APOD: The Unexpected Trajectory of... (2018 Nov 20)

[neufer]

Avi Loeb has said that the observed acceleration requires 10% of the mass of 'Oumuamua to have been outgassed. So yes, very confusing.

That assertion by itself makes no sense, since any delta-V is determined by momentum, not mass.

If one knows the albedo of an asteroid/comet then one has a rough idea of its temperature at any given time.

Assuming the jets are primarily water vapor then one know the average velocity/momentum of a water molecule at the stated temperature.

[Chris Peterson]

Avi Loeb has said that the observed acceleration requires 10% of the mass of 'Oumuamua to have been outgassed. So yes, very confusing.

That assertion by itself makes no sense, since any delta-V is determined by momentum, not mass.

If one knows the albedo of an asteroid/comet then one has a rough idea of its temperature at any given time.

Assuming the jets are primarily water vapor then one know the average velocity/momentum of a water molecule at the stated temperature.

We rarely have a good idea of the velocity of outgassing, even with well observed comets in our own system. There are too many unknowns. The temperature of the outer centimeters is not enough information. Also, outgassing often ejects much more massive material, from dust up to boulders, so that's another unknown.

(Anyway, the point was that the statement by itself makes no sense- a lot of underlying assumptions were unstated.)

[hamilton1]

That assertion by itself makes no sense, since any delta-V is determined by momentum, not mass.

If one knows the albedo of an asteroid/comet then one has a rough idea of its temperature at any given time.

Assuming the jets are primarily water vapor then one know the average velocity/momentum of a water molecule at the stated temperature.

We rarely have a good idea of the velocity of outgassing, even with well observed comets in our own system. There are too many unknowns. The temperature of the outer centimeters is not enough information. Also, outgassing often ejects much more massive material, from dust up to boulders, so that's another unknown.

(Anyway, the point was that the statement by itself makes no sense- a lot of underlying assumptions were unstated.)

Those who postulate invisible cometary outgassing would know all about underlying assumptions being left unstated.

[MarkBour]

\[Summary\]I would summarize my conjecture as "fortuitously-timed ejection of large dust, or gravel, or bigger chunks".
And I wondered about any detectable changes in rotational rate, which would presumably be the rate of change in the light curve.

Thanks for that extended treatment of my question Mark. One clarification I would seek in what you are saying is what you mean by "increase in velocity." This object has deviated somewhat from its projected path, but is it really moving away from the sun faster than it would have without the suspected outgassing, or has it just moved laterally off the expected trajectory? Naturally, it must in fact be slowing down as it climbs out of our system's gravity well.

However, I would naively expect a tumbling outgassing object to have the delta Vs it experiences largely cancel out due to its rotations and not cause a significant course alteration.
...
Bruce

Thanks, Bruce. I am pretty intrigued by Oumuamua. I was trying to sort out what possibilities could remain from the information in the APOD and directly-referenced sources. I can now add the following, though I am too "cheap" to buy access to the article in the first bullet point, at least for now.

• The source at the center of this is evidently:
  "Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua)" Micheli, M., Farnocchia, D., Meech, K. J., et al. 2018, Nature, 559, 223
  Accessible at: https://www.nature.com/articles/s41586-018-0254-4 ... if I want to pay for it.
• Oumuamua (as Art pointed out) was only observed past perihelion, on its outward journey, we have nothing before nor near perihelion. So, all of the discussion would have to involve only changes to its velocity on this outward journey.
• Oumuamua is no longer observable by any of our instruments. So, short of someone mounting a special mission, which seems highly unlikely, I guess all of the data-gathering on it is done.
• The sources I followed did consistently say it has higher outward velocity from the Sun than expected, not just a different angle or changing angle. You are correct in pointing out that overall, it is decelerating while moving away from the Sun. The deviation was not dramatic, but it was significant. Evidently it was just not decelerating quite as much as an inert rock ought to.
• One of the secondary references I found indicated that the trajectory was also inconsistent with a single added impulse, which would have been the case in my guess that a big chunk may have broken off and may have been thrown backward. I'd have to find out more before I could know why they say that.

[Chris Peterson]

[*]The sources I followed did consistently say it has higher outward velocity from the Sun than expected, not just a different angle or changing angle.

You mean orbital speed? Velocity is a vector.

