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Originally Posted by Swift
Agreed. And while the odds might be low for any single object, there may be many such objects out there, which we have not detected, and so the odds of one such object having a close approach to Earth, out of the many, is not low.
Sure, if you throw 14,000 darts at a dartboard, you're bound to hit dead center at least once. But the thing is, there are two bullseyes in a row, and it just so happens to be the first one we see.

It would be as if someone who had never thrown darts before, walks up to the line, and throws a bullseye--and not just within the inner ring, it is smack dab in the middle. And then for her next throw hits the triple 20--and not just within the triple 20, but smack dab in the dead center of it, with an error less than one tip width.

Like Phil Plait said, "It's..... weird...." The path looks as if it were designed.

I suppose such things are possible. I have only sank the 8-ball on the break twice in my entire life. The weird thing is, I did it twice in a row. So anything is possible--I guess....
Last edited by Warren Platts; 2017-Dec-14 at 04:36 PM.

2. Originally Posted by Warren Platts
It would be as if someone who had never thrown darts before,
I think your analogy is not appropriate. For all we know, that "someone" has been throwing darts for millions of years and has hundreds of darts a year. How much is the fact that we saw this object (and not the supposed others) biased by the fact that this particular object came in on this path and how much our observing technology has improved over the years?

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Originally Posted by Swift
I think your analogy is not appropriate. For all we know, that "someone" has been throwing darts for millions of years and has hundreds of darts a year. How much is the fact that we saw this object (and not the supposed others) biased by the fact that this particular object came in on this path and how much our observing technology has improved over the years?
Well, that's an assumption--that the dart thrower has lots of darts--but lets follow that rabbit hole and see where it leads. I guess the canonical paper on that is Zwart et al. (2017), available for free at the arxiv (I'm looking at preprint dated 13 November).

So the factors that affect detection are the density of the objects, and the volume sampled by the pan-STARRS telescope. Assuming Oumuamua is a typical (natural) object, then the volume sampled is a function the detection radius (adjusted for gravitational focusing), time, and speed of a typical object:

V = A * v * dt

where V is the volume, v is the relative velocity WRT to the sun (26 km/s), and dt is the time (5 years), and A is the cross-section sampled, and is given by:

A = pi * rdetect^2 * (1 + vesc/v) and vesc at 1 au is 42 km/s

where r is the detection limit, and since Ouamuamua was barely detectable at 0.16 au, we go for that.

And here is where I get stuck on the first page. Maybe you guys can help.

They get a density of 1 object/0.8 au^3 (1.7 x 10^14 obj/parsec^3), but I get a different figure:

First the area A: first term is pi * 0.16^2 = .08 au^2. Correction factor is 1 + 26/42 = 3.6; thus A = 0.08 * 3.6 = ~0.3 au^2

Now the length factor is 26 km/s * 5 years, so we have to convert to au:

26 km/s * 5 yr * 3.154 x 10^7 s/yr / 1.496 x 10^8 km/au = 27.4 au

Thus, 0.3 au^2 * 27.4 au = ~8 au^3

And since there is one object in our detection volume, that is our density: 1 object/8 au = 10^12 objects/parsec^3.

So I'm off by a factor of 100, or they are. Could it be that they are wrong? It sure wouldn't be the first time I've found a paper that dropped a zero or two (this would be #4, as far as I can remember). What do you guys think?

ETA: Never mind: I see their density is 0.08 per au^3; thus 1 object per 8 au^3 equals a density of 0.125 per au^3, which is in the right ball park. They must have used a somewhat higher detection radius than 0.16 au.

ETA 2: I see if you use a rdetect of 0.2 au, you get a density of 0.08
Last edited by Warren Platts; 2017-Dec-14 at 08:13 PM.

4. I don't know what you did with the gravitational focusing correction factor, but somehow the right figure comes out of that one.

I do see that Zwart et al arrive at a similar number as the Jewitt at all paper I referenced earlier, but they assume an only 2 year period of observing at full efficiency.

5. There have been billions of dart throwers (all of the stars in the galaxy which have planetary systems) throwing billions of darts (all the planitesimals ejected when giant planets migrated inward and outward in their home planetary systems) for billions of years (since planetary systems started forming).

What's surprising to me is that it's taken us so long to notice one of them. I suspect some contributing factors are their sizes (most are going to be small), dark coloration (most of the outer small bodies in our solar system are dark, so maybe interlopers would be, too), and trajectories which don't coincide with the Ecliptic (where we tend to look for small bodies).

Solar observatories like SOHO see an incredible number of sungrazing "comets". I wouldn't be a bit surprised if a substantial number of them have originated in other planetary systems.
Last edited by selden; 2017-Dec-14 at 09:30 PM.

