Hyperbolic Hyperbole or Interstellar Interloper? ('Oumuamua)

[Mongo62]

I assume that we should have a good idea of this object's trajectory, if not yet, then surely after it has been tracked for a while longer. Is anybody calculating its trajectory backwards, comparing its location in the past with the calculated location of nearly stars at the same time?

With Gaia, we should certainly be able to backtrack the paths of nearby stars to considerable accuracy for many millions of years in the past, so the main source of uncertainty in my opinion might be the exact trajectory of the object itself. Obviously there would be some degree of uncertainty, increasing the further back in time you look, but if we find that the object was within say 0.01 pc of some red dwarf star five million years ago, with an uncertainty of 0.02 pc, that should significantly increase out understanding of the object.

[zubenelgenubi]

How rare is this object really? Is it a 1 in a century sort of thing or have plenty of others probably passed by and we just didn't spot them?

The consensus among astronomers: the discovery of a minor body on an "extreme" hyperbolic trajectory is a type that we've been waiting for, for a long time:
1) a new comet is discovered,
2) observations are integrated into preliminary orbital elements,
3) what does e = ? (e is eccentricity)

Someone would have to do a literature search to find the earliest printed mentions of the realization that finding a body on an hyperbolic solar orbit, with e = significantly greater than 1.000 means it's an extra-solar interloper.  I wonder if Newton or Halley or Herschel (William or Catherine or John) had such a realization.

I also think that the astronomical consensus is that our solar system is penetrated by small interstellar wanderers all the time.  It's only now, with deep-magnitude, large-scale or all-sky frequent surveys, and the computer hardware and software to search lots and lots of data to correlate moving bodies, that we are rewarded with such a discovery.

Again, a literature search might turn up a paper with the math determining the frequency of "penetration" of a given solar-centered volume, given an assumed flux of minor bodies.
***

I assume that we should have a good idea of this object's trajectory, if not yet, then surely after it has been tracked for a while longer. Is anybody calculating its trajectory backwards, comparing its location in the past with the calculated location of nearly stars at the same time?

<snip>

Re-quoting my post up-thread--there's significance to the fact that the object came from the direction near to the solar apex.

We may not deduce a progenitor system, only that our Solar System sort-of "ran over" A/2017 U1 as our Sun goes on its 250 million year orbit around/within our Galaxy.
 

Pretty good article here:

http://www.skyandtelescope.com/astronomy-news/astronomers-spot-first-known-interstellar-comet/

What gave this object away as an interstellar visitor wasn't its very high inclination (122°) with respect to Earth's orbit, which isn't particularly rare, but more critically its extremely hyperbolic eccentricity (1.19).
<snip>
More intriguing is the fact that A/2017 U1 is coming from a spot only 6° from the solar apex, the direction that our Sun is moving (at about 20 km/s) through its interstellar neighborhood and thus, statistically, the most likely incoming direction for an interstellar visitor.

[ChrisWilson68]

and the extrasolar body was made of anti-matter."

if the object was made of anti-matter it would have followed the other branch of the hyperbola, the one without the Sun at its focus

However, if this object were somehow anti-matter, the charged particles of the solar wind hitting it would annihilate themselves and the anti-matter they hit, releasing enormous amounts of energy.  So there's no chance we would miss the fact that it was anti-matter.

This is not clear - space is pretty sparse.  According to Wikipedia, interplanetary space has about 5-10 protons per cubic cm.  That's 10^7 protons per cubic meter.  According to JPL, the speed relative to the sun is about 60 km/sec, so each m^2 of surface sweeps up 10^7 x 6x10^4 = 6x10^11 protons.  Divide by Avagadro's number (6x10^23) to get 10^-12 grams/sec or 10^-15 kg/sec/m^2.  Now you need to multiply by 2*c^2 to get energy.  That's about 2*10^17, so it's releasing about 200 w/m^2, in assorted charged particles and gamma rays.  About 1/2 will head away from the object, and half into it and get absorbed.  So it's getting about 100 w/m^2 of extra heating.   But it's already absorbing most of the sunlight that hits it (estimated 92%) from 1300 w/m^2.  So it adds only a few percent to the heating.

