been discovered as of mid-1999. Most have orbital periods in
excess of 250 years and thus have been tracked for less than 1
percent of their orbits. Nevertheless, reasonably reliable orbital
parameters have been determined for about 45 of the known KBOs
[see illustration below]. Their orbital distribution is not a
pattern of uniform, nearly circular, low-inclination orbits, as
would be expected for a pristine, unperturbed planetesimal
population. Instead one finds strong evidence of gaps and
concentrations in the distribution. A large fraction of these KBOs
travel in eccentric 3:2 resonant orbits similar to Pluto's, and
KBOs in orbits interior to the 3:2 orbit are nearly absent--which
is consistent with the predictions of the resonance sweeping

Still, one outstanding question remains: Are there KBOs in the 2:1
resonance comparable in number to those found in the 3:2, as the
planet migration theory would suggest? And what is the orbital
distribution at even greater distances from the sun? At present,
the census of the Kuiper belt is too incomplete to answer this
question fully. But on Christmas Eve 1998 the Minor Planet Center
in Cambridge, Mass., announced the identification of the first KBO
orbiting in 2:1 resonance with Neptune. Two days later the center
revealed that another KBO was traveling in a 2:1 resonant orbit.
Both these objects have large orbital eccentricities, and they may
turn out to be members of a substantial population of KBOs in
similar orbits. They had previously been identified as orbiting in
the 3:2 and 5:3 resonances, respectively, but new observations
made last year strongly indicated that the original
identifications were incorrect. This episode underscored the need
for continued tracking of known KBOs in order to map their orbital
distribution correctly. We must also acknowledge the dangers of
overinterpreting a still small data set of KBO orbits.

In short, although other explanations cannot be ruled out yet, the
orbital distribution of KBOs provides increasingly strong evidence
for planetary migration. The data suggest that Neptune was born
about 3.3 billion kilometers from the sun and then moved about 1.2
billion kilometers outward--a journey of almost 30 percent of its
present orbital radius. For Uranus, Saturn and Jupiter, the
magnitude of migration was smaller, perhaps 15, 10 and 2 percent,
respectively; the estimates are less certain for these planets
because, unlike Neptune, they could not leave a direct imprint on
the Kuiper belt population.

Most of this migration took place over a period shorter than 100
million years. That is long compared with the timescale for the
formation of the planets--which most likely took less than 10
million years--but short compared with the 4.5-billion-year age of
the solar system. In other words, the planetary migration occurred
in the early history of the solar system but during the later
stages of planet formation. The total mass of the scattered
planetesimals was about three times Neptune's mass. The question
arises whether even more drastic orbital changes might occur in
planetary systems at earlier times, when the primordial disk of
dust and gas contains more matter and perhaps many protoplanets in
nearby orbits competing in the accretion process.

Other Planetary Systems?

In the early 1980s theoretical studies by Peter Goldreich and
Scott Tremaine, both then at the California Institute of
Technology, and others concluded that the gravitational forces
between a protoplanet and the surrounding disk of gas, as well as
the energy losses caused by viscous forces in a gaseous medium,
could lead to very large exchanges of energy and angular momentum
between the protoplanet and the disk. If the torques exerted on
the protoplanet by the disk matter just inside the planet's orbit
and by the matter just beyond it were slightly unbalanced, rapid
and drastic changes in the planet's orbit could happen. But again,
this theoretical possibility received little attention from other
astronomers at the time. Having only our solar system as an
example, planet formation theorists continued to assume that the
planets were born in their currently observed orbits.

In the past five years, however, the search for extrasolar planets
has yielded possible signs of planetary migration. By measuring
the telltale wobbles of nearby stars--within 50 light-years of our
solar system--astronomers have found evidence of more than a dozen
Jupiter-mass companions in surprisingly small orbits around
main-sequence stars. The first putative planet was detected
orbiting the star 51 Pegasi in 1995 by two Swiss astronomers,
Michel Mayor and Didier Queloz of the Geneva Observatory, who were
actually surveying for binary stars. Their observations were
quickly confirmed by Geoffrey W. Marcy and R. Paul Butler, two
American astronomers working at Lick Observatory near San Jose,
Calif. As of June 1999, 20 extrasolar planetary candidates have
been identified, most by Marcy and Butler, in search programs that
have surveyed almost 500 nearby sunlike stars over the past 10
years. The technique used in these searches--measuring the Doppler
shifts in the stars' spectral lines to determine periodic
variations in stellar velocities--yields only a lower limit on the
masses of the stars' companions. Most of the candidate planets
have minimum masses of about one Jupiter-mass and orbital radii
shorter than 0.5 AU.

