First Interstellar Asteroid Found

[CygnusX-1]

The discovery of the interstellar asteroid is now on the radio: Listen free at The John Batchelor Show.

 

[Bandersnatch]

What does one expect? I'd expect ejected bodies to be going a little faster (relative to the LSR) than the bodies they were ejected from. So maybe 30 km/s would be most probable. Now, 8 km/s is not an impossible number, but it is unusual - phase space considerations (an idealization - maybe even an oversimplification - to be sure) would suggest maybe 1 in 10 or so such objects would be going that fast or slower.

So 1km/s would be more puzzling. Not impossible, but slower than ~99.9% of the expected objects. 15 km/s would be less puzzling - maybe a quarter of the objects would be that slow or slower. 1000 km/s would be very surprising, as it is faster than the galaxy's escape velocity.

First, there seems to be a matter of language use: when you say puzzling, it implies that the cause is somehow unknown, perplexing, completely unexpected. As in, 'how in the world did it get to have that velocity?' kind of puzzling.
But from your post it looks like what you meant is that it is just an outlier in some distribution. I was objecting to the former (all of those velocities can be produced by known interactions, hence they're not puzzling), not to the latter.

But let's look at whether 8 km/s w/r to LSR is really an outlier. There are two things to take into consideration here:
1.
Ejection events are more likely to be caused by gradual (over many orbits) changes to orbital angular momentum of an asteroid whose orbit is coupled to some massive planet, rather than a one-off close encounter resulting in large delta V - simply because there will always be more asteroids affected at long range than in close encounters.
Since the most likely ejection is by incremental boosting of orbital momentum, the velocities of ejecta should be clustered around the escape velocity, i.e. one would expect rogue asteroids to have velocities close to the peculiar velocity of their parent star, with similar distribution.

2.
Peculiar velocities of stars w/r to LSR are, by definition, directed every which way. An asteroid ejected with some velocity in a random direction w/r to its parent system will then have its velocity w/r to LSR be a nett result of the two.
E.g. even in a fantasy scenario where all stars have peculiar velocities equal to 20 km/s, and all asteroids are ejected with 20 km/s over escape velocity, the expected LSR velocity of an asteroid would be anywhere in the range of [0, 40] km/s.
That is, one doesn't even need a slow-moving star, nor slow ejection, to get a slow-moving rogue asteroid.

This is the velocity distribution in the galactic plane of some 20 thousand stars in the solar neighbourhood:

Where the first dark contour contains approx 50% of stars. Vx is radial, Vy is in the direction or galactic rotation. The triangle is LSR.
(Taken from: https://arxiv.org/abs/0912.3262, fig 1. This earlier paper contains contours for plane-normal direction as well: https://arxiv.org/abs/0905.2980 - they're narrower, but similarly clustered around 0 km/s w/r to LSR)

So, taking into account both points mentioned above, as well as the velocity distribution of nearby stars, I think the 8 km/s is not only within the expected range (i.e. not-puzzling), but also in the most likely range.

250 would NOT be puzzling, because that happens to be the peculiar speed of Kapteyn´s star, for example.
The lecture presentation gives velocity dispersion of 9 km/s for A stars, compared to 20 of Sun.

The dispersion you mentioned is for plane-normal velocities only. It is not the nett peculiar velocity which you compare it with. The plane-normal velocity of the Sun is approx. +7 km/s.

Same thing as with Vanadium's post, if by 'not puzzling' you mean 'likely', then 250 km/s would be unlikely, as velocities like that of Kapteyn's star are strong outliers.

 

[Vanadium 50]

Yes, by "puzzling" I mean "an outlyer". An it's not a yes or no thing. A 1 in 2 outlyer doesn't bother me at all. A 1 in 10 bothers me some, 1 in 100 more, and so on.

E.g. even in a fantasy scenario where all stars have peculiar velocities equal to 20 km/s, and all asteroids are ejected with 20 km/s over escape velocity, the expected LSR velocity of an asteroid would be anywhere in the range of [0, 40] km/s.