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Happy to read that loads of people including leading scientists agree with with my view that Oumuamua is a spacecraft

http://www.nzherald.co.nz/technology...ectid=11959710

"
Today, scientists led by Stephen Hawking are using high-tech scanners to discover if a huge, cigar-shaped "comet" is in fact, an alien probe.

Now, one astronomer claims that the space rock, named Oumuamua, could be an alien spacecraft with broken engines that is tumbling through our solar system.

Dr Jason Wright from Penn State University suggests that a broken alien spacecraft would move in exactly the same way as the interstellar comet, according to the Daily Mail."

More from space.com

http://www.space.com/39100-interstel...fe-search.html

"An initial search for artificial signals coming from 'Oumuamua, the needle-shaped interloper that zoomed past Earth two months ago, have come up empty, scientists with the \$100 million Breakthrough Listen project announced today (Dec. 14).

But researchers aren't done analyzing the data that came in from the Robert C. Byrd Green Bank Telescope in West Virginia yesterday (Dec. 13), and they also plan to conduct three more "blocks" of observations, team members said. ['Oumuamua: Our 1st Interstellar Visitor Explained in Photos]"

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Last edited by selvaarchi; 2017-Dec-14 at 10:17 PM.

7. Originally Posted by selvaarchi
Happy to read that loads of people including leading scientists agree with with my view that Oumuamua is a spacecraft
I wish you'd take a little more time to read before re-posting stuff like that. You might notice if you read carefully that the headlines are bad. They say "claims" but if you actually look at the article, it makes it clear that they are suggesting the possibility. If you take the time to actually go to Jason Wright's blog, you will see at the very top that he writes:
[note: As I wrote in November, I don’t think ‘Oumuamua is an alien spacecraft.
So no, lots of scientists do not agree with you. There are lots of scientists who are intrigued by it and find it very interesting and would like to consider the possibility that it is an alien spacecraft. I agree with that, but it doesn't mean that I think it is a spaceship. I think it probably isn't, but it's such an exciting possibility that I would support efforts to investigate.

8. Aw come on, it's just a joke. There's even a big fat smiley there to say so. If you're going to be annoyed at anything, be annoyed at the constant misspelling of the name `Oumuamua without the single quote mark.

9. Originally Posted by slang
Aw come on, it's just a joke. There's even a big fat smiley there to say so. If you're going to be annoyed at anything, be annoyed at the constant misspelling of the name `Oumuamua without the single quote mark.
Oops, I guess that went over my head. I thought the smiley was one of happiness rather than humor. Sorry for the misunderstanding.

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Originally Posted by Jens
Oops, I guess that went over my head. I thought the smiley was one of happiness rather than humor. Sorry for the misunderstanding.
Second article does say no signals been detected so far but they are still looking out just in case. Would give us a big jolt if that is the case as it would indicate a civilisation streets ahead of us.

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What would you guys do if the message received was: "di-di-dit da-da-dah di-di-dit"?

If it was a derelict spacecraft, however, the message wouldn't be that there is a civilization streets ahead of us. The message would be that space travel is really, really hard to do, and that one had better be prepared to spend hundreds of thousands of years to get to the next star system....

The scariest thing was Tony's post where a small brown dwarf on the same path would have disrupted our orbit to halfway between Jupiter and Saturn. We would all be in the process of freezing to death at this point....

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Originally Posted by slang
I don't know what you did with the gravitational focusing correction factor, but somehow the right figure comes out of that one.
The gravitational focusing correction factor is simply (1 + (vesc/v)2) where vesc at 1 au is 42 km/s and v is 26 km/s = 3.6 if I did it right.

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Originally Posted by selden
There have been billions of dart throwers (all of the stars in the galaxy which have planetary systems) throwing billions of darts (all the planitesimals ejected when giant planets migrated inward and outward in their home planetary systems) for billions of years (since planetary systems started forming).

What's surprising to me is that it's taken us so long to notice one of them.
The thing is, if we accept 'Oumuamua as typical, then we have to accept a density of these objects at something like 1.7 x 1014 per cubic parsec. To put that in perspective, that is supposedly much higher than the suspected density of icy objects in the Oort cloud. That doesn't make sense IMHO: interstellar space should have fewer objects per cubic parsec than objects bound to a solar system, and we should expect that most objects ejected into interstellar space should be icy, not bone dry rocky/metallic objects. YMMV

14. Originally Posted by Warren Platts
The gravitational focusing correction factor is simply (1 + (vesc/v)2) where vesc at 1 au is 42 km/s and v is 26 km/s = 3.6 if I did it right.
Yeah, you switched the velocities in the post. But not in your calculation.