Could we see the gamma rays?   We have at most 100W of gamma rays, so about 100/(1.6 x 10^-19 J/ev * 0.6 Mev) = 10^15 gamma rays per second.   If we are 10^10 meters away (much less than one AU) these are spread over an area of 4 x pi x 10^20 or about 10^21 m^2.  So at most 10^-6 gamma rays per square meter per second.  A gamma ray detecting satellite of 10 m^2 would see one per day.  No way we could see that either.

So it could be made of antimatter, without obvious consequences.

Thanks for doing the calculation!  The numbers are less than I thought they would be.

However, I think you're underestimating things somewhat.  You're calculating the number of protons hitting the object based on the object's relative speed of 60 km/s.  But the solar wind itself has a speed of 400 km/s.  You the number of proton hits per second is around an order of magnitude greater than your estimate.  So instead of 100 W/m^2, it's getting more like 700 W/m^2.

https://solarscience.msfc.nasa.gov/SolarWind.shtml

[hop]

Again, a literature search might turn up a paper with the math determining the frequency of "penetration" of a given solar-centered volume, given an assumed flux of minor bodies.

The Engelhardt paper I linked earlier does exactly that, and has references to earlier work.

Another related paper Realistic Detectability of Close Interstellar Comets Nathaniel V. Cook, Darin Ragozzine, Mikael Granvik, Denise C. Stephens

As you say, this has been an active area of research / speculation for a long time.

My non-professional impressions
* Spotting this one isn't a huge surprise. We know there has to be a lot of stuff ejected from other solar systems.
* The uncertainties on estimates to date are very large, some models suggested we should have seen interstellar comets already, others predicted that LSST has little chance of finding any.
* The fact that this one was an asteroid rather than a comet is somewhat surprising. Both are expected to exist, but comets are easier to detect and generally expected to be more common.

We may not deduce a progenitor system, only that our Solar System sort-of "ran over" A/2017 U1 as our Sun goes on its 250 million year orbit around/within our Galaxy.

Eric Mamajek (who has done a lot of work on nearby stars) wrote a series of tweets on this

https://twitter.com/EricMamajek/status/924061457280507904

1/n #A2017U1 interstellar asteroid: object came from RA,Dec=281.09,+34.59 deg; est. "inbound" Galactic velocity U,V,W =-11.0,-22.4,-7.3 km/s

2/n #A2017U1 interstellar asteroid: velocity doesn't match any nearby individ stars -doesn't belong to Oort cloud of nearby stars (<12 ly)

3/n #A2017U1: However, velocity is w/i ~5km/s of median velocity of stars within 25pc, i.e. vel completely typical for interstellar "stuff"

4/n #A2017U1 NO evidence it came from Vega -entrd sol sys frm that dirctn, but Vega nowhre near it thsnds of yrs ago

Bill Gray also wrote a nice general FAQ on this object
https://projectpluto.com/temp/2017u1.htm

[LouScheffer]

So it could be made of antimatter, without obvious consequences.

You skipped dust. Dust flux in inner solar system would conservativly mean 100s of partcles of mass around 10^-16 kgs or more hitting per second (flux would be much higher for nanodust many orders of magnitude more.) I think we oould detect the sparkling.

From the paper Interplanetary Dust, the mass density of dust is comparable to that of the solar wind.  Also, the reaction may be less efficient -  the first edge of a dust particle may hit and the reaction might blow the rest of the particle away before it can annihilate.   And even several times the solar wind number would be hard to detect, especially since the object's intrinsic albedo is unknown.

[zubenelgenubi]

Again, a literature search might turn up a paper with the math determining the frequency of "penetration" of a given solar-centered volume, given an assumed flux of minor bodies.

The Engelhardt paper I linked earlier does exactly that, and has references to earlier work.
<snip>

Thanks, hop.  I should have started my "literature search" up-thread at your earlier posting! 

Thank you also for the other links and your impressions in this most recent posting.

[nacnud]

and the extrasolar body was made of anti-matter."

if the object was made of anti-matter it would have followed the other branch of the hyperbola, the one without the Sun at its focus

However, if this object were somehow anti-matter, the charged particles of the solar wind hitting it would annihilate themselves and the anti-matter they hit, releasing enormous amounts of energy.  So there's no chance we would miss the fact that it was anti-matter.

