How often do we get close passages?

[Tom Mazanec]

Gliese 710 is now expected to miss the solar system by 13,000 AU about the year 1,352,000 AD https://en.wikipedia.org/wiki/Gliese_710
How often does a star come this close to the sun, per billion years?

[antoniseb]

I'm going to guess about once every million years.
If you want a more precise number, tell us what the lower bound for what you'd call a star (15 Jupiter masses?), and do you mean exactly 13,000 AU as the threshold?
Once we have those numbers, the number you ask for should be easy to calculate.

[Tom Mazanec]

Let's cut it at the "classical" red dwarf limit (0.08 solar masses) and have it 13,000 AU or closer.

[kzb]

There was a list of close approaches published just a few years ago. I remember linking to it on here.

However, the closest approaches in that article were in the light-year range, I think there is one due in about 35,000 years or so.

I looked at the Wikipedia link where it says Gliese 710 will approach to 0.2 light years.

But the reference given on that wiki page does not say this as far as I can see. So where has this information come from?

It also says there is a more-than-zero probability of it approaching to 1000 AU !

Anyhow, this GL 710 approach is much closer than in the list I referred to.

I have also linked in the past a paper which gave estimates for close stellar encounter rates in different regions of the galaxy and in globular clusters.

The upshot was that stellar encounters in the central few kpc of the galaxy and in globular clusters are too frequent to allow complex life to evolve in those regions.

[kzb]

Gliese 710 is now expected to miss the solar system by 13,000 AU about the year 1,352,000 AD https://en.wikipedia.org/wiki/Gliese_710
How often does a star come this close to the sun, per billion years?

It's about once every 1 or 2 billion years.

t = 3.3E+07 years * (100 pc^-3/rho) * (mean relative speed of objects km/s) * (1000 AU/r) * (Msun/Mtotal)

rho is the stellar density (0.12/pc^-3 in our locality)

r is the encounter distance, in this case 13000 AU.

The last term is the ratio of the sun's mass to the sum of the masses of the two stars.

The one question I have is the mean relative speed of objects. I have assumed 10 km/s as an order of mag but others may know better.

[grant hutchison]

Twenty years ago, based on Hipparcos data, Sanchez et al. set a lower bound of 4.2 * D^2.02/Myr, where D is the distance of closest approach in parsecs. Since 13000AU = 0.063pc, that comes to 0.016/Myr = 16 per billion years. They expected more because Hip was incomplete for low mass stars.

Grant Hutchison

[George]

Twenty years ago, based on Hipparcos data, Sanchez et al. set a lower bound of 4.2 * D^2.02/Myr, where D is the distance of closest approach in parsecs. Since 13000AU = 0.063pc, that comes to 0.016/Myr = 16 per billion years. They expected more because Hip was incomplete for low mass stars.

Nice find, Grant. [That paper shows a slightly different equation (3.5D^2.12/Myr).] I assume the extrapolation to a billion years would be fairly speculative, but the best available.

[grant hutchison]

[That paper shows a slightly different equation (3.5D^2.12/Myr).]

That's interesting. The preprint in my file drawer has the equation I gave. They must have responded to reviewer feedback before publication. The revision takes it down to 10 per billion.

Grant Hutchison

[Tom Mazanec]

This is another star I look forward to seeing in the GAIA release coming up. See if the estimated distance gets any closer.

[kzb]

<deleted post>

[kzb]

Whichever equation you believe, it does seem that encounters similar to this are not uncommon.

Travel between stellar systems that are 0.2 light years apart is less difficult than systems 5 light years apart.

As I've said before on here, this is an argument against that proposed solution to the Fermi paradox, that interstellar travel is just too difficult.

[selden]

FWIW, "Scholz’s star" (WISE J072003.20-084651.2) probably came within 52,000 AU about 70,000 years ago.
See http://www.rochester.edu/newscenter/scholz-star/ and

The Closest Known Flyby of a Star to the Solar System
Eric E. Mamajek, et al.
https://arxiv.org/abs/1502.04655

That paper estimates passages within 0.25 pc happen at a rate of about 0.1/Myr.