15. Originally Posted by Warren Platts
we should expect that most objects ejected into interstellar space should be icy, not bone dry rocky/metallic objects. YMMV
How do you come to that conclusion?

I'm not arguing against it, rather I'm wondering what factors might have been used to determine the relative proportion of icy vs rocky planetesimals. For example, I would expect an extremely large number of "dry" planetesimals would have had to have been present and then ejected from their systems in order for all the "hot jupiters" to become such close star-huggers. However, I don't know how to determine what the initial mass distributions would have had to have been in those systems, and thus what the proportion of icy planetesimals would have been.

16. Originally Posted by slang
If you're going to be annoyed at anything, be annoyed at the constant misspelling of the name `Oumuamua without the single quote mark.
Now, that there's a ʻokina, not a single quote - it marks a glottal stop in Hawaiian (and some other languages). So the first syllable is pronounced starting with the vocal cords closed.

Grant Hutchison
Last edited by grant hutchison; 2017-Dec-16 at 01:33 AM.

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"Breakthrough Listen - the initiative to find signs of intelligent life in the universe - is reporting preliminary results and making initial data available from its observations of the "interstellar visitor" 'Oumuamua."

http://www.spacedaily.com/reports/In...uamua_999.html

The initial block of observations (the first of a planned four blocks) ran from 3:45 pm to 9:45 pm ET on Wednesday, December 13, using the Breakthrough Listen backend instrument on the Robert C. Byrd Green Bank Telescope in West Virginia. Listen observed 'Oumuamua across four radio bands (corresponding to four of the radio receivers available at Green Bank, denoted L, S, X, and C), spanning billions of individual channels across the 1 to 12 GHz range.

In addition to calibration observations, the instrument accumulated 90 TB of raw data over a 2-hour observation of 'Oumuamua itself. A search for signals that may be of artificial origin has begun, but despite the impressive computational power of the Breakthrough Listen computing cluster at Green Bank, the large data volumes mean that this will take some time to complete.

18. Originally Posted by selvaarchi
"Breakthrough Listen - the initiative to find signs of intelligent life in the universe - is reporting preliminary results and making initial data available from its observations of the "interstellar visitor" 'Oumuamua."

http://www.spacedaily.com/reports/In...uamua_999.html
What if we are looking in the wrong direction? Oumuamua could be a giant burned out rocket booster or jettisoned fuel tank. The much smaller probe it brought, could be nearby, surveying the inner solar system. Maybe even sending us a greeting, while we watch a piece of space junk sail away into interstellar space.

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Originally Posted by grant hutchison
Now, that there's a ʻokina, not a single quote - it marks a glottal stop in Hawaiian (and some other languages). So the first syllable is pronounced starting with the vocal cords closed.

Grant Hutchison
A glottal stop at the beginning of a word is kind of redundant, isn 'it?
Last edited by Warren Platts; 2017-Dec-18 at 10:52 PM.

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And now we're seeing the bending-over-backwards to explain things that don't add up. If 'Ouamuamua was a typical object, that entails that rocky objects are the most common interstellar interlopers; that goes against all common sense (albeit common sense has been known to be wrong in the past).

So now they're saying that the rock is actually a "baked Alaska": icy core with protective covering on the outside that withstands 27 kW of heat per square meter, perfectly keeping any gases on the inside from sublimating....

https://www.wired.com/story/oumuamua...ve-passengers/

It doesn't add up. The assumption that Oumuamua is typical is just that: an assumption. If they were as common as the simple BOTE suggests: (a) we should have seen such hyperbolic "comets" before; and (b) the theory requires special theories on how to protect ice from sun grazing orbits....

21. Originally Posted by Warren Platts
A glottal stop at the beginning of a word is kind of redundant, isn 'it?

No, because a “stop” simply refers to a sound where you block the air and then release it, as in P and B and M. I think we also use (unmarked) glottal stops in English at the beginning of words that start with a vowel.

22. Originally Posted by Jens

No, because a “stop” simply refers to a sound where you block the air and then release it, as in P and B and M.
And the Hawaiian glottal stop probably evolved from other stopped consonants that are present in some other Polynesian languages, but which have been glottalized in Hawaiian. It's an important phoneme, making the difference between ʻahi ("tuna") and ahi ("fire"), for instance.

Originally Posted by Jens
I think we also use (unmarked) glottal stops in English at the beginning of words that start with a vowel.
Yes, English speakers effectively pronounce the initial stop in ʻOumuamua when they speak it in isolation - they would find it difficult to omit it, in fact.