This is not clear - space is pretty sparse.  According to Wikipedia, interplanetary space has about 5-10 protons per cubic cm.  That's 10^7 protons per cubic meter.  According to JPL, the speed relative to the sun is about 60 km/sec, so each m^2 of surface sweeps up 10^7 x 6x10^4 = 6x10^11 protons.  Divide by Avagadro's number (6x10^23) to get 10^-12 grams/sec or 10^-15 kg/sec/m^2.  Now you need to multiply by 2*c^2 to get energy.  That's about 2*10^17, so it's releasing about 200 w/m^2, in assorted charged particles and gamma rays.  About 1/2 will head away from the object, and half into it and get absorbed.  So it's getting about 100 w/m^2 of extra heating.   But it's already absorbing most of the sunlight that hits it (estimated 92%) from 1300 w/m^2.  So it adds only a few percent to the heating.

Could we see the gamma rays?   We have at most 100W of gamma rays, so about 100/(1.6 x 10^-19 J/ev * 0.6 Mev) = 10^15 gamma rays per second.   If we are 10^10 meters away (much less than one AU) these are spread over an area of 4 x pi x 10^20 or about 10^21 m^2.  So at most 10^-6 gamma rays per square meter per second.  A gamma ray detecting satellite of 10 m^2 would see one per day.  No way we could see that either.

So it could be made of antimatter, without obvious consequences.

Thanks for doing the calculation!  The numbers are less than I thought they would be.

However, I think you're underestimating things somewhat.  You're calculating the number of protons hitting the object based on the object's relative speed of 60 km/s.  But the solar wind itself has a speed of 400 km/s.  You the number of proton hits per second is around an order of magnitude greater than your estimate.  So instead of 100 W/m^2, it's getting more like 700 W/m^2.

https://solarscience.msfc.nasa.gov/SolarWind.shtml

I'm sure you'd see a whole lot of 0.5 MeV gamma rays spewing off the thing, it'd be pretty obvious something was fishy.

[sanman]

Is there any spectroscopic analysis of this object?

Maybe it came from a collision in our solar system that sent it onto a crazy orbital trajectory that it's just now returning from.

[nacnud]

You can't get a hyperbolic trajectory from a collision and have the hyperbolic fragments return.

Hyperbolic = Escape velocity or more

[zubenelgenubi]

Is there any spectroscopic analysis of this object?

Up-thread:

Alan Fitzsimmons
@FitzsimmonsAlan
Spectrum of A/2017 U1 obtained on Wednesday night with the @INGLaPalma 4.2m WHT. Colour is red like Kuiper Belt Objects, featureless.

https://mobile.twitter.com/FitzsimmonsAlan/status/923928561722982400

Mike Brown @plutokiller responded in two separate replies that it's "kind of normal-KBO-red type color" and then "a little redder than even the reddest trojans."

I wonder if the extreme color is a result of a greater exposure to stellar ultraviolet than the Sun?  That is, was the parent star an "earlier" spectral class than the Sun?  Hotter, therefore producing more ultraviolet flux, therefore photochemical reactions progress at a faster rate?

Or, have the slow chemical reactions producing tholins or such had longer than the age of our Solar System to progress?  (Do those reactions continue in the dark and cold of interstellar space?)  Is A/2017 U1 older than our Solar System, because it was formed around another star older than the Sun?

[sanman]

You can't get a hyperbolic trajectory from a collision and have the hyperbolic fragments return.

Hyperbolic = Escape velocity or more

What if it was from more than one collision?

[as58]

You can't get a hyperbolic trajectory from a collision and have the hyperbolic fragments return.

Hyperbolic = Escape velocity or more

What if it was from more than one collision?

A collision, or more likely gravitational scattering without actually colliding, in the outer Solar System could send an object on a hyperbolic trajectory. However, getting as large v_inf as this object has by gravitational scattering of a bound Solar System object seems very unlikely to me.

edit: as explained in Bill Gray's FAQ that hop linked to, the thing that makes people confident that A/2017 U1 really is an interstellar object is that that the eccentricity is significantly greater than one. On the other hand, if the eccentricity were something like 1.005, it would probably be a Solar System object.

[hop]

What if it was from more than one collision?