What is the relationship between these objects and the planets in
our solar system? According to the prevailing model of planet
formation, the giant planets in our solar system coalesced in a
two-step process. In the first step, solid planetesimals clumped
together to form a protoplanetary core. Then this core
gravitationally attracted a massive gaseous envelope from the
surrounding nebula. This process must have been completed within
about 10 million years of the formation of the solar nebula
itself, as inferred from astronomical observations of the lifetime
of protoplanetary disks around young sunlike stars.

At distances of less than 0.5 AU from a star, there is 
insufficient mass in the primordial disk for solid protoplanetary
cores to condense. Furthermore, it is questionable whether a
protoplanet in a close orbit could attract enough ambient gas to
provide the massive envelope of a Jupiter-like planet. One reason
is simple geometry: an object in a tight orbit travels through a
smaller volume of space than one in a large orbit does. Also, the
gas disk is hotter close to the star and hence less likely to
condense onto a protoplanetary core. These considerations have
argued against the formation of giant planets in very short-period

Instead several theorists have suggested that the putative
extrasolar giant planets may have formed at distances of several
AU from the star and subsequently migrated inward. Three
mechanisms for planetary orbital migration are under discussion.
Two involve disk-protoplanet interactions that allow planets to
move long distances from their birthplaces as long as a massive
disk remains.

With the disk-protoplanet interactions theorized by Goldreich and
Tremaine, the planet would be virtually locked to the inward flow
of gas accreting onto the protostar and might either plunge into
the star or decouple from the gas when it drew close to the star.
The second mechanism is interaction with a planetesimal disk
rather than a gas disk: a giant planet embedded in a very massive
planetesimal disk would exchange energy and angular momentum with
the disk through gravitational scattering and resonant
interactions, and its orbit would shrink all the way to the disk's
inner edge, just a few stellar radii from the star.

The third mechanism is the scattering of large planets that either
formed in or moved into orbits too close to one another for
long-term stability. In this process, the outcomes would be quite
unpredictable but generally would yield very eccentric orbits for
both planets. In some fortuitous cases, one of the scattered
planets would move to an eccentric orbit that would come so near
the star at its closest approach that tidal friction would
eventually circularize its orbit; the other planet, meanwhile,
would be scattered to a distant eccentric orbit. All the
mechanisms accommodate a broad range of final orbital radii and
orbital eccentricities for the surviving planets.

These ideas are more than a simple tweak of the standard model of
planet formation. They challenge the widely held expectation that
protoplanetary disks around sunlike stars commonly evolve into
regular planetary systems like our own. It is possible that most
planets are born in unstable configurations and that subsequent
planet migration can lead to quite different results in each
system, depending sensitively on initial disk properties. An
elucidation of the relation between the newly discovered
extrasolar companions and the planets in our solar system awaits
further theoretical and observational developments. Nevertheless,
one thing is certain: the idea that planets can change their
orbits dramatically is here to stay.

Further Reading

Freeman and Company, 1993.

Paul Butler in Annual Review of Astronomy and Astrophysics, Vol.
36, pages 57-98; 1998.

Dynamics of the Kuiper Belt. Renu Malhotra et al. in Protostars
and Planets IV. Edited by V. Mannings et al. University of Arizona
Press (in press).

The Author

RENU MALHOTRA did her undergraduate studies at the Indian Institute of
Technology in Delhi and received a Ph.D. in physics from Cornell
University in 1988. After completing postdoctoral research at the
California Institute of Technology, she moved to her current
position as a staff scientist at the Lunar and Planetary Institute
in Houston. In her research, she has followed her passionate
interest in the dynamics and evolution of the solar system and
other planetary systems. She also immensely enjoys playing with her
four-year-old daughter, Mira.

Copyright 1999, Scientific American

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