[George]

The Closest Known Flyby of a Star to the Solar System
Eric E. Mamajek, et al.
https://arxiv.org/abs/1502.04655

That paper estimates passages within 0.25 pc happen at a rate of about 0.1/Myr.

I think that estimate comes from the same folks (Garcia-Sanchez et al) Grant references, but in an updated 2001 paper. [I failed to snag it.]

[selden]

I think that estimate comes from the same folks (Garcia-Sanchez et al) Grant references, but in an updated 2001 paper. [I failed to snag it.]

You're right. Sorry for my sloppy attribution.

[George]

You're right. Sorry for my sloppy attribution.

Just a nit.

But it's interesting that there seems to be a significant change in their earlier equation. I'm not that great at finding articles and I had no luck in my novice attempt to find this newer one. Perhaps someone will find this 2001 paper so we can update the equation.

[selden]

The full text of the published paper is behind The American Astronomical Society's paywall, which probably is why you couldn't find it by a Web search.

If you have access to a library which subscribes to the journal, you should be able to get it for free from them.

STELLAR ENCOUNTERS WITH THE OORT CLOUD BASED ON HIPPARCOS DATA
JOAN GARCI�A-SA�NCHEZ, et.al.

(c) 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.
Received 1998 May 15 ; accepted 1998 September 4

However, its abstract is available for free at http://adsabs.harvard.edu/abs/1999AJ....117.1042G and says

We find that the rate of close approaches by star systems (single or multiple stars) within a distance D (in parsecs) from the Sun is given by N = 3.5D^2.12 Myr^-1, less than the number predicted by a simple stellar dynamics model.

I also found a couple of copies of the preprint. This one has handwritten figure numbers, so it's probably the oldest: http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

They say

We find that the rate of close approaches by star systems (single or multiple stars) within a distance D (in parsecs) from t h e Sun is given by N = 4.2 D^2.02M yr^-1, less than the numbers predicted by simple stellar dynamics models.

Note that "numbers" and "models" were changed, too, from plural to singular.

[Tom Mazanec]

When will I find out the refined estimate from GAIA DR2? How long does it take for such data to result in a published prediction?

[selden]

That's entirely up to whoever is interested in doing the research and whatever other priorities they might have. You could try contacting one of the authors of the papers and ask about it.

[Tom Mazanec]

That's entirely up to whoever is interested in doing the research and whatever other priorities they might have. You could try contacting one of the authors of the papers and ask about it.

How would I go about doing that? Should I try to find their email or phone number? And how would I find that information?
What is the best way of doing this?

[selden]

All published scientific articles include the names and institutional affiliations of their authors.

The Web sites of most research and educational institutions include directories of everyone affiliated with them.

Another location method is to do a generic Web search specifying the author's name and the name of the institution.

[Jens]

And just to add, published papers will have at least one ‘corresponding author’ listed, with the email and address.


Sent from my iPhone using Tapatalk

[selden]

https://arxiv.org/pdf/1805.02644


An independent confirmation of the future flyby of Gliese 710 to the solar system using Gaia DR2

Authors: R. de la Fuente Marcos, C. de la Fuente Marcos

7-May-2018

[kzb]

https://arxiv.org/pdf/1805.02644


An independent confirmation of the future flyby of Gliese 710 to the solar system using Gaia DR2

Authors: R. de la Fuente Marcos, C. de la Fuente Marcos

7-May-2018

Thanks for posting that!

This paper finds the Gliese 710 approach will be even closer than the 13,000AU in the previous article. The central expectation is 10,721 +/- 2114 AU.

There is a small chance it could be as close as 4303 AU.

[Tom Mazanec]

Will future data Releases refine this further?

[selden]

Yes: future Gaia data releases are intended to have better accuracy in their measurements. Unfortunately, though, the next one (DR3) won't be published for another two years.

[Roger E. Moore]

The most recent paper on stellar flybys of the Solar System is below. It will be interesting to see a large-scale map of objects in the Oort Cloud (if we are ever so lucky) to see if "damage" to it from past flybys can be discerned.

Also, it occurs to me that there should be a lot of material from other stars' Oort Clouds out there in our own, a sort of interstellar mixing of materials. Perhaps we've had exocomets in the past made from different materials than our usual ones--heavier in metals, lighter in metals, different rock compositions, etc.