Yes, but in this fantasy I would expect the distribution to peak at ~30 km/s (20 and 20 in quadrature). 8 would be relatively unlikely, 4 even more so, and so on.

 

[Bandersnatch]

Yes, but in this fantasy I would expect the distribution to peak at ~30 km/s (20 and 20 in quadrature). 8 would be relatively unlikely, 4 even more so, and so on.

But that's a fantasy scenario, meant to illustrate a specific point. In reality, stellar velocities cluster around 0 km/s w/r to LSR, as shown on the graph above. Couple that with ejection velocities predominantly being expected to be close to escape velocity, and 8 km/s is within 1 sigma for the population. As is 1 km/s, or 0.

 

[stefan r]

Yes, by "puzzling" I mean "an outlyer". An it's not a yes or no thing. A 1 in 2 outlyer doesn't bother me at all. A 1 in 10 bothers me some, 1 in 100 more, and so on.
Yes, but in this fantasy I would expect the distribution to peak at ~30 km/s (20 and 20 in quadrature). 8 would be relatively unlikely, 4 even more so, and so on.

Wikipedia says oxygen has a critical ionization velocity of 12 km/s. Atoms in interstellar space frequently have thermal velocities of several km/s. Silicon and iron have lower ionization energies and more atomic mass. Higher velocity objects will build up more surface charges. Ionizing radiation and plasma can levitate dust. The effect has been observed on the moon. Charged particles interact with plasma in a wider radius than the particle radius. Asteroids with velocities 30 km/s would build up a lot more charge.

The milky way averages around 5 x 10⁵ molecules/m³ of interstellar gas. That is not enough to drag down the velocity of planets or stars. Suppose we guestimate the asteroid to have mass 3 x 10⁶ kg and 2 x 10⁴ m² cross section. If it moves at 10⁴m/s it will hit 10¹⁴ molecules per second. Assuming inelastic collisions _{(probably wrong)}, 1.7 x 10^-13 kg/s at 10⁴ m/s should decelerate a 3 x 10⁶ kg asteroid at 5.5 x 10^-16 ms^-2. In one billion years that becomes 17 m/s.

A one micro gram piece of dust orbiting (or following) the asteroid 7500 meters (100x radius) would have gravitational acceleration F=G(m₁m₂)r^-2= 6.674 x 10^-11 x 3 x 10⁵ x 1 x 10^-6 x 1.8 x 10-8 = 3.6 x 10^-19 Newtons.
The piece of dust could have diameter 10^-4 m, cross sectional area 8 x 10^-9 m². The dust hits about 40 molecules per second or 6.6 x 10-26 kg/s. Traveling at 10⁴ m/s (simplified) drag force would be around 6.6 x 10-22 Newtons. The force of interstellar gas on an orbiting grain of dust is 500x less than the force of the asteroid's gravity. So the grain could stay in orbit. A comet's dust tail could have much higher drag force than the comet's body.
If the dust tail has 1000x the total surface area then it could drop velocity from 10 km/s to 8 km/s in 120 million years. I am not sure if a dust trail would be detectable. It also would not need to be currently present.

I have not seen any evidence that A/2017 U1 has a dusty regolith. Just throwing out the possibility that high velocity meteors could slow down.

 

[Vanadium 50]

But that's a fantasy scenario, meant to illustrate a specific point.

But it was your scenario. That's why I responded to it.

In reality, stellar velocities cluster around 0 km/s w/r to LSR

But stellar speeds do not.

 

[r8chard]

Good discussion all around.
I am of the opinion that for all the highfalutin' mathematics and continuously evolving technology of the last few centuries?

It still remains a viable rule of thumb "That all action produces an equal and opposite reaction".

 

[Vanadium 50]

r8chard, was this the thread you intended to post this to?

 

[Bandersnatch]

But stellar speeds do not.

