Generally brightness in radar-derived images corresponds to reflectivity. It is possible that what we’re seeing as the top of the asteroid in this perspective is simply more reflective after signal processing to account for the graze angle.
It’s us humans that are interpretting that reflectivity as meaning “lit from above”.
Back in the day in the military we used ground mapping radar to navigate and to target. They had apps that could generate a pretty good simulation of what would be “seen” by the radar and how it would appear on the scope. All while driving in at a very low altitude and hence low look angle.
Like interpreting IR imagery, it took awhile to really grok what light and dark or smooth or mottled corresponded to in the naturally lit visual scene. And what borders between different sorts of signal return really meant.
IMHO that is a good reason if there is no intention to fund scientific research outside of American territory with virtually no representation.
As an aside, the observatory is not