More to come, a likeable topic. Papers are dated from most recent to further past.

=========================================

http://adsabs.harvard.edu/abs/2018arXiv180702960P

Outer solar system possibly shaped by a stellar fly-by

Pfalzner, Susanne; Bhandare, Asmita; Vincke, Kirsten; Lacerda, Pedro
07/2018

The planets of our solar system formed from a gas-dust disk. However, there are some properties of the solar system that are peculiar in this context. First, the cumulative mass of all objects beyond Neptune (TNOs) is only a fraction of what one would expect. Second, unlike the planets themselves, the TNOs do not orbit on coplanar, circular orbits around the Sun, but move mostly on inclined, eccentric orbits and are distributed in a complex way. This implies that some process restructured the outer solar system after its formation. However, some of TNOs, referred to as Sednoids, move outside the zone of influence of the planets. Thus external forces must have played an important part in the restructuring of the outer solar system. The study presented here shows that a close fly-by of a neighbouring star can simultaneously lead to the observed lower mass density outside 30 AU and excite the TNOs onto eccentric, inclined orbits, including the family of Sednoids. In the past it was estimated that such close fly-bys are rare during the relevant development stage. However, our numerical simulations show that such a scenario is much more likely than previously anticipated. A fly-by also naturally explains the puzzling fact that Neptune has a higher mass than Uranus. Our simulations suggest that many additional Sednoids at high inclinations still await discovery, perhaps including bodies like the postulated planet X.

[Roger E. Moore]

Gliese 710 gets the lion's share of reporting on flyby stars. The reporting comes in spikes, starting around 1997, with smaller spikes in academic output since.

==================================

http://adsabs.harvard.edu/abs/2018RNAAS...2b..30D

An Independent Confirmation of the Future Flyby of Gliese 710 to the Solar System Using Gaia DR2

de la Fuente Marcos, Raúl; de la Fuente Marcos, Carlos
05/2018

Gliese 710 is a K7V star located 19 pc from the Sun in the constellation of Serpens Cauda, which is headed straight for the solar system. Berski & Dybczynski (2016) used data from Gaia DR1 to show that this star will be 13366 AU from the Sun in 1.35 Myr from now. Here, we present an independent confirmation of this remarkable result using Gaia DR2. Our approach is first validated using as test case that of the closest known stellar flyby, by the binary WISE J072003.20-084651.2 or Scholz's star. Our results confirm, within errors, those in Berski & Dybczynski (2016), but suggest a somewhat closer, both in terms of distance and time, flyby of Gliese 710 to the solar system. Such an interaction might not significantly affect the region inside 40 au as the gravitational coupling among the known planets against external perturbation can absorb efficiently such a perturbation, but it may trigger a major comet shower that will affect the inner solar system.

==========================================

http://adsabs.harvard.edu/abs/2017MNRAS.472.4634K

Oort spike comets with large perihelion distances

Królikowska, Malgorzata; Dybczynski, Piotr A.
12/2017

The complete sample of large-perihelion nearly-parabolic comets discovered during the period 1901-2010 is studied, starting with their orbit determination. Next, an orbital evolution that includes three perihelion passages (previous-observed-next) is investigated in which a full model of Galactic perturbations and perturbations from passing stars is incorporated. We show that the distribution of planetary perturbations suffered by actual large-perihelion comets during their passage through the Solar system has a deep, unexpected minimum around zero, which indicates a lack of 'almost unperturbed' comets. Using a series of simulations we show that this deep well is moderately resistant to some diffusion of the orbital elements of the analysed comets. It seems reasonable to assert that the observed stream of these large-perihelion comets experienced a series of specific planetary configurations when passing through the planetary zone. An analysis of the past dynamics of these comets clearly shows that dynamically new comets can appear only when their original semimajor axes are greater than 20 000 au. On the other hand, dynamically old comets are completely absent for semimajor axes longer than 40 000 au. We demonstrate that the observed 1/aori-distribution exhibits a local minimum separating dynamically new from dynamically old comets. Long-term dynamical studies reveal a wide variety of orbital behaviour. Several interesting examples of the action of passing stars are also described, in particular the impact of Gliese 710, which will pass close to the Sun in the future. However, none of the obtained stellar perturbations is sufficient to change the dynamical status of the analysed comets.