I've been trying to understand what you meant by that, but couldn't. No matter how I look at the graph in post #37, I see low speeds in the most likely range.
Same when I look at the W velocity dispersion from the presentation linked to in post #33.
Maybe I'm missing something obvious here.

 

[mfb]

The difference here is between speed and velocity.

As an example, consider a uniform velocity distribution from -20 km/s to +20 km/s in both directions. What is the speed distribution for speeds below 20 km/s?It is a linearly increasing function. Regions of fixed speed are circles, and the circle corresponding to 1 km/s is much smaller than the circle corresponding to 10 km/s. Slower speeds are less likely. This stays true even if the velocity distribution is not completely uniform. The probability that both velocities are very close to zero at the same time is very small.

 

[Bandersnatch]

Ok, I can appreciate how there would be a dip around the zero point. But if the distribution in the x-y plane is such that 50% of the population falls within a circle of radius 20, then any speed of less than 20 still makes the star fall within that 50% range, no?

 

[mfb]

But if the distribution in the x-y plane is such that 50% of the population falls within a circle of radius 20, then any speed of less than 20 still makes the star fall within that 50% range, no?

In that case 20 km/s is the median speed, sure. The mean can be higher.

 

[tony873004]

This asteroid, now called ʻOumuamua, is thought to be about 150 meters across.
If it was a red dwarf star instead, with a mass of 0.1 solar masses, it would have perturbed all the planets' orbits.
Earth would have a semi-major axis of over 7 AU.

Here is a simulation of our new solar system:
http://orbitsimulator.com/gravitySi...ns/1510265575945_afterInstellarRedDwarf..html

 

[Vanadium 50]

Nice! (Pun intended)

At 7 AU global warming becomes something we aspire to.

What happens to the moon in this scenario?

Why does A2017U1 keep coming back? I thought its orbit was hyperbolic.

Did you run this with a Jupiter-sized object? Anything interesting happen?

 

[tony873004]

The Moon stays with the Earth, but its eccentricity is nearly quadrupled.

In the simulation, the "camera" is in an unrealistically-fast orbit around the solar system just to show it to you from all sides. That makes it appear as if A2017U1 keeps coming back. If you check "Trails" and run it, you will see that's its spiraling away in this rotating frame of reference.

With a Jupiter-mass, it wasn't nearly as interesting. Earth's orbit got ever-so-slightly rounder. The Moon's eccentricity jumped from about 0. 55 to about 0.1.

 

[tony873004]

As this object continues to get observed, its trajectory gets refined. The good news is that with the latest data, Earth only gets pushed out to 2.6 AU.

 

[Charles Bell]

The IAU posted an announcement on 2017-Nov-14 concerning the newly discovered interstellar object that explained its discovery circumstances, its naming and the new designation system. The new object is now officially known as 1I/2017 U1 and named 'Oumuamua which in Hawaiian means “a messenger from afar arriving first”. This was approved by the IAU Executive Committee. It is being called a prototype of a new class of objects, an “interstellar asteroid” which is
not gravitationally bound to the Solar System. https://www.iau.org/news/announcements/detail/ann17045/

 

[Charles Bell]

A search on "2017 U1" in arxiv will find the recently submitted papers on 1I/2017 U1
Today I find 10 such new papers.

https://arxiv.org/find/all/1/all:+AND+2017+U1/0/1/0/all/0/1

 

[Charles Bell]

The orbit of 1I/2017 U1 was updated on 2017-Nov-13 with the issuance of
MPEC 2017-V63 (2017 Nov. 13) 1I/`OUMUAMUA
The latest value of the eccentricity e = 1.1992920
The values of the orbital parameters are only changing out at the 4th or 5th decimal place.

Observers W. H. Ryan and E. V. Ryan. submitted astrometry on 1I/2017 U1 with magnitude near 24 using the 2.4-m f/8.9 reflector telescope at Magdalena Ridge Observatory, Socorro.

