Some Interesting Details of How the Hubble Space Telescope Was Used to Discover Neptune's 7th Moon.
The paper I'll discuss in this brief post is this one: The seventh inner moon of Neptune (M. R. Showalter, I. de Pater, J. J. Lissauer & R. S. French, Nature 566, pages 350–353 (2019))
One of the greatest inventions of my overly long lifetime has been the CCD (charge coupled device) camera that has made it possible to convert light into digital data.
My own most immediate experience of this kind of device concerns immunogenicity testing, wherein a person's immune response to a protein therapeutic drug can either inactivate the drug, or even worse, cause a strong anaphylactic - at times life threatening - shock. In these cases the body's immune system generates antibodies to the drug, "anti-drug antibodies" and to detect these, one (proprietary) technology uses some nano/micro technology to affix copies of the drug onto a carbon surface, followed by treatment with the patient's blood, serum or plasma. The biological fluid is washed off, and the antibodies, developed to attach to the drug itself, attach. Antibodies have a "Y" shape, and attach to antigens at either arm of the "Y" meaning that if they attach to the bound drug, one arm (generally) remains free. In the next step, the drug, this time attached to a chemical species complexing a ruthenium atom, binds to the other arm of the antibody, whereupon, the application of an electrical charge causes the ruthenium to emit a tiny amount of light, probably not visible to the naked eye, but available to be detected by a CCD camera. This is a key technology in saving human lives.
One may ask what this has to do with the moons of Neptune, but in a remote sense it does have a connection, since the Hubble Telescope is chock full of CCD cameras, and the spec of light reflected off of Neptune's seventh moon, Hippocamp.
What is cool is that the detection of Neptune's seventh moon depended not just on optical detection, but on the processing of the digital data on which the tiny optical signal was abstracted from the "noise" light of the universe.
From the introductory text:
We have devoted three Hubble Space Telescope (HST) observing programmes to studies of the rings, ring arcs and small inner moons of Neptune. We used the High Resolution Channel (HRC) of the Advanced Camera for Surveys (ACS) in 2004–2005 and the Ultraviolet/Visual Imager (UVIS) of Wide Field Channel 3 (WFC3) in 2009 and 2016. Hippocamp, also designated4 as S/2004 N 1 and Neptune XIV, was discovered during a reanalysis of the first two datasets (Fig. 1a–c) and confirmed in the third (Fig. 1d).
Here is the figure to which the text refers:
Here is the caption:
a, View from Visit 04 of programme GO-10398, showing the earliest detection of Hippocamp, on 2004 December 9. Neptune is behind the HRC occulting mask. b, Visit 08 of programme GO-10398, on 2005 May 12. c, View from the first orbit during Visit 01 of programme GO-11656, on 2009 August 19. The grey vertical band is due to Neptune’s saturation bloom, in which the heavily saturated pixels of the charge-coupled device tend to saturate adjacent pixels above and below. d, Visit 03 of programme GO-14217, on 2016 September 2. Panels a and b have been rotated 90° anticlockwise. In each panel a small square locates Hippocamp, and a close-up is shown in the inset. Other moons and the outline of Neptune are indicated.
Some more text:
The long delay between the first image acquisition and the discovery of Hippocamp arose because of the specialized image processing techniques required. To detect a small moon in an image, motion smear should be limited to the scale of the point-spread function. For Neptune’s inner system, this limits exposure times to 200–300 s before smear dominates and the signal-to-noise ratio (SNR) ceases to grow. We have developed an image processing technique to push integration times well beyond this limit. Although the moons of Neptune move rapidly across the detector, that motion is predictable and can be described by a distortion model. Our procedure involves deriving a pair of functions r(x) and θ (x) that return the orbital radius and inertial longitude, respectively, as a function of the two-dimensional (2D) pixel coordinate x. The inverse function x (r, θ ) can also be readily defined. We derive the mean motion function n (r) from Neptune’s gravity field, including its higher moments5 J2 and J4. One can use these functions to transform an image taken at time t0 to match the appearance of another image obtained at time t1 by relocating each pixel x 0 in the original image to a new location x1:
x1=x (r(x0),θ (x0)+n(r(x o))(t1−t0))
(I had to suppress the conversion of equation notation into smiley's here, and the editor here no longer allows subscripts for some reason, following the election day attack on DU in elections Putin's orange nightmare puppy was installed in our White House. Imagine where the subscripts go.)