[BDanielMayfield]

\[Summary\]I would summarize my conjecture as "fortuitously-timed ejection of large dust, or gravel, or bigger chunks".
And I wondered about any detectable changes in rotational rate, which would presumably be the rate of change in the light curve.

Thanks for that extended treatment of my question Mark. One clarification I would seek in what you are saying is what you mean by "increase in velocity." This object has deviated somewhat from its projected path, but is it really moving away from the sun faster than it would have without the suspected outgassing, or has it just moved laterally off the expected trajectory? Naturally, it must in fact be slowing down as it climbs out of our system's gravity well.

However, I would naively expect a tumbling outgassing object to have the delta Vs it experiences largely cancel out due to its rotations and not cause a significant course alteration.
...
Bruce

Thanks, Bruce. I am pretty intrigued by Oumuamua. I was trying to sort out what possibilities could remain from the information in the APOD and directly-referenced sources. I can now add the following, though I am too "cheap" to buy access to the article in the first bullet point, at least for now.

• The source at the center of this is evidently:
  "Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua)" Micheli, M., Farnocchia, D., Meech, K. J., et al. 2018, Nature, 559, 223
  Accessible at: https://www.nature.com/articles/s41586-018-0254-4 ... if I want to pay for it.
• Oumuamua (as Art pointed out) was only observed past perihelion, on its outward journey, we have nothing before nor near perihelion. So, all of the discussion would have to involve only changes to its velocity on this outward journey.
• Oumuamua is no longer observable by any of our instruments. So, short of someone mounting a special mission, which seems highly unlikely, I guess all of the data-gathering on it is done.
• The sources I followed did consistently say it has higher outward velocity from the Sun than expected, not just a different angle or changing angle. You are correct in pointing out that overall, it is decelerating while moving away from the Sun. The deviation was not dramatic, but it was significant. Evidently it was just not decelerating quite as much as an inert rock ought to.
• One of the secondary references I found indicated that the trajectory was also inconsistent with a single added impulse, which would have been the case in my guess that a big chunk may have broken off and may have been thrown backward. I'd have to find out more before I could know why they say that.

Nice response Mark. It seems to me that this object is behaving somewhat like an out of control remnant of a designed lightsail might be expected to behave, being pushed by possibly both outgassing and solar wind/radiation pressure, or maybe just solar wind and radiation with no outgassing.

Note that I say it is behaving like an artifact might, not that it is an artifact. I still think this is a natural object, but there is a non-zero probability of it being a construction of some sort.

Bruce

[R_Saggio]

But, isn't the solar system, as whole, moving as well? Can the difference in trajectory be explained simply by the relative velocities impact to relative distances between the Solar System and 'Oumuamua? Just asking. I know it sounds silly, but, hey, some one forgot to convert metric into imperial and pile-drove a probe into Mars. Someone should put the Solar System velocity vector in the animation.

[FooJetSuu3]

because you know the actual trajectory and speed etc, don't you know the mass? isn't the unexpected trajectory and hindsight proof that it wasn't known what the composition actually was before it passed the sun?

[Chris Peterson]

because you know the actual trajectory and speed etc, don't you know the mass?

No. The orbital parameters are independent of the mass.

[MarkBour]

[*]The sources I followed did consistently say it has higher outward velocity from the Sun than expected, not just a different angle or changing angle.

You mean orbital speed? Velocity is a vector.

Thanks, yes. I think that is the correct term, and is understood by astrophysicists. But as an amateur, I do find the term a bit misleading for this interstellar object, in particular the word "orbital". It is heading right out of here. Indeed, it is already way out of the invariable plane, even though it is not yet farther from the Sun than is Saturn.

How about this long-winded try? The observations put its velocity vector, relative to the Sun as a fixed frame of reference with v = 0 (how to draw a "hat" for the vector?) as having a higher magnitude than their celestial mechanics solutions produced based only on gravitational forces. (... and the magnitude of a velocity vector is speed.)

[MarkBour]

... It seems to me that this object is behaving somewhat like an out of control remnant of a designed lightsail might be expected to behave, being pushed by possibly both outgassing and solar wind/radiation pressure, or maybe just solar wind and radiation with no outgassing.

Note that I say it is behaving like an artifact might, not that it is an artifact. I still think this is a natural object, but there is a non-zero probability of it being a construction of some sort.

Bruce

Well, you're in good company in entertaining that thought, Bruce. I assume you know that Abraham (Avi) Loeb at Harvard co-authored a paper on Oumuamua, exploring that hypothesis.