Karen Meech is PI of Hubble Space Telescope proposal 15405 titled "Which way home? Finding the origin of our Solar System's first interstellar visitor".
Hubble is going to be used to observe 1I/2017 U1 possibly until 2018-Jan-01 when it will have faded to magnitude 27.5

Details of the observing plan can be found online at
http://www.stsci.edu/cgi-bin/get-proposal-info?id=15405&observatory=HST
When observations are completed, they will show up under the HST Archive link.
They will be used to extend the observation arc and orbit and gather light curve data.

Spitzer Space Telescope has an approved plan to observe 1I/2017 U1.

 

[rootone]

I know it's not possible but it sure would be interesting to get a probe to it,
To see if it's composition is similar or not to solar system asteroids.

 

[DaveC426913]

http://digg.com/2017/interstellar-object-oumuamua

Apparently, it's a spindle - more than five times longer than it is wide.

The article (or is it Wiki) says that some suggestions are that it is a contact binary.

How would 2 (or more) smaller asteroids manage to make contact (and then stay in contact long enough to adhere) in such an arrangement?

Seems to me, it's essentially two (or more) long, thin asteroids balancing on their tips against gravity.

 

[jedishrfu]

Magnets!

 

[newjerseyrunner]

I've also read that it's possible those long spindles are created by molten rock being flung out from an impact and frozen like that.

 

[Ernest S Walton]

An object with a trajectory never seen before and a shape never seen before? That's like winning the lottery 2 weeks in a row. Folks, this is an alien craft.

 

[Filip Larsen]

Press release from ESO on 1I/2017 U1: http://www.eso.org/public/unitedkingdom/news/eso1737/

 

[DaveC426913]

According to this other article, its length to width ratio is more like 10:1.
https://www.sciencedaily.com/releases/2017/11/171120120935.htm
So that "artist's impression" if a spearoid is not nearly as "artist's impression" as it would seem.

 

[litup]

An object with a trajectory never seen before and a shape never seen before? That's like winning the lottery 2 weeks in a row. Folks, this is an alien craft.

Well it ain't stopping here, so don't worry about an alien attack:)

 

[rootone]

Well it ain't stopping here, so don't worry about an alien attack:)

Yeah but the hyperbolic trajectory past our Sun might be just to fool us.

 

[DaveC426913]

It is already on its way out of the system.

 

[stefan r]

Long, hard, no belt, and flying free.

Seems to me, it's essentially two (or more) long, thin asteroids balancing on their tips against gravity.

Stalactites are long and thin. So are hoodoos.

This article suggests asteroids in the solar system started out with odd shapes with many elongated. They get chipped and bumped into spherical shapes over time.

 

[newjerseyrunner]

An object with a trajectory never seen before and a shape never seen before? That's like winning the lottery 2 weeks in a row. Folks, this is an alien craft.

There are several known objects with this “cigar” shape. They’re simply rare.

 

[Ernest S Walton]

There are several known objects with this “cigar” shape. They’re simply rare.

According to Sky and Telescope, this is the only asteroid - out of 750,000 - to have a light curve range of 2.5 magnitudes :

http://www.skyandtelescope.com/astronomy-news/meet-oumuamua-the-interstellar-cigar/

 

[stefan r]

According to Sky and Telescope, this is the only asteroid - out of 750,000 - to have a light curve range of 2.5 magnitudes :

http://www.skyandtelescope.com/astronomy-news/meet-oumuamua-the-interstellar-cigar/

How do we differentiate the light curve of a "cigar shape" from the light curve of a "poker chip shape"?

 

[Ernest S Walton]

How do we differentiate the light curve of a "cigar shape" from the light curve of a "poker chip shape"?

A picture tells a thousand words, remembering that the rotation takes just over 7 hours :

 

[stefan r]

A picture tells a thousand words, remembering that the rotation takes just over 7 hours :

I looked at that picture. A cigar 10 cm by 1 cm by 1cm would be at the peak when the long axis is perpendicular to the sun. 10x luminosity because 10x surface. A cookie with a face surface area 10x the edge surface area (3.2 x 3.2x1, or 5 x 2 x1) would have the same peak and trough.