View Full Version here: : 3 months of observing and 2 very slow spinning Asteroids
02-01-2011, 01:26 AM
Technically they should not exist. After 3 months of observing these targets I have one sitting on 1332 hrs and the other at 622 hrs (oh and a 3rd at a paltry 54hrs)
The theory says that the solar system is not old enough for 'normal' damping to have applied to slow them this far and so far only tumblers have been seen with such slow spin rates but these 2 do not exhibit the traits associated with tumblers (lightcurve wise)
What a dilema. I have to report on my findings but with no explaination for my results other than they could be wrong....
Is there any statistics on asteroid spin rates, generally, to suggest the odds of such low spin rates happening by chance?
Also - if an asteroid's mass deviated significantly from uniform distribution such that one side was significantly more dense than the other then I suspect (but do not know) that damping will kick in quicker.
And - of course - your data could be wrong (but hopefully not).
Keep us updated - this is interesting.
02-01-2011, 08:38 AM
Other than collisional, YORP is the only effect that will dampen an asteroids spin rate. Current thinking is that YORP affects asteroids on the inverse square of their size and the inverse square of their semi major axis. The 1332 hr target is not less than 83km (spectrally a C type but measured albedo is considered too high for a C type so the diameter measurement is a limiting minimum) and an outer main belt asteroid and as a slow rotator "challenges" the current paradym!
Given the current understanding of the formation of asteroids, significant non uniform distribution is unlikely, even in a contact binary case.
If the objects spin rate was artificially reduced by collision, one would expect tumbling as a result. However, for an object large enough to cause the spin rate to reduce significantly one would expect a large impact crater and with a large crater one would also expect a large amplitude in the lightcurve. This target has only shown amplitudes of 0.02 to 0.14mag in 5 sets of observations since 2000 by 4 different observers. Previous observations were less than complete generating periods from 6 to 48 hrs with low reliability.
02-01-2011, 08:39 AM
Why would this be the only explanation ?
There must be limitations in what you can detect with your observation/measurement setup. (??)
If you can't detect something, how can you explain the observations you have as being incorrect ?
02-01-2011, 09:37 AM
As you suggest Craig there are limitations to what I can detect. The spin parameters of several thousand asteroids are known and the current scientific explanation for very slow spinners is YORP or Tumbling. But the YORP theory only works for small inner main belt (or closer) asteroids and then the amount of dampening is limited to the age of the solar system.
My observations are in contrast to the paradigm which, given the situation that the target is low amplitude (only 3 times the catalogue error - on the lower limit of acceptable) and the noise in my data has a typical RMS of 0.015 - 0.018mag, means the periodicy that I see could just be coincidence or convenient patterns in the "noise".
If I can't explain "how", my data is then called into question (such is the life of an amateur) which is rather annoying after 3 months work.....
Within the next 2 years the current 2MASS catalogue will be replaced by the APASS catalogue and the catalogue errors will be reduced from 0.05 to 0.02mag. A re-measure with the new catalogue will provide more reliable results.
(FYI - the 2MASS catalogue and algorithms inside MPO Canopus allow observers to take unfiltered observations and link nightly sessions without the need for absolute photometry. The results are not on the standard system but they are close and it also allows different observers using the same process and catalogue to link their sessions to within 0.05mag. Gone are the days of artificially altering the magnitude zero points of data sets to align sessions to fit curves and determine periods)
02-01-2011, 10:06 AM
Have your masters ever considered outgassing ?
How about the breakup of a larger body? Surely there would be some fragments that would be left turning very slowly?
02-01-2011, 10:50 AM
Outgassing - no. In my view that would be pure speculation. The problem with outgassing is it is rare (only 7 known MB objects have exhibited this behaviour) and we would expect it to occur randomly and over a very short duration (months only) after some sort of collision with another body (also rare), it's not like there would be a sufficient 'localised source' that would act over a period of My to dramatically slow the spin rate. Mind you it will be interesting to measure the spin period for 596 Scheila after its current activity is over to see if there was any impact though it's current spin period determination is not well established.
