7,560 research outputs found
Jupiter - Friend or Foe? IV: The influence of orbital eccentricity and inclination
For many years, it was assumed that Jupiter prevented the Earth from being
subject to a punishing impact regime that would greatly hinder the development
of life. Here, we present the 4th in a series of studies investigating this
hypothesis. Previously, we examined the effect of Jupiter's mass on the impact
rate experienced by Earth. Here, we extend that approach to consider the
influence of Jupiter's orbital eccentricity and inclination on the impact rate.
We first consider scenarios in which Jupiter's orbital eccentricity was
somewhat higher and somewhat lower than that in our Solar System. We find that
Jupiter's orbital eccentricity plays a moderate role in determining the impact
flux at Earth, with more eccentric orbits resulting in a higher impact rate of
asteroids than for more circular orbits. This is particularly pronounced at
high "Jupiter" masses. For short-period comets, the same effect is clearly
apparent, albeit to a lesser degree. The flux of short-period comets impacting
the Earth is slightly higher for more eccentric Jovian orbits.
We also consider scenarios in which Jupiter's orbital inclination was greater
than that in our Solar System. Increasing Jupiter's orbital inclination greatly
increased the flux of asteroidal impactors. However, at the highest tested
inclination, the disruption to the Asteroid belt was so great that the belt
would be entirely depleted after an astronomically short period of time. In
such a system, the impact flux from asteroid bodies would therefore be very
low, after an initial period of intense bombardment. By contrast, the influence
of Jovian inclination on impacts from short-period comets was very small. A
slight reduction in the impact flux was noted for the moderate and high
inclination scenarios considered in this work - the results for inclinations of
five and twenty-five degrees were essentially identical.Comment: 5 figures, plus 12 as an appendi
Optical-noise supression unit: A concept
Device is used with coherent optical-processing spatial-filtering computer. It is isexpensive to manufacture and is made from readily available standard components. Its alignment is not critical
Jupiter - friend or foe? II: the Centaurs
It has long been assumed that the planet Jupiter acts as a giant shield,
significantly lowering the impact rate of minor bodies upon the Earth, and thus
enabling the development and evolution of life in a collisional environment
which is not overly hostile. However, in the past, little work has been carried
out to examine the validity of this idea. In the second of a series of papers,
we examine the degree to which the impact risk resulting from objects on
Centaur-like orbits is affected by the presence of a giant planet, in a
continuing attempt to fully understand the impact regime under which life on
Earth has developed. The Centaurs, which occupy orbits beyond Jupiter, have
their origins in the Edgeworth-Kuiper belt that extends beyond Neptune. The
giant planets peturb the Centaurs, sending a significanr fraction into the
inner Solar System where they become visible as short-period comets. In this
work we present results which show that the presence of a giant planet can act
to significantly change the impact rate of short-period comets on the Earth,
and that a giant planet often actually increases the impact flux greatly over
that which would be expected were it not present. (Shortened version of
abstract.)Comment: 13 pages, 1 Figur
The Capture of Centaurs as Trojans
Large scale simulations of Centaurs have yielded vast amounts of data, the
analysis of which allows interesting but uncommon scenarios to be studied. One
such rare phenomenon is the temporary capture of Centaurs as Trojans of the
giant planets. Such captures are generally short (10 kyr to 100 kyr), but occur
with sufficient frequency (about 40 objects larger than 1 km in diameter every
Myr) that they may well contribute to the present-day populations. Uranus and
Neptune seem to have great difficulty capturing Centaurs into the 1:1
resonance, while Jupiter captures some, and Saturn the most (80 %). We
conjecture that such temporary capture from the Centaur population may be the
dominant delivery route into the Saturnian Trojans. Photometric studies of the
Jovian Trojans may reveal outliers with Centaur-like as opposed to asteroidal
characteristics, and these would be prime candidates for captured Centaurs.Comment: 5 pages, 2 figures, submitted to MNRAS (Letters
Planetary Trojans - the main source of short period comets?
We present a short review of the impact regime experienced by the terrestrial
planets within our own Solar system, describing the three populations of
potentially hazardous objects which move on orbits that take them through the
inner Solar system. Of these populations, the origins of two (the Near-Earth
Asteroids and the Long-Period Comets) are well understood, with members
originating in the Asteroid belt and Oort cloud, respectively. By contrast, the
source of the third population, the Short-Period Comets, is still under debate.
