The Astroprof’s Page takes a look at the interesting star Gliese 710, a K7 dwarf with a particular claim to distinction: it’s headed in the direction of our Sun at about 24 kilometers per second. Give it 1.4 million years and the star will have closed to within a light year of Sol, shining at a magnitude of 1.2 and disturbing the icy debris out in the Oort Cloud. A rain of comets moving into the inner system is the probable result.
Barnard’s Star is moving towards us too, closing to within four light years around 10000 AD, but we needn’t wait for a close stellar pass to start worrying about catastrophic collisions. As the battered surface of the Moon suggests, the Solar System can be a hostile place, making a space-based infrastructure to prevent future disaster an imperative.
On human timescales it does not look like a rain of comets.
García-Sánchez, et al studied this and other close pass candidates in the Hipparcos data:
http://www.journals.uchicago.edu/AJ/journal/issues/v117n2/980216/980216.html
(I also copied the URL in the ‘website’ field.)
From the article: “For the future passage of Gl 710, the star with the closest approach in our sample, we predict that about 2.4 × 106 new comets will be thrown into Earth-crossing orbits, arriving over a period of about 2 × 106 yr. Many of these comets will return repeatedly to the planetary system, though about one-half will be ejected on the first passage. These comets represent an approximately 50% increase in the flux of long-period comets crossing Earth’s orbit.”
The increase of the impact rate is much lower, because most impactors are short period comets and asteroids.
Hmm, sorry. SHould have checked other’s comments before. Now it looks, like I’m the owner of that page. I’m not.
This is quite an interesting paper, Hans. Thanks for the link. My comment about a ‘rain of comets’ from Gliese 710 certainly looks questionable in this light. From the paper: “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.” Nice catch!
We must also realize that (maybe most) part of the current comet flux is the result of similar events in the past. We’re not at level zero.
Maybe we need a star passing every few hundred thousand years to keep fresh comets coming.
What I miss in both articles is that (probably) a complete solar system (instead of just a star) will encounter our solar system. Maybe it’s irrelevant, because a solar system is mostly empty space?
I’d think that when 2 Oort clouds + planetary systems intersect there will be more perturbation than from the gravity of the star alone. Or maybe the mass of the other bodies is too low to have any effect with the high relative velocity and hence short perturbation time?
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Centauri-dreams question: could our descendants use such a passing star as a slingshot, much like the planet assisted boosts often used by solar system probles?
The relative velocity of these stars is quite low, so there seems to be no utility to using them as a slingshot.
Question: We have a couple of stars that we know will get close within thousands or a million or two years. Surely we have measured recessional velocities of stars that similarly passed within ~1 ly or so in the past million years or so. Are any known?
1.4 million years. I’m willing to wait.
Hi Hans & Ron
Stars aren’t all slow pokes. Barnard’s star’s relative velocity is a hefty ~ 150 km/s or so. Some hyper-velocity stars are edging towards 1,000 km/s. So if you want to get a boost for slow interstellar probes there are a few options. The ultimate practical boost would be from binary white-dwarfs which can boost a vehicle to about 3,000 km/s. Theoretically binary neutron-stars and black-holes could boost vehicles to relativistic speeds BUT such rapidly rotating systems put out huge amounts of gravity waves and have horribly strong tidal forces.
As for stellar close-approaches the Garcia-Sanchez article which mentions Gliese 710 is available free online…
http://www.journals.uchicago.edu/AJ/journal/issues/v117n2/980216/980216.html
…courtesy of NASA’s ADS and U.Chicago’s journals.
Adam,
I was aware of those high-velocity stars when I made my comment. I was thinking more along the lines of what would be practically accessible in our vicinity (time and distance). To reach those interesting ‘boosters’ would require relativistic velocities just to get there in anything less than millions of years. Once you get relativistic to bring the travel time down to thousands of years those options are less useful.
Once you get over, say 0.01c (3,000 km/s), even Barnard’s Star has no value as a slingshot. A close fly-by would barely alter the vehicle’s direction and would have even less affect on the magnitude of the velocity.