Break up of a larger body would be the result of collision and thus expected to induce tumbling rather than 'simple' primary axis rotation which appears to be what I have observed.
02-01-2011, 10:58 AM
Craig, I'm intrigued by the outgassing idea, how good is your maths? Lets take an 83km sphere at 2g/cm^3 density and work out how much energy would be required to slow the asteroids rotation from 3/d to 0.02/d over a period of say 4By.
02-01-2011, 06:03 PM
The difference in Rotational Energy between 3 rev/24hrs and 0.02 rev/24hrs is approx 9.83 x 10⌃18 Joules, by my reckoning.
The question would seem to be what outgassing thrust would be required, over what period of time.
The bigger questions are how should the thrust be aligned relative to the axis of rotation, what type of molecules should be assumed and what outgassing rate seems reasonable.
(Don't ask me for the answers to these though … ;) :shrug: )
PS: This is a pretty big Asteroid !! Had it been smaller, outgassing alone, would seem to be reasonable. As it stands, I think something else would have to be at play.
02-01-2011, 08:24 PM
Thats 78 joules per second or in terms we can all picture, the energy of a tennis ball moving at 500 metres per second, for every second over the past 4 billion years. I can only assume that the energy calculated would be what is required in direct opposition to the asteroids spin. Given that jets would be very unlikey to outgas at such an angle, the energy output required would be considerably greater...
02-01-2011, 08:27 PM
Its a pretty massive asteroid.
.. Gravitational disturbance ???
Breakup could also be from gravitational tidal forces , ie a close approach to one of the other planets... ? Not just from collisions...
Another thought... also gravity based, if the asteroids are very irregular in shape then they're susceptible to tidal-locking effects when they pass by other objects... over a long period, and regular close approaches to other large objects the rotation period might change? Just a random thought...
03-01-2011, 12:25 AM
Those objects would be expected to have a high eccentricity. If the target was that irregular then it would have a large lightcurve amplitude. Although possibilities for NEOs neither scenario seems very likely here.
But it is an outer main belt object...... some sort of gravitational tugging from Jupiter? I wonder how close it is to a resonance orbit? (thinking out loud here)
03-01-2011, 09:03 AM
So, the spin rate distribution is just that .. a distribution. Whilst it seems that perhaps, size-dependent mechanisms (such as YORP), appear to result in population of the outer regions of the distribution, there still seems to be unexplained phenomena, such as the occurrence of large asteroids having slowed rotation.
It would seem that 253 Mathilde is one example.
Perhaps the 'Higgins anomaly' has other cousins out there .. both discovered and undiscovered. (?)
PS: My point here, is that just because David's observations don't confirm the theory, doesn't mean they are 'in error'. I don't believe anyone should accept this (provided of course, you are absolutely confident that they aren't in error). As I mentioned, all measurements are subject to 'uncertainties'. This doesn't mean they are in 'error'. Uncertainty is quantifiable.
03-01-2011, 05:15 PM
Some questions to ask:
1. Is a 3 month period long enough to establish such long periods, especially since the amplitude is near the lower limit of measurement based on catalogue error.
2. Can you exclude a precessional effect over a 3 month arc. How has the asteroids observation geometry changed in that time?
3. Why do we assume that the asteriod had a defined starting rotational speed at formation? I'm not up to speed on this theory.
03-01-2011, 07:18 PM
The 1332hr period was covered 1.5 times, enough to establish a period, just not refine it.
Given we know nothing about the targets shape or spin axis, all we know is the variation in phase angle. In this case there was only a 15 degree phase angle change over the period.
All asteroids started spinning but I don't know the theory or reasoning.
Note that My 'masters' have ruled out the effect of gravity from planets.
I should point out that I am an observer and not very strong on the theory/science.
03-01-2011, 07:26 PM
253 Mathilde is in the sub 500hrs category, there are a couple of dozen slower objects, at least 4 slower than 1000 hrs and I have another new one sitting at 620hrs. As my masters say.... more data!