[alter-ego]

But, isn't the solar system, as whole, moving as well? Can the difference in trajectory be explained simply by the relative velocities impact to relative distances between the Solar System and 'Oumuamua? Just asking. I know it sounds silly, but, hey, some one forgot to convert metric into imperial and pile-drove a probe into Mars. Someone should put the Solar System velocity vector in the animation.

No, determining an orbit requires accurate positions as a function of time, and as long as the model includes accurate ephemerides of all the known (interacting) solar system bodies, an orbit can be precisely determined. These positions are measured against the stellar background which is stationary wrt the sun, so the measured positions inherently include the combined motions of the solar system and the object. A couple things: First, the orbital deviation is asymmetric. It was not observed entering the solar system, only exiting it. That fact alone means another force is action on the body now that wasn't before. Second, consider a simple 2-body problem (almost the case here) being only the sun and a passer-by object. The gravitational interaction from the point of view of either the moving sun or separately moving body will map to a normal conic, even though to a wrt a stationary 3rd distant observer the motion may not be simple due independent motions. In fact, I wonder if Oumuamua's trajectory is deviating from a plane - seems likely it would be. Without a changing, perturbing force (e.g. outgassing after passing the sun), it's trajectory would stay in a plane.

[Chris Peterson]

[*]The sources I followed did consistently say it has higher outward velocity from the Sun than expected, not just a different angle or changing angle.

You mean orbital speed? Velocity is a vector.

Thanks, yes. I think that is the correct term, and is understood by astrophysicists. But as an amateur, I do find the term a bit misleading for this interstellar object, in particular the word "orbital".

"Orbit" describes the ballistic motion of a body. We are most familiar with closed orbits, where a body follows a closed, cyclic path around another. In a two body scenario, such an orbit is always an ellipse. But an orbit can also be open. In the two-body case, such an orbit is hyperbolic. That's what we have with Oumuamua. Its orbit is dominated by the Sun- very close to a simple two-body problem.

How about this long-winded try? The observations put its velocity vector, relative to the Sun as a fixed frame of reference with v = 0 (how to draw a "hat" for the vector?) as having a higher magnitude than their celestial mechanics solutions produced based only on gravitational forces. (... and the magnitude of a velocity vector is speed.)

Sure, but both the magnitude and the direction deviate from their expected values under a purely ballistic analysis. There's no reason to just focus on speed.

[Galaxian]

... Oumuamua doesn't have an unnatural shape, but is consistent with other bodies in our solar system.

You keep making this point. Can you show us any evidence of natural objects from our solar system with a 5 to 1 or larger aspect ratio?

<snip>

Thus, my point, that the light curve of this body is far from convincing if intended to send the message "I am artificial".

Would anything convince the "scientific establishment"? If the thing were perfectly smooth, perfectly cylindrical and flashing a sign that read (in Ancient) "I am an alien artefact. No, really, I really, really AM!" wouldn't you still be looking to hedge your bets, cover your tenure and protect your reputations by claiming that all of those are reproducible by bioluminecscent bacteria reflecting off of swamp gas in a trillion-to-one freakily accidental but barely possible way?

Not that I imagine it's true but wouldn't it be a neat idea were the "Wow!" signal a call home handshake from the first of these probes? A signal that the Earth just happened to dip into the cone of for a few beats before moving away from it.

I wonder how much evidence, apart from "a huge, great lot" would make that conjecture "convincing"? More than we will ever get from the poor efforts at SETI we are currently doing, I expect.

[Chris Peterson]

You keep making this point. Can you show us any evidence of natural objects from our solar system with a 5 to 1 or larger aspect ratio?

<snip>

Thus, my point, that the light curve of this body is far from convincing if intended to send the message "I am artificial".

Would anything convince the "scientific establishment"?

Sure, there are lots of things. Like some kind of radio or optical beacon. Or a significant non-ballistic orbital change. But something passive, like an odd light curve? That would be properly looked upon with a high level of skepticism. And really, why would an artificial probe that was intended to advertise itself use an ambiguous method of doing so?

[MarkBour]

... wouldn't you still be looking to hedge your bets, cover your tenure and protect your reputations by claiming that all of those are reproducible by bioluminecscent bacteria ...

I wonder how much evidence, apart from "a huge, great lot" would make that conjecture "convincing"? More than we will ever get from the poor efforts at SETI we are currently doing, I expect.