The proximate source of these objects is the Centaurs, a population of
dynamically unstable objects that pass perihelion between the orbits of Jupiter
and Neptune. However, a variety of different origins have been suggested for
the Centaur population. Here, we present evidence that at least a significant
fraction of the Centaur population can be sourced from the planetary Trojan
clouds, stable reservoirs of objects moving in 1:1 mean-motion resonance with
the giant planets (primarily Jupiter and Neptune). Focusing on simulations of
the Neptunian Trojan population, we show that an ongoing flux of objects should
be leaving that region to move on orbits within the Centaur population. With
conservative estimates of the flux from the Neptunian Trojan clouds, we show
that their contribution to that population could be of order ~3%, while more
realistic estimates suggest that the Neptune Trojans could even be the main
source of fresh Centaurs. We suggest that further observational work is needed
to constrain the contribution made by the Neptune Trojans to the ongoing flux
of material to the inner Solar system, and believe that future studies of the
habitability of exoplanetary systems should take care not to neglect the
contribution of resonant objects (such as planetary Trojans) to the impact flux
that could be experienced by potentially habitable worlds.Comment: 16 pages, 4 figures, published in the International Journal of
Astrobiology (the arXiv.org's abstract was shortened, but the original one
can be found in the manuscript file
(1173) Anchises - Thermophysical and Dynamical Studies of a Dynamically Unstable Jovian Trojan
We have performed detailed thermophysical and dynamical modelling of Jovian
Trojan (1173) Anchises. Our results reveal a most unusual object. By examining
observational data taken by IRAS, Akari and WISE between 11.5 and 60 microns,
along with variations in its optical lightcurve, we find Anchises is most
likely an elongated body, with an axes-ratio of ~1.4. This yields calculated
best-fit dimensions of 170x121x121km (an equivalent diameter of 136+18/-11km).
We find the observations are best fit by Anchises having a retrograde sense of
rotation, and an unusually high thermal inertia (25 to 100 Jm-2s-0.5K-1). The
geometric albedo is found to be 0.027 (+0.006/-0.007). Anchises therefore has
one of the highest published thermal inertias of any object larger than 100km
in diameter, at such large heliocentric distances, and is one of the lowest
albedo objects ever observed. More observations are needed to see if there is a
link between the very shallow phase curve, with almost no opposition effect,
and the derived thermal properties for this large Trojan asteroid. Our
dynamical investigation of Anchises' orbit has revealed it to be dynamically
unstable on timescales of hundreds of Myr, similar to the unstable Neptunian
Trojans 2001 QR322 and 2008 LC18. Unlike those objects, we find that Anchises'
dynamical stability is not a function of its initial orbital elements, the
result of the exceptional precision with which its orbit is known. This is the
first time that a Jovian Trojan has been shown to be dynamically unstable, and
adds weight to the idea that planetary Trojans represent a significant ongoing
contribution to the Centaur population, the parents of the short-period comets.
The observed instability does not rule out a primordial origin for Anchises,
but when taken in concert with the result of our thermophysical analysis,
suggest that it would be a fascinating target for future study.Comment: 5 figures, 3 tables, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
A Detailed Investigation of the Proposed NN Serpentis Planetary System
The post-main sequence eclipsing binary NN Serpentis was recently announced
as the potential host of at least two massive planetary companions. In that
work, the authors put forward two potential architectures that fit the
observations of the eclipsing binary with almost identical precision. In this
work, we present the results of a dynamical investigation of the orbital
stability of both proposed system architectures, finding that they are only
stable for scenarios in which the planets are locked in mutual mean motion
resonance. In the discovery work, the authors artificially fixed the orbital
eccentricity of the more massive planet, NN Ser(AB) c, at 0. Here, we reanalyse
the observational data on NN Serpentis without this artificial constraint, and
derive a new orbital solution for the two proposed planets. We detail the
results of further dynamical simulations investigating the stability of our new
orbital solution, and find that allowing a small non-zero eccentricity for the
outer planet renders the system unstable. We conclude that, although the
original orbits proposed for the NN Serpentis planetary system prove
dynamically feasible, further observations of the system are vital in order to
better constrain the system's true architecture.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society; 5 figures, 2 table
Mediation, arbitration and negotiation
We compare three common dispute resolution processes { negotiation, mediation, and arbitration { in the framework of Crawford and Sobel (1982). Under negotiation, the two parties engage in (possibly arbitrarily long) face-to-face cheap talk. Under mediation, the parties communicate with a neutral third party who makes a non-binding recommendation. Under arbitration, the two parties commit
to conform to the third party recommendation. We characterize and compare the optimal mediation and arbitration procedures. Both mediators and arbitrators should optimally filter information, but mediators should also add noise to it. We find that unmediated negotiation performs as well as mediation if and only if the degree of
conflict between the parties is low
Simulations of the Population of Centaurs II: Individual Objects
Detailed orbit integrations of clones of five Centaurs -- namely, 1996 AR20,
2060 Chiron, 1995 SN55, 2000 FZ53 and 2002 FY36 -- for durations of 3 Myr are
presented. One of our Centaur sample starts with perihelion initially under the
control of Jupiter (1996 AR20), two start under the control of Saturn (Chiron
and 1995 SN55) and one each starts under the control of Uranus (2000 FZ53) and
Neptune (2002 FY36) respectively. A variety of interesting pathways are
illustrated with detailed examples including: capture into the Jovian Trojans,
repeated bursts of short-period comet behaviour, capture into mean-motion
resonances with the giant planets and into Kozai resonances, as well as
traversals of the entire Solar system. For each of the Centaurs, we provide
statistics on the numbers (i) ejected, (ii) showing short-period comet
behaviour and (iii) becoming Earth and Mars crossing. For example, Chiron has
over 60 % of its clones becoming short-period objects, whilst 1995 SN55 has
over 35 %. Clones of these two Centaurs typically make numerous close
approaches to Jupiter. At the other extreme, 2000 FZ53 has roughly 2 % of its
clones becoming short-period objects. In our simulations, typically 20 % of the
clones which become short-period comets subsequently evolve into
Earth-crossers.Comment: 10 pages, in press at MNRA
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