04-01-2011, 01:32 AM
All data should be called into question and it is the quality of your data which is your legacy, not any conclusions that you may draw. Actually, that is true for all observational astronomers, myself included.
I don't believe that non full time physicists (paid or otherwise) are often in a position to explain how. It is important to present the data and method to others in a timely manner so that others can contribute, analyse or criticise as they see fit.
What's the difference between spin and tumble?
And possibly a silly question - is it possible that we have an aliasing effect with the measurements - ie observations are seperated at a frequency close to a multiple of the asteroid spin rate such that we get curve which isn't true. I could be off on a complete tangent here but what, as an example, if the spin rate were some large - but reasonable multiple of 24hr and we just managed to sample it at close to varying multiples 23hr 56min - is it possible that we get a banjaxed - but convincing long-period light curve?
I have no experiencing with this so I don't want to sound like I actually know what I'm talking about but present the question in case it has any value.
04-01-2011, 11:26 AM
99.99999% of asteroids are principal axis rotators. That is, they spin on one axis only. Tumblers spin on 2 or more axis'.
As to your second point - yes that is a very valid point and has been considered and why we review the raw data to see if the calculated periodicy seems valid. In this case, the raw plot of data showed 3 well spaced maxima and minima so I would expect the bimodal period to be approx 2/3rds the observational period.
The aliases are more sinister than your 24hr example. One observer who I am mentoring derived a solution for a target very close to 6hrs. Unfortunately his observational sessions also spanned 6hrs and they were spaced 24hrs apart. In this case the fourier analysis picked up on the wrong period (6hrs) because from the 3 sessions of data it was the strongest 'beat'.
04-01-2011, 11:37 AM
Agreed, it just leaves one in a 'precarious' position. I don't do this for a living, so have I overlooked something, have I made a mistake somewhere... The work we (amateurs) are doing on these long period targets is 'borderline' (bleeding edge analysis routines and catalogue and on the limits of data acceptability error wise). We need to be careful not to overstep these boundaries as we lack, in general, the training and skills to justify ourselves (and in some cases to realise that we have overstepped the boundaries)
It is this lack of, for the want of a better word, 'ability' to identify that they have 'gone too far' that puts a lot of pro's in the professional 'dog house' and why I think many Pro's dismiss the work of amateurs.
Davd thanks for your response.
If you can indulge me on the the spin vs tumbler idea a little more.
I can't visualise any difference between a tumble on two axes as being [distinguishable] from a spin on a common axis.
If I spin a sphere in space such that it spins - then graze the sphere such that I impart an additional spin about a different axis then surely the sphere will have one common spin about an axis different from the original two.
Could it be that with an irregular shaped asteroid one would normally expect a spin coincidental with a geometric feature - but the tumbler is one in which this is not the case?
If this gets too involved then don't worry about it - I'm not going to dog you - but if there is a simple answer I think more than I will profit from it.
04-01-2011, 03:31 PM
Actually you are talking about shifting the spin axis rather than a tumble. A tumble will induce a secondary rotation on a second axis.
Very few asteroids are spherical. If they were then there would be insufficient detail to generate a lightcurve. Asteroids, in general, are potatoe shaped and cratered. As such, there is enough 'detail' to detect variations in amplitude as low as 0.02mag, even when viewing the asteroids spin axis directly.
Now a potatoe shaped object will usually result in a bimodal curve. If its a tumbler there will be 2 overlapping periods that results in a 3rd combined period, but it depends on how strong the second axis rotation is. I have done a dual period search on my target and uncovered a possible combination of 287 hr and 130 hr. After subtracting the 130 h period, the 287 h period fits a nice curve, but the 130 h component is a bit ugly.
The primary spin axis is usually through the shortest axis. The secondary component will damp out over time, generally not that much time is required.
I did a bit more research and found another mechanism to slow an asteroids spin rate. Its called angular momentum drain and is the result of a spinning object losing surface ejecta over long periods of time. For my target, the expected dampening limit is between 600 and 700 hrs - gets me halfway :)
The other interesting thing is that there is no current working model describing what effect collisions may have on spin rates.
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