==========================================

http://adsabs.harvard.edu/abs/2016A%26A...595L..10B

Gliese 710 will pass the Sun even closer. Close approach parameters recalculated based on the first Gaia data release

Berski, Filip; Dybczynski, Piotr A.
11/2016

Context. First results based on Gaia data show that the well-known star Gliese 710 will be the closest flyby star in the next several Myrs and its minimum distance from the Sun will be almost five times smaller than that suggested by pre-Gaia solution.
Aims: The aim of this work is to investigate the proximity parameters and the influence of the close approach of Gliese 710 on the basis of Gaia DR1. Furthermore, we compare new results with previous works based on HIP2 and Tycho 2 catalogues to demonstrate how Gaia improves the accuracy of determination of such phenomena.
Methods: Using a numerical integration in an axisymmetric Galactic model, we determine new parameters of the close encounter for Gliese 710. Adding ten thousand clones drawn with the use of a covariance matrix, we estimate the most probable position and velocity of this star at the minimum distance from the Sun.
Results: Our calculations show that Gliese 710 will pass 13365 AU from the Sun in 1.35 Myr from now. At this proximity it will have the brightness of -2.7 mag and a total proper motion of 52.28 arcsec per year. After the passage of Gliese 710 we will observe a large flux of new long-period comets. Thanks to the Gaia mission, the uncertainties of the minimum distance and time of the close approach are several times smaller than suggested by previous works based on data from earlier observations.

==========================================

http://adsabs.harvard.edu/abs/2015MNRAS.454.3267F

Finding the imprints of stellar encounters in long-period comets

Feng, Fabo; Bailer-Jones, C. A. L.
12/2015

The Solar system's Oort cloud can be perturbed by the Galactic tide and by individual passing stars. These perturbations can inject Oort cloud objects into the inner parts of the Solar system, where they may be observed as the long-period comets (periods longer than 200 yr). Using dynamical simulations of the Oort cloud under the perturbing effects of the tide and 61 known stellar encounters, we investigate the link between long-period comets and encounters. We find that past encounters were responsible for injecting at least 5 per cent of the currently known long-period comets. This is a lower limit due to the incompleteness of known encounters. Although the Galactic tide seems to play the dominant role in producing the observed long-period comets, the non-uniform longitude distribution of the cometary perihelia suggests the existence of strong - but as yet unidentified - stellar encounters or other impulses. The strongest individual future and past encounters are probably HIP 89825 (Gliese 710) and HIP 14473, which contribute at most 8 and 6 per cent to the total flux of long-period comets, respectively. Our results show that the strength of an encounter can be approximated well by a simple proxy, which will be convenient for quickly identifying significant encounters in large data sets. Our analysis also indicates a smaller population of the Oort cloud than is usually assumed, which would bring the mass of the solar nebula into line with planet formation theories.

[Roger E. Moore]

http://adsabs.harvard.edu/abs/2015MNRAS.448..588D

Near-parabolic comets observed in 2006-2010 - II. Their past and future motion under the influence of the Galaxy field and known nearby stars

Dybczynski, Piotr A.; Królikowska, Malgorzata
03/2015

In the first part of this research we extensively investigated and carefully determined osculating, original (when entering Solar system) and future (when leaving it), orbits of 22 near-parabolic comets with small perihelion distance (qosc < 3.1 au), discovered in years 2006-2010. Here, we continue this research with a detailed study of their past and future motion during previous and next orbital periods under the perturbing action of our Galactic environment. At all stages of our dynamical study, we precisely propagate in time the observational uncertainties of cometary orbits. For the first time in our calculations, we fully take into account individual perturbations from all known stars or stellar systems that closely (less than 3.5 pc) approach the Sun during the cometary motion in the investigated time interval of several million years. This is done by means of a direct numerical integration of the N-body system comprising of a comet, the Sun and 90 potential stellar perturbers. We show a full review of various examples of individual stellar action on cometary motion. We conclude that perturbations from all known stars or stellar systems do not change the overall picture of the past orbit evolution of long-period comets. Their future motion might be seriously perturbed during the predicted close approach of Gliese 710 star but we do not observe significant energy changes. The importance of stellar perturbations is tested on the whole sample of 108 comets investigated by us so far and our previous results, obtained with only Galactic perturbations included, are fully confirmed. We present how our results can be used to discriminate between dynamically new and old near-parabolic comets and discuss the relevance of the so-called Jupiter-Saturn barrier phenomenon. Finally, we show how the Oort spike in the 1/a-distribution of near-parabolic comets is built from both dynamically new and old comets. We also point out that C/2007 W1 seems to be the first serious candidate for interstellar provenance.

