My understanding was that they didn't yet have the telemetry for the last 200m or so of the descent, so they didn't know. Everything had looked nominal up to that point. It was an excellent and informative press conference, as has been said. The small team means that the top guys know the lower-levels details. And they didn't mind sharing them.

What is the very small, roundish bright thing (floating against the black background) visible in three of the images?

I think the first three octal digits are the exponent (power of two) with an offset applied (which I'm too lazy to work out) followed by the mantissa. The examples do not seem to be the exact number of days I was initially expecting; the bit patterns seem to match the number of seconds part-way through the specified days. They do not seem to be an exact number of seconds, either, so presumably this format recorded higher-precision times than mere seconds.

Thanks for the tip, Alan, I've just tried it.

There might be a problem with the logic that led me to believe this was a promising line of thought. I was inspired by the detection of Ultima Thule, which is one of a very numerous type of object, and by the later occultation observations that pinned down its position and shape. I figured that meant that occultations by such objects must be visible using fairly cheap telescopes very frequently... and that even a fixed line of scopes would have occasional occultations visible from their location. That still sounds right. Crucially, though, observers of Ultima Thule knew WHICH star to look at for that occultation, thanks to Hubble. To monitor lots of stars at once, you might well need better telescopes.

However, I'm intrigued by a recent report in Nature Astronomy of the detection of a 1km KBO using two cheap telescopes - so perhaps I'm again confused! Unfortunately I cannot read the paper, but if anyone is able to share the gist of it that would be great. Many thanks for the patient clarifications I've had here, by the way.

Got it. I was confused by an earlier post that I now realise was to do with background galaxies. And I understand that an occultation from one spot can only give you position, not direction. But if you observed the same occultation from two places, and the second one happened, say, 20 seconds later, because the places were far apart, don't you get two position estimates that reflect the distance the KBO travelled in that time? I guess that would be only about 100kms, and I'm not sure what accuracy your position estimates would have, but at least in principle that would give you directional information. Or am I missing something?

One aspect of signal extraction from the noise that may help a little, is that we have some idea what to look for. It reminds me in some ways of LIGO's extraction of gravitational wave signals from their raw data. They had modelled (if I recall correctly) thousands of possible waveforms that they could look for, and they knew that a similar waveform would appear within a fairly narrow time window at their second detector. This helped detection a lot.

If a KBO passes over a star, there will be a probably quite small change in brightness in the detector pixels that are influenced by that star, but it will be abrupt, and it will be followed (probably, and dependant on typical shadow paths) by changes due to other stars in predictable positions in relation to the first star, being occulted. (I'm assuming here that we have a good star catalogue that allows simulation of such events.) As the trailing edge of the shadow passes, we get a predictable reversal of the changes. In neighbouring telescopes, we get related changes, which will not be exactly the same, but will have some spatial and temporal correlations to the changes seen by the first telescope. My hope is that the combination of multiple correlated small changes in meaningful patterns may be detectable, in situations where any one change on its own would not be.

Will our estimate of the rotation rate ever become good enough to look back at the occultation results from last year and work out exactly how many rotations have happened since?

Would that still be true if multiple observers saw the same occultation?

If we had an elongated array of small telescopes (with the long axis at right angles to the most likely shadow paths) would we expect to get some detections with a good enough velocity accuracy (and fortunate orbit) that we could say, "we are in luck, this object may occult another detectable star from our array in about X weeks, so we will have the array look at that star at that time"? Obviously it would help if the array was very long....

Thinking about this a bit more, probably the best strategy would be to have two long 'picket lines' of telescopes, with the lines as far apart from each other as you could get whilst retaining a good chance that both would detect the same KBOs. That would maximise the time difference between your two position fixes and allow the greatest movement of the KBO between detections. (Which would be at what speed? 5 km/sec or so?) I assume that the greatest contributor to the shadow passage speed is actually the Earth's orbital velocity, not the KBO's speed.... but this will vary with the height above the horizon of the observation, I guess. I'm thinking that the times between position fixes could be of the order of 10s of seconds, which isn't much motion for the KBO. On the other hand, I was impressed by how well the now-known outline of Ultima Thule fitted the occultation data, so I'm hoping that position estimates can be pretty good.

Would it be feasible to deploy a whole matrix of many small telescopes in order to determine a trajectory as an occultation event swept across them?

Thanks fredk. Pity about the accuracy change. One factor in our favour (but unlikely to be big enough) is that the sky areas most likely to contain planet nine are densely populated - a nuisance for
most searches but an advantage with this one.

If Gaia is aiming to get proper motions for all the stars it looks at (with 70 observations of each one) I guess we could distinguish that from the lensing effect, if there is any.

Thanks for the info. The size of the gravitational lens is indeed small.
I have two questions:
Why are we talking about mag 14 stars? Gaia can go down to about mag 20.5, there ought to be a lot more of those stars.
Also, why twice the mass of the Earth? - I thought planet nine might be 10 times the mass of the Earth.

Two relevant things:
First, Mike Brown and Konstantin Batygin comment on the Malhotra, Volk & Wang ArXiv e-print here:

http://web.gps.caltech.edu/~mbrown/papers/ps/findp9.pdf

(Near the end of the paper.) Their final relevant sentence is this: “Thus, it appears that no useful constraint on the orbit or position can be drawn from this method.”
I’m not qualified to evaluate the arguments, but from their blog postings I’ve formed the impression that Brown & Batygin are very competent and cautious scientists and I’m biased in their favour.

The second thing is something that I discovered when I was fortunate enough to attend the British Astronomical Association one-day meeting on Robotic exploration of the Solar System on the 30th
April. (Incidentally James Canvin of UMSF gave a great talk on “Amateur use of Solar System spacecraft data”.) What attracted my notice Planet Nine-wise was a talk by Prof. Mark McCaughrean
on ESA’s Solar System exploration programme. (He is the Senior Science Advisor in the Directorate of Science & Robotic Exploration at ESA.) When talking about the Gaia astrometry mission,
he mentioned that the gravitational lensing effects of the sun and planets have to be removed as part of the data processing – the sun’s effects are practically 360 degree, and those for Jupiter are
very large, but I believe they do it for all the planets.

After the talk, I asked him if they could potentially detect the gravitational lensing effects of Planet Nine. He thought that they probably could – if they already knew where it was!
However, he thought that the volume of the data and the small signal would make it very difficult to extract without knowing where to look.

He was answering after just a few seconds thought, and I still wonder if there might be some hope in this method – after all, we should be able to calculate roughly what the lensing signal
would look like, and it’s moving, so Gaia, with multiple observations of the same patches of sky, should have observations with and without the planet, at multiple locations that have a
roughly predictable separation from each other. But I don’t know anything about the signal to noise ratio, or the data set. For a billion stars, I guess it’s pretty big!

Can anyone give a more informed opinion on this?

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