I'm not sure why you would have this attitude toward the "scientific establishment". They seem to be very reasonable people, and most especially balanced when taken as a whole. If there is any bias in the system that I sense, it would be that scientists experience pressure to say things that are new and disruptive, not the opposite.

[BDanielMayfield]

... wouldn't you still be looking to hedge your bets, cover your tenure and protect your reputations by claiming that all of those are reproducible by bioluminecscent bacteria ...

I wonder how much evidence, apart from "a huge, great lot" would make that conjecture "convincing"? More than we will ever get from the poor efforts at SETI we are currently doing, I expect.

I'm not sure why you would have this attitude toward the "scientific establishment". They seem to be very reasonable people, and most especially balanced when taken as a whole. If there is any bias in the system that I sense, it would be that scientists experience pressure to say things that are new and disruptive, not the opposite.

Yes Mark, they "seem to be very reasonable", right up until one makes a point in favor of a different explanation as to our origin.

Bruce

[geckzilla]

... wouldn't you still be looking to hedge your bets, cover your tenure and protect your reputations by claiming that all of those are reproducible by bioluminecscent bacteria ...

I wonder how much evidence, apart from "a huge, great lot" would make that conjecture "convincing"? More than we will ever get from the poor efforts at SETI we are currently doing, I expect.

I'm not sure why you would have this attitude toward the "scientific establishment". They seem to be very reasonable people, and most especially balanced when taken as a whole. If there is any bias in the system that I sense, it would be that scientists experience pressure to say things that are new and disruptive, not the opposite.

Yes Mark, they "seem to be very reasonable", right up until one makes a point in favor of a different explanation as to our origin.

Hi.

[Chris Peterson]

... wouldn't you still be looking to hedge your bets, cover your tenure and protect your reputations by claiming that all of those are reproducible by bioluminecscent bacteria ...

I wonder how much evidence, apart from "a huge, great lot" would make that conjecture "convincing"? More than we will ever get from the poor efforts at SETI we are currently doing, I expect.

I'm not sure why you would have this attitude toward the "scientific establishment". They seem to be very reasonable people, and most especially balanced when taken as a whole. If there is any bias in the system that I sense, it would be that scientists experience pressure to say things that are new and disruptive, not the opposite.

Yes Mark, they "seem to be very reasonable", right up until one makes a point in favor of a different explanation as to our origin.

What does that mean? People who study this have all kinds of theories they're looking at, and they shift around- the migration patterns of early humans, whether we left and returned to Africa, how we mixed genes with other human subspecies, etc. All of these are taken seriously and debated openly.

[geckzilla]

I think y'all know where this is going, and I'm asking nicely that you stop.

[GoshOGeeOGolly]

Avi Loeb has said that the observed acceleration requires 10% of the mass of 'Oumuamua to have been outgassed. So yes, very confusing.

Thanks for this remarkable and ultra significant bit of information. (As in Ultra .. you know .. the 'secret' Ultra.)

[Chris Peterson]

Avi Loeb has said that the observed acceleration requires 10% of the mass of 'Oumuamua to have been outgassed. So yes, very confusing.

Thanks for this remarkable and ultra significant bit of information. (As in Ultra .. you know .. the 'secret' Ultra.)

But it's still wrong as stated.

[GoshOGeeOGolly]

Would anything convince the "scientific establishment"? If the thing were perfectly smooth, perfectly cylindrical and flashing a sign that read .....

Galaxian please don't mistake the Scientific Establishment (which has landed a robot geologist on Mars) from a gang of argue junkies on the internet. The difference can be seen in the attitudes of those ON internet discussions who genuinely DO have advanced education on the topics .. they are far less firm in their opinions, far more considerate of other opinions in case that opinion does have or will someday have validity in them, more open to new evidence and ideas.

[GoshOGeeOGolly]

Thanks for this remarkable and ultra significant bit of information. (As in Ultra .. you know .. the 'secret' Ultra.)

But it's still wrong as stated.

Perhaps you would like to say why?

[Chris Peterson]

Thanks for this remarkable and ultra significant bit of information. (As in Ultra .. you know .. the 'secret' Ultra.)

But it's still wrong as stated.

Perhaps you would like to say why?

Because as previously noted, you cannot relate a change in mass to a change in orbit. The relevant parameter is momentum.