A collision in the far outer solar system could send something inward on a slightly hyperbolic trajectory, but this thing has a hyperbolic excess of ~25 km/s. Getting that from a collision in the outer solar system seems like a tall order.

edit:
Bill Gray's FAQ addresses this, suggestion #4
https://projectpluto.com/temp/2017u1.htm#alt_ideas

The fourth suggestion was that two objects collided, and a bit flew off at really high speed, and that's what we're seeing. Aside from the problems in getting two objects to hit that hard, these are not rubber balls; when rocks or ice chunks hit, you get rubble not moving much faster or slower than the incoming objects. You don't have some bits coming out at interstellar speeds.

[Bynaus]

Very interesting find. Too bad we don't have a spaceprobe ready (and a drive to match) to go catch it and investigate it... But next time one of these comes around, we might be ready.

Regarding orgins, there is a simple short-hand to estimate travel distances: for each km/s at infinity, the object will travel about a parsec per million years. So for this one, 25 parsecs (~82 light years) per million year. Even if it travelled only a few million years, it could have come from very far away!

[KelvinZero]

How old could this thing plausibly be?
eg what is the average and the extreme 1% of the expected age distribution?

[LouScheffer]

How old could this thing plausibly be?
eg what is the average and the extreme 1% of the expected age distribution?

Using the estimate above, if was a billion years old it could have come from anywhere in the galaxy. And it could easily be older than that (there are lots of stars billions of years older than the sun).  But it did not come from some other galaxy, since otherwise the galactic field would have made it much faster.  Likewise probably not from the center, where even if escaped its star it would not get out this far.

[jgoldader]

Very interesting find. Too bad we don't have a spaceprobe ready (and a drive to match) to go catch it and investigate it... But next time one of these comes around, we might be ready.

I keep thinking that as well.  There've been a number of close approaches of interesting objects, and it's a pity we've no way to get a small probe into the path of one to get a few images and spectra as they pass.  You'd need to have a smallest with an upper stage, already in Earth orbit (high orbit?  GEO?) ready to go; but perhaps orbital mechanics are so cruel that such things are the stuff of fantasy.

It's a pity the engine on Japan's Procyon satellite failed.  That was a nice mission, a smallsat designed for an asteroid flyby, and who knows when we'll get to test that concept again.

[KelvinZero]

Looks like a rock from this galaxy could be pretty close to as old as the universe??!

Methuselah star, only 190 ly away, estimated age 14.46 ± 0.8 billion years. (assumed not actually older than the universe, of course)
population II so metal poor but possibly has oxygen, silicon etc, so I guess it is at least slightly possible for an icy, rocky object in this galaxy to be as old.

https://en.wikipedia.org/wiki/HD_140283

[Star One]

Looks like a rock from this galaxy could be pretty close to as old as the universe??!

Methuselah star, only 190 ly away, estimated age 14.46 ± 0.8 billion years. (assumed not actually older than the universe, of course)
population II so metal poor but possibly has oxygen, silicon etc, so I guess it is at least slightly possible for an icy, rocky object in this galaxy to be as old.

https://en.wikipedia.org/wiki/HD_140283

And coincidentally there’s this new video about this very star. Though he’s not a scientist I think this guy does some of the best short form videos on topics like this.

[hop]

Very interesting find. Too bad we don't have a spaceprobe ready (and a drive to match) to go catch it and investigate it... But next time one of these comes around, we might be ready.

Eric Mamajek had an interesting take on that https://twitter.com/EricMamajek/status/924010707271553026

Flyby or sample return to interstellar asteroid A/2017 U1 = exploring small exoplanetary world in our backyard:10s of km/s easier than 0.1c!

Yes, it's pretty ridiculous to think about chasing this thing down, but compared to something like Breakthrough Starshot it would be a total piece of cake. Launching at mere 0.001c to chase something like this down could be a great test run

In practice, there's another problem though: It will be invisible to our biggest telescopes very soon, and the uncertainties on the trajectory would make it very, very hard to find years down the road.

Having an interceptor ready for the next one seems much more practical. We often see Oort cloud comets years out, so even with the extra interstellar velocity there's a chance of having a reasonable amount of notice for other objects. OTOH, it may prove difficult to convince anyone to fund a mission on the argument that a suitable target will eventually appear...