3 months of observing and 2 very slow spinning Asteroids

[higginsdj]

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'.

Cheers

[higginsdj]

Originally Posted by AstroJunk

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.

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.

Cheers

[mjc (Mark)]

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.

Mark C.

[higginsdj]

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.

Cheers