The next figure shows the image processing steps:
The caption:
a, Image ib2e02ziq_flt, the first in a sequence of eight long-exposure images from the second HST orbit of Visit 02 in programme GO-11656 (2009 August 19). b, Image ib2e02zmq_flt, taken 21 min later. Despina, Galatea and Larissa have shifted noticeably in position. c, Image from a, transformed to match the geometry of the image in b. d, The result of co-adding all eight images, revealing Hippocamp and Thalassa. The outline of Neptune’s disk, as distorted by the camera, is shown in each panel.
A little more cool stuff on the imagining procedure:
Although we were able to control Neptune’s saturation using the methods described above, glare from Neptune was ever-present and, as with all long exposures on HST, cosmic rays created a smattering of ‘snow’ atop most images (Extended Data Fig. 5a). Hot pixels fall at known locations in each image and are catalogued for each detector. Cosmic-ray hits were recognized as clusters of pixels in one image that differ by more than three standard deviations from the median of identical exposures from the same HST orbit. For cosmetic purposes, we overwrote these pixels with the median of the adjacent pixels (Extended Data Fig. 5b). However, we also kept track of overwritten pixels using a boolean mask and ensured that masked pixels were ignored in the subsequent data analysis (Extended Data Fig. 5c). We suppressed the glare and diffraction spikes by aligning the centre of Neptune in all the images from each HST visit that shared a common filter. We constructed a background image from the median value of all the pixels after aligning on the centre of Neptune. Unlike the mean, the median is not affected by moons (which move rapidly) or cosmic-ray hits (which are transient). The resulting images were therefore a smooth representation of Neptune’s glare and diffraction spikes. Subtracting the backgrounds yielded individual images that were almost free of distracting gradients (Extended Data Fig. 5d).
This process involves mathematical modeling of the orbital parameters, and the author's check the viability of this procedure using the previously discovered moons of Neptune, which are more or less satisfactory:
All orbits are in good agreement for Despina, Galatea, Larissa and Proteus. Naiad’s orbit agrees with the Voyager-era solution7 if one increases its mean motion by 1σ; the 2004 solution6 disagrees with this work because it includes an erroneous measurement. We also note that the orbit solutions for Thalassa appear to be diverging, although all solutions agree at the Voyager epoch.
The authors comment on the possibility of other moons:
The Voyager images established an upper limit of about 5 km on the radius of any undiscovered moons1 (assuming k = 0.09). That search was complete inside r = 65,000 km and partially complete inside 90,000 km. Between the limits of the Voyager search and the orbit of Proteus, we can now rule out any moons that are half as bright as Hippocamp, which corresponds to R ≈ 12 km. Beyond Proteus, our images are freer from Neptune’s glare and orbital motion is slower, making it possible to co-add larger sets of images (Extended Data Fig. 3).
The full paper contains some interesting commentary on the history of Neptune and its moons.
From the extended data, a graphic showing the "recovery" of Naiad, the first image on that moon since the Voyager flyby:
a, b, Portions of an HST image after processing and co-adding as described in the text. The location of Naiad in each panel is indicated by a small square; close-ups are shown in the upper-right insets. The outline of Neptune’s disk is indicated by a blue ellipse. a, View from Visit 01, orbit 1 of HST programme GO-11656, obtained on 2009 August 19. The image shows the first unambiguous detection of Naiad since the 1989 Voyager flyby of Neptune. b, View from Visit 08, orbit 2 of programme GO-14217, taken on 2016 September 2.
Wonderful science I think.
It is, albeit, an artifact of another age, a time when our country could do things like build the Hubble Space Telescope, before a fool set out to destroy this country aided by sick little racists and traitors.
It is strange to be surrounded by so much ignorance at the precise time when humanity has extended it's vision to the most incredibly small dimensions, and the most incredibly vast dimensions, literally, quite literally across the universe.
How odd it is that this is so that we can see so far out and so far in and so much in between still, even in a country ruled by a lump of mindless orange lipids with it's greasy, greedy, plasticine eyes fixated on its own ugly porcine navel and oblivious of the ugliness of its little nerves and oozing petty bigotry.
Enough of pain, enjoy the new moon.
Have a pleasant weekend.