=========================================

http://adsabs.harvard.edu/abs/2015A%26A...575A..35B

Close encounters of the stellar kind

Bailer-Jones, C. A. L.
03/2015

Stars which pass close to the Sun can perturb the Oort cloud, injecting comets into the inner solar system where they may collide with the Earth. Using van Leeuwen's re-reduction of the Hipparcos data complemented by the original Hipparcos and Tycho-2 catalogues, along with recent radial velocity surveys, I integrate the orbits of over 50 000 stars through the Galaxy to look for close encounters. The search uses a Monte Carlo sampling of the covariance of the data in order to properly characterize the uncertainties in the times, distances, and speeds of the encounters. I show that modelling stellar encounters by assuming instead a linear relative motion produces, for many encounters, inaccurate and biased results. I find 42, 14, and 4 stars which have encounter distances below 2, 1, and 0.5 pc respectively, although some of these stars have questionable data. Of the 14 stars coming within 1 pc, 5 were found by at least one of three previous studies (which found a total of 7 coming within 1 pc). The closest encounter appears to be Hip 85605, a K or M star, which has a 90% probability of coming between 0.04 and 0.20 pc between 240 and 470 kyr from now (90% Bayesian confidence interval). However, its astrometry may be incorrect, in which case the closest encounter found is the K7 dwarf GL 710, which has a 90% probability of coming within 0.10-0.44 pc in about 1.3 Myr. A larger perturbation may have been caused by gamma Microscopii, a G6 giant with a mass of about 2.5 M&sun;, which came within 0.35-1.34 pc (90% confidence interval) around 3.8 Myr ago.

==========================================

http://adsabs.harvard.edu/abs/2011MNRAS.418.1272J

Effect of different stellar galactic environments on planetary discs - I. The solar neighbourhood and the birth cloud of the Sun

Jiménez-Torres, Juan J.; Pichardo, Barbara; Lake, George; Throop, Henry
12/2011

We have computed trajectories, distances and times of closest approaches to the Sun by stars in the solar neighbourhood with known position, radial velocity and proper motions. For this purpose, we have used a full potential model of the Galaxy that reproduces the local z-force, the Oort constants, the local escape velocity and the rotation curve of the Galaxy. From our sample, we constructed initial conditions, within observational uncertainties, with a Monte Carlo scheme for the 12 most suspicious candidates because of their small tangential motion. We find that the star Gliese 710 will have the closest approach to the Sun, with a distance of approximately 0.34 pc in 1.36 Myr in the future. We show that the effect of a flyby with the characteristics of Gliese 710 on a 100 au test particle disc representing the Solar system is negligible. However, since there is a lack of 6D data for a large percentage of stars in the solar neighbourhood, closer approaches may exist. We calculate parameters of passing stars that would cause notable effects on the solar disc. Regarding the birth cloud of the Sun, we performed experiments to reproduce roughly the observed orbital parameters such as eccentricities and inclinations of the Kuiper belt. It is now known that in Galactic environments, such as stellar formation regions, the stellar densities of new born stars are high enough to produce close encounters within 200 au. Moreover, in these Galactic environments, the velocity dispersion is relatively low, typically sigma˜ 1-3 km s-1. We find that with a velocity dispersion of ˜1 km s-1 and an approach distance of about 150 au, typical of these regions, we obtain approximately the eccentricities and inclinations seen in the current Solar system. Simple analytical calculations of stellar encounters effects on the Oort Cloud are presented.

==========================================

http://adsabs.harvard.edu/abs/2011AN....332..831S

The error ellipsoid in phase space of stellar encounters and its evolution in time

Serafin, R. A.; Birkenbach, B.
10/2011

We systematically investigate stellar encounters in their six-dimensional phase space, including also the error ellipsoid and its evolution in time. It allows to give not only a mathematical model of stellar encounters but also an error model for this process. On that occasion we derive fundamental formulae for the positional error in {R}n: the probability of a position X falling into the error ellipsoid, the standard deviation in an arbitrary direction, and we briefly discuss the mean positional error in {R}n. The case of the close encounter of the star GL 710 (HIP 89825) is an illuminating example of applying the derived results. Moreover, we show that the error ellipsoid can also be successfully applied to approximate the confidence region.

[Roger E. Moore]

http://adsabs.harvard.edu/abs/2010AstL...36..220B

Searching for stars closely encountering with the solar system

Bobylev, V. V.
03/2010

Based on a new version of the Hipparcos catalog and currently available radial velocity data, we have searched for stars that either have encountered or will encounter the solar neighborhood within less than 3 pc in the time interval from -2 Myr to +2 Myr. Nine new candidates within 30 pc of the Sun have been found. To construct the stellar orbits relative to the solar orbit, we have used the epicyclic approximation. We show that, given the errors in the observational data, the probability that the well-known star HIP 89 825 (GL 710) encountering with the Sun most closely falls into the Oort cloud is 0.86 in the time interval 1.45 ± 0.06 Myr. This star also has a nonzero probability, 1 × 10-4, of falling into the region d < 1000 AU, where its influence on Kuiper Belt objects becomes possible.

============================================

http://adsabs.harvard.edu/abs/2002Icar..157..228M

Characteristics and Frequency of Weak Stellar Impulses of the Oort Cloud

Matese, John J.; Lissauer, Jack J.
05/2002

We have developed a model of the response of the outer Oort cloud of comets to simultaneous tidal perturbations of the adiabatic galactic force and a stellar impulse. The six-dimensional phase space of near-parabolic comet orbital elements has been subdivided into cells. A mapping of the evolution of these elements from beyond the loss cylinder boundary into the inner planetary region over the course of a single orbit is possible. This is done by treating each perturbation separately, and in combination, during a time interval of 5 Myr. We then obtain the time dependence of a wide range of observable comet flux characteristics, which provides a fingerprint of the dynamics. These include the flux distributions of energy, perihelion distance, major axis orientation, and angular momentum orientation. Correlations between these variables are also determined. We show that substantive errors occur if one superposes the separately obtained flux results of the galactic tide and the stellar impulse rather than superposing the tidal and impulsive perturbations in a single analysis. Detailed illustrations are given for an example case where the stellar mass and relative velocity have the ratio M*/ Vrel=0.043 M&sun;/km s -1 and the solar impact parameter is 45,000 AU. This case has features similar to the impending Gliese 710 impulse with the impact parameter selected to be close to the low end of the predicted range. We find that the peak in the observable comet flux exceeds that due to the galactic tide alone by ≈41%. We also present results for the time dependence of the flux enhancements and for the mean encounter frequency of weak stellar impulse events as functions of M*/ Vrel and solar impact parameter.

============================================

http://adsabs.harvard.edu/abs/1999AJ....117.1042G

Stellar Encounters with the Oort Cloud Based on HIPPARCOS Data [ Erratum: 1999AJ....118..600G ]

García-Sánchez, Joan; Preston, Robert A.; Jones, Dayton L.; Weissman, Paul R.; Lestrade, Jean-François; Latham, David W.; Stefanik, Robert P.
02/1999

We have combined Hipparcos proper-motion and parallax data for nearby stars with ground-based radial velocity measurements to find stars that may have passed (or will pass) close enough to the Sun to perturb the Oort cloud. Close stellar encounters could deflect large numbers of comets into the inner solar system, which would increase the impact hazard at Earth. We find that the rate of close approaches by star systems (single or multiple stars) within a distance D (in parsecs) from the Sun is given by N= 3.5D^2.12 Myr^-1, less than the number predicted by a simple stellar dynamics model. However, this value is clearly a lower limit because of observational incompleteness in the Hipparcos data set. One star, Gliese 710, is estimated to have a closest approach of less than 0.4 pc 1.4 Myr in the future, and several stars come within 1 pc during a +/-10 Myr interval. We have performed dynamical simulations that show that none of the passing stars perturb the Oort cloud sufficiently to create a substantial increase in the long-period comet flux at Earth's orbit.