Grant Hutchison

23. Originally Posted by Warren Platts
And now we're seeing the bending-over-backwards to explain things that don't add up.
When something is mysterious, I think it's natural that people will try to explain it in various ways. What's important is that you want to test to determine what it is. The possibility that it is some kind of artificial body has been suggested as well, and people have done observations to see if it is emitting anything. It would not be good science to just throw our hands up and say, well we can't explain it, so it must be the thing that can fit any parameter (artificial things). Because an artificial object could have any shape, and requires no explanation. Is it rectangular? They built it that way! Is it a torus? They built it that way! Is it a sphere? They built it that way! So you have to find something that will demonstrate that it is artificial. I think it would be great if we could send a probe to chase it down, but I think it would be pretty tough. It's going 26 km/s, so it would have to be a fast probe.

24. Originally Posted by Jens
When something is mysterious, I think it's natural that people will try to explain it in various ways. What's important is that you want to test to determine what it is. The possibility that it is some kind of artificial body has been suggested as well, and people have done observations to see if it is emitting anything. It would not be good science to just throw our hands up and say, well we can't explain it, so it must be the thing that can fit any parameter (artificial things). Because an artificial object could have any shape, and requires no explanation. Is it rectangular? They built it that way! Is it a torus? They built it that way! Is it a sphere? They built it that way! So you have to find something that will demonstrate that it is artificial. I think it would be great if we could send a probe to chase it down, but I think it would be pretty tough. It's going 26 km/s, so it would have to be a fast probe.
I've been thinking a little about this, and it just may be (extremely unlikely, of course) that a once in a lifetime of Earth chance to see an artificial, extraterrestrial object will pass us by and there's almost nothing we can do about it. If this thing had come 100 years later, or we were "100 years" more "advanced", maybe it'd be different. But thems the breaks.

CJSF

25. Well, luckily. the Ramans do everything in threes.

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Now we have gone one full circle - as the start of this thread said - it could be a comet.

http://www.esa.int/Our_Activities/Sp...really_a_comet

An object from another star system that made a brief appearance in our skies guised as an asteroid turns out to be a tiny interstellar comet.

‘Oumuamua, a name that reflects the Hawaiian meaning for ‘a messenger from afar, arriving first’, was discovered by astronomers working with the Pan-STARRS survey in Hawaii in October last year as the object came close to Earth’s orbit. Follow-up observations by ESA’s Optical Ground Station telescope in Tenerife, Canary Islands, and other telescopes around the world helped determine its trajectory.

The astronomers had detected ‘Oumuamua about a month after its closest approach to the Sun, which took it on a highly inclined path within the orbit of Mercury. Travelling at a speed of about 114 000 km/h as of 1 June 2018, ‘Oumuamua is fast enough to escape the Sun’s gravitational pull and will eventually break free of our Solar System.

Since interstellar comets are thought to be more numerous than interstellar asteroids, the mystery visitor was first assumed to be a comet. However, no evidence of gas emissions or a dusty environment were seen in the images, leading to its classification as the first interstellar asteroid.

But the story has another surprising twist.

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Now it appears to be escaping a bit too fast:
http://www.spacetelescope.org/static.../heic1813a.pdf
A hyperbolic encounter with the sun may have reproduced the anomalous flyby effect sometimes seen in spacecraft making a hyperbolic gravity assist with earth.

28. Is that the “Pioneer Effect”?
I thought they had resolved the cause of that.

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The Pioneer Anomaly and the Flyby Anomaly are not the same though it would be useful to find a connection between them. Not everyone agrees that the Pioneer anomaly is resolved which includes the discoverer, John Anderson et al.The Pioneer Anomaly uses a model of a constant small acceleration towards the sun and possibly beyond which makes the craft slow down more than normal gravity would accomplish. This has been true for all craft suspected of exhibiting the anomaly that may include Galileo and Ulysses and possibly New Horizons. It appears as a persistent force although Turyshev and Toth make a case for linear decay in Pioneer 11 data.

The Flyby Anomaly seems to appear near perigee in the hyperbolic interaction giving a Delta V in some cases. This is in addition to the larger classical Delta V from gravity assist. The Flyby Anomaly can be plus or minus and is independent of the sign of the gravity assist. It is not always monitored in all flybys. There have been a number of attempts to give a model for it, but then predictions do not always succeed.

The important difference is that the Pioneer Anomaly is always negative (attractive) where the Flyby anomaly can be either plus or minus. The comet has a plus delta V anomaly which suggests that is not related to the Pioneer Anomaly but does not immediately rule out the flyby anomaly. The paper suggests a possible explanation but might be faced with the issue of where the anomaly would be on the incoming path.

30. Rather than Pioneer anomaly, maybe it’s better to say Flyby Anomaly. Is the magnitude of the acceleration close to what has been seen in planetary flybys, like 10 mm/s or so? If it is it would be quite infesting.

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