KBO encounters Options

[HSchirmer]

Found some prior info about NH using at Pluto expecting to use radar from Arecibo

Transmission, in a word wow
The observatory has four radar transmitters, with effective isotropic radiated powers of 20 TW (continuous) at 2380 MHz, 2.5 TW (pulse peak) at 430 MHz, 300 MW at 47 MHz, and 6 MW at 8 MHz.

I never realized Arecibo could hit 20 terawatts at the 12cm wavelength, was that an upgrade for near-earth-asteroid searches?!?

[scalbers]

TheSkyNow is not really indicative of what a deeper, more sensitive digital survey "looks like" -- where things like saturation and blooming of CCDs become worse, due to longer exposure times, CCD thermal and read noise and shot noise become problematic, images of individual objects can fall into the boundaries between CCD cells, etc. etc.

True enough. Here's a loosely related example of a Zooniverse search project for new distant and/or moving objects that has digital imagery, though using an IR detector.

[siravan]

Transmission, in a word wow
The observatory has four radar transmitters, with effective isotropic radiated powers of 20 TW (continuous) at 2380 MHz, 2.5 TW (pulse peak) at 430 MHz, 300 MW at 47 MHz, and 6 MW at 8 MHz.

This is the effective isotropic power, i.e., it assumes that the radiation was emitted uniformly in all directions. Arecibo emits a very narrow beam, so the actual power is this number multiplied by the ratio of the beam area in steradian to 4pi.

[fredk]

And of course Arecibo is limited to roughly the celestial equator, while NH is well south of that at around -20 deg Dec.

[Explorer1]

Looks like occultation searches for new objects are actually quite plausible, seems a 1.3 km candidate may have been detected in data from a large number of telescopes: https://www.nature.com/articles/s41550-018-0685-8

[JRehling]

The occultation detection is quite exciting, but aiming the search to the cone that New Horizons can reach would require a much higher density of KBOs than the detection of one, once. Also, detection via occultation is not a repeatable event, and it would be remarkable if that gave enough information about the orbital parameters to allow the planning of a flyby.

[HSchirmer]

The occultation detection is quite exciting, but aiming the search to the cone that New Horizons can reach would require a much higher density of KBOs than the detection of one, once. Also, detection via occultation is not a repeatable event, and it would be remarkable if that gave enough information about the orbital parameters to allow the planning of a flyby.

But, still good to see that observers are moving from "Olber's Paradox" towards "Olber's Parrallax".

Seems counter-intuitive, we ARE aligned to see a finite number of dark KBOs move across a galaxy of (eh) ~300 billion observable stars, you'd kinda-sorta figure we'd have enough "back lighting" to see more objects...

[AJAW]

The occultation detection is quite exciting, but aiming the search to the cone that New Horizons can reach would require a much higher density of KBOs than the detection of one, once. Also, detection via occultation is not a repeatable event, and it would be remarkable if that gave enough information about the orbital parameters to allow the planning of a flyby.

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?

[ngunn]

If you are looking for kilometre sized objects you would need a telescope in every square kilometre or so over a wide area. You would then get good values for the object's likely size and shape, along with a snapshot of its position and angular velocity. That's not enough to determine the orbit, though. For that you need observations over a much longer period than the time it takes for an occultation to sweep Earth's surface.

As to the availability of 'back lighting' just look at the apparent surface brightness of the Milky Way and compare it to the surface of the Sun. That's how much our sky is undersampled with light sources for occultation. A huge marority of dark objects of whatever size will simply slip through unnoticed, like neutrinos through a planet.

[Gerald]

An essentially one-dimensional bright line in the backgound would result in a black mean background. Nevertheless, we could theoretically find all black foreground objects crossing this line with a probability of 1 with just one telescope. The telescope would need to be designed in a way, that it can observe the whole bright line more or less continuously. So, the question of how to distribute a set of telescopes the most efficient way for a known set of background stars and for an unknown number of dark foreground objects appears to be a little less trivial.

[ngunn]

It's an interesting question - if all the stars in the Milky Way were collapsed onto a single line in the sky how optically dense would that line be? Also (arguing against myself and with Gerald here) the probability of an occultation will be enhanced by the looping track of the KBO produced by parallax. Here is a nice diagram of the path of Pluto:
http://www.nakedeyeplanets.com/path-of-plu...06-22-s-hem.png

[AJAW]

An essentially one-dimensional bright line in the backgound would result in a black mean background. Nevertheless, we could theoretically find all black foreground objects crossing this line with a probability of 1 with just one telescope. The telescope would need to be designed in a way, that it can observe the whole bright line more or less continuously. So, the question of how to distribute a set of telescopes the most efficient way for a known set of background stars and for an unknown number of dark foreground objects appears to be a little less trivial.

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.

[JRehling]

Great discussion. Little bit of astrophysics background (literally and figuratively): Unlike in the solar system, or looking out with naked-eye resolution into the night sky, galactic-scale empty space in the direction of Sagittarius is not optically null and void. The background does not consist of an idealized plane of pointlike sources of light but, in principle, a blotchy continuum of variable illumination. Individual stars co-align but also are hidden to varying degrees by dust and gas. A picture of this is here:

https://upload.wikimedia.org/wikipedia/comm...e_Milky_Way.jpg

In some cases, you almost have an inversion of the usual case: You're looking for a dark pixel moving across a known background.

A real quirk of the first KBO mission is that it is headed out into this small patch of sky with the brightest background possible. If the first Pluto mission had occurred when Pluto was basically anywhere else in its orbit, the circumstances of the KBO searches would have been quite different.

[ngunn]

Can you elaborate on the statement 'Individual stars co-align'? Do you mean that we see mutual eclipses of random pairs of stars in these star clouds? I was not aware of this and would find it very surprising if true. My understanding has been that the apparent crowding of stars in images such as the one you post has to do mainly with each star appearing enlarged by diffraction and other effects. As to background illumination from dust and gas, surely a putative KBO a few km across would not be observable in silhouette against this ???

[JRehling]

Sorry to be unclear. What I'm referring to showed up in the Kepler analysis, though the specifics were different: Sometimes two stars' alignment will be close enough that they cannot be distinguished, with ground telescopes of any kind, from a single star. Effectively, you have a pixel of a certain brightness and a transit, instead of blacking a star out to zero will diminish the brightness of the pixel from a nonzero value to some lesser nonzero value. In principle, the occultation could become non-boolean: a given observer could see an occultation begin as one star is occulted, then deepen as the second is, then slack off as the first ceases to be occulted. And this could be true of more that two stars – four, six, eight – there is no limit.

In this paradigm, with an unknown astrophysical reality behind the observation, it could be very challenging or even impossible to interpret what observers recorded. Perhaps the KBO could be tracked as it moved from pixel to pixel – but I suspect that signal to noise ratios would make a conclusive interpretation challenging.