=================================================

http://adsabs.harvard.edu/abs/1997AAS...191.6906M

Close Approaches of Stars to the Oort Cloud: Algol and Gliese 710

Molnar, L. A.; Mutel, R. L.
12/1997

The gravitational impulse of close approaches of stars to the Oort comet cloud are thought to be responsible for randomizing the orientations and eccentricities of their orbits. Particularly close encounters or combinations of more distant encounters can affect the distribution of comets observed at Earth. We present calculations of the recent encounter with Algol and the future encounter with Gliese 710. The Algol computation uses a new measurement of the proper motion made with Very Long Baseline Interferometry, which has uncertainties six times less than the Hipparcos measurements. We discuss in detail the various sources of uncertainty. We find that including the galactic rotation and oscillation about the midplane is essential to accurately compute not only the distance of closest encounter, but also which side of the Sun the stars will pass. Finally, we discuss the impact of the Algol passage on the observed distribution of comet orbits.

[Roger E. Moore]

http://adsabs.harvard.edu/abs/1997ESASP.402..617G

A Search for Stars Passing Close to the Sun

Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Weissman, P. R.; Lestrade, J.-F.; Latham, D. W.; Stefanik, R. P.
08/1997

We have combined Hipparcos proper motion and parallax data for nearby stars with ground-based radial velocity measurements to find stars which may have passed (or will pass) close enough to the Sun to perturb the Oort cloud. Close stellar encounters could deflect large numbers of comets into the inner solar system, with possibly serious consequences for the impact hazard on the Earth. Only one star (Gliese 710) is found with a predicted closest approach of less than 0.5 parsec, although several stars come within about 1 parsec during a +/- 8.5 Myr interval. In most cases the uncertainty in closest approach distance is dominated either by uncertainties in published radial velocity measurements or by uncertainties in the barycentric motion of binary systems. We have begun a program to obtain new radial velocities for stars in our sample with no previously published values.

================================================== ==

http://adsabs.harvard.edu/abs/1997DPS....29.2501W

Close Approaches of Stars to the Solar System

Weissman, P. R.; Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Latham, D. W.
07/1997

We have combined Hipparcos proper motion and parallax data for nearby stars with ground-based radial velocity measurements to find stars which may have passed (or will pass) close enough to the Sun to perturb the Oort cloud. Close stellar encounters could deflect large numbers of comets into the planetary region and raise impact rates on the planets and their satellites, with possible consequences for biological evolution on Earth. From the data analyzed to date, we find that the number N of close stellar approaches within a distance D from the Sun (measured in parsecs) is given by N ~ 5 D(2) Myr(-1) , in agreement with previously predicted values (Weissman, 1980 Nature 288, 242). Only one star, Gliese 710, is found with a predicted closest approach distance < 10(5) AU (0.5 parsecs), although several stars come within about 1 parsec during a +/-8.5 Myr interval. The predicted minimum distance for Gliese 710 is 53,000 to 71,000 AU, approximately 1.0 to 1.4 Myr in the future. Gliese 710 is a late-type dwarf star (dM1 or K7 V) with an estimated mass of 0.42 M{_sun}, and is currently about 19 parsecs from the Sun. The star may be a binary. The absence of close stellar approaches in the recent past is consistent with analyses of the orbital element distributions of long-period comets by Weissman (1993 BAAS 25, 1063) which determined that we are not currently in a cometary shower. The expected dynamical effects of the closest encounters on the Oort cloud will be discussed. In most cases the uncertainty in closest approach distance is dominated either by uncertainties in published radial velocity measurements or by uncertainties in the barycentric motion of binary systems. We have begun a program to obtain radial velocities for stars in our sample with no previously published values.

================================================== ==

http://adsabs.harvard.edu/abs/1997xmm..pres...14.