I’d also be more inclined to hang around Barnard’s Star anyway rather than flip by in a few minutes – at least the first time we get there!
Are there any tightly-orbiting white or red dwarfs nearby? Maybe even a red dwarf with a ‘hot’ Jupiter could work.
Gentlemen:
Skipping Gliese 710’s effect on our Oort Cloud, how about the sun’s effect on Gliese 710’s Oort Cloud? Granted we do not know that it has one, but if such debris collections are normal to star formation, one would think the solar system could pass right through that one.
John, if the closest approach is 1 ly (as described) I’d expect little impact either way. A quick calculation shows 1 ly = 62,000 AU. Those Oort clouds are no where near that far out from the primary.
A second, statistical argument might go as follows. We’ve been here some 4.5 billion years, and there have likely been a number of close encounters with other systems. Yet our Oort cloud still seems fairly well populated.
If a slow space craft were aimed at the star sirius at say 300 km/sec it would
take 8620 years to get there , but once it arrived it could use the gravity of
sirius b possibly to booost its velocity to perhaps 3000 km/sec by gravitational sling shot.
tim
Boost to 3,000 km/s using Sirius B? Did you check the relative velocity of the Sirius system, and even the orbital velocities of A and B? I wish it could be done, but no way. At a craft velocity of 300 km/s, Sirius appears to have no utility as a slingshot. My quick calc based on Sirius’ proper motion and radial velocity shows something like 10 km/s relative motion to us, give or take a few km/s.
The velocity boost would come from the intense gravitation of Sirius B
not from the relative velocity of Sirius A it self.
Hmm, looks like I was wrong about my Mar 8 14:38 comment. The Oort cloud could extend much further than I thought. I realized this when I read the following note in Scientific American:
http://www.sciam.com/askexpert_question.cfm?chanID=sa005&articleID=000BDB70-8C70-1CD1-B4A8809EC588EEDF
This means the Oort cloud is severely modified by passing stars. I thought it worth pointing out my error in case anyone was misled.
Hi Ron S.
You were right about the difficulty of getting a boost from Sirius B. Tim is possibly confusing a binary white dwarf system with a binary with a white dwarf. The former can be used as a ‘gravity machine’ for very high speed boosts, but compact objects like Sirius B can be used for more modest boosts – a 10 km/s burn at periapsis at the 3,000 km/s level can boost the vehicle by 88 km/s if its hyperbolic speed is 300 km/s already.
Extremal white dwarfs can be even more compact. The mass-radius relationship varies with the inverse cube-root of the mass until the mass is very close to the Chandrasekhar limit of 1.44 solar masses, then the radius gets really small, as though the star was on the verge of becoming a neutron star – which it is. But the internal structure of such a white dwarf is very dependent on nuclear physics that is still being explored, full of uncertainty. It might be unstable, liable to collapse.
A binary neutron star would be an incredible natural accelerator, but the tidal forces and gravity waves would make it hazardous to anything but very strong nanoprobes. It would also be on the verge of collapse into a GRB.
Hi All
To give some idea of the difficulty a neutron star poses – a 1 solar mass white dwarf is about 7,160 km in radius, while a 1 solar mass neutron star is about 16 km in radius. Tidal forces increase with the inverse cube of the separation, thus for a low pass over the white-dwarf the lateral (compressive) tidal acceleration is just 0.36 m/s^2, but for the neutron star it’s (7160/16)^3 times that or 32,400,000 m.s^2 (3.3 million gees.)
Quite close approaches might be allowed if we can withstand say 1,000 gees – though I’ve no idea just how to protect against the stretching, radial forces – thus we might get to a periapsis of 238 km around that same neutron star, gaining a boost to about 0.066 c. For a nanoprobe made of diamondoid a speed-boost to 0.25 c is physically possible.
Solar-mass black-holes would allow even higher boosts, but a periapsis within 1.5 times the Schwarzschild radius is within the photon sphere and would be non-Newtonian in overall orbital motion. The gravity waves from such a binary arrangement would be powerful and rather nasty so close.