The impact of Hipparcos star-fixing extends to life's evolution

05/1997

"ESA's Hipparcos brings the greatest step forward in star measurements since Tycho Brahe," Bonnet says. "When the Danish astronomer died in 1601, the German astronomer Johannes Kepler inherited his careful observations. Kepler used them to discover the laws of the motions of planets, and paved the way for Isaac Newton's gravitational theory. Now we have another multinational success story from European astronomy".

"Hipparcos began as an imaginative French concept to chart the stars by satellite," Bonnet continues. "ESA adopted the idea and many astronomers in our member states collaborated in the mission. A hundred-fold improvement in the accuracy of star positions may already alter the size of the Universe and the ages of stars. So don't be surprised if the results from Hipparcos are as revolutionary as Tycho Brahe's, in their impact on our knowledge of the cosmos."

The study of the Earth itself will benefit from the new star data. Wobbles of the Earth and variations in its rate of rotation can now be measured far more accurately. The ozone layer will be monitored by ESA's Envisat environmental mission, by looking for chemical alterations in the light from 1000 Hipparcos stars, when seen on lines of sight slanting through the atmosphere.

Even the erratic evolution of life on Earth may make more sense, as Hipparcos picks out stars that passed close enough to cause trouble here. Reliable identifications of stars heading towards or away from our vicinity were impossible before Hipparcos. The satellite measured shifts in the directions of stars in the sky with such high precision that astronomers can now pick out those few stars that scarcely change their bearings. Such stars are probably moving almost directly towards or away from us. A US-European team, led by Robert Preston at the Jet Propulsion Laboratory in California, used Hipparcos to search for nearby stars with very small shifts in position. They were, or will be, passers-by.

Gliese 710, an inconspicuous star in the constellation Ophiuchus, is currently 63 light-years away and approaching at about 14 kilometres per second. From the Hipparcos data, it will pass within about one light-year, one million years from now. Joan Garcia-Sanchez, a doctoral student in Preston's team, identifies Gliese 710 in one of the scientific posters that display Hipparcos results in Venice. Garcia-Sanchez has found evidence that Gliese 710 is today moving more slowly towards the Sun than it was several decades ago. That may mean it is orbiting around another star, so far unidentified. If so, the closest distance to which Gliese 710 will approach may be nearer or farther than in the team's initial estimate.

The stars of the Alpha Centauri system, at 4 light-years, are the nearest at present. Several stars investigated by Preston and his colleagues will come within 3 light-years during the next 8,500,000 years. Others have already passed by during a similar time-span and are now travelling away from us.

"A star coming too near could put the Earth at risk," Bob Preston explains. "It might dislodge comets from a swarm that surrounds the Sun in the Oort Cloud, and send them into the inner Solar System. Some comets could then collide with our planet. The fossils tell us of past disasters, in extinctions of many species, and we hope to identify culprits among stars now hurrying away from the scene. The theory isn't new, but only now can we check it, thanks to the amazing precision of Hipparcos."

Uncertainty about the timing of the stellar visits arises from inadequate information about the speed of approach or recession. That is measured from ground-based observatories, by shifts in the wavelengths of light (blueshifts and redshifts). A team led by Dave Latham at the Center for Astrophysics in Cambridge, Massachusetts, is busy making fresh observations to improve the ground-based data on the visitors, past and future.


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http://adsabs.harvard.edu/abs/1997IAUJD..14E..51G

A Search for Stars Passing Close to the Sun

Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Weissman, P. R.; Latham, D. W.
00/1997

We have combined HIPPARCOS proper motion and parallax data for nearby stars with ground-based radial velocity measurements to find stars which may have passed (or will pass) close enough to the Sun to disrupt the Oort cloud. Such close encounters could deflect large numbers of comets into the inner solar system, with possibly serious consequences for biological evolution. From the data analyzed to date, we find the number N of close stellar approaches within a distance D from the Sun (in pc) is given by N ~5 {D^2} {{Myr}^{-1}}, in good agreement with previously predicted values. Only one star (Gliese 710) is found with a predicted closest distance of less than 0.5 parsec, although several stars come within about 1 parsec during a +/- 8.5 Myr interval. In most cases the uncertainty in closest approach distance is dominated either by uncertainties in published radial velocity measurements or by uncertainties in the barycentric motion of binary systems. We have started a program to obtain new radial velocities for stars in our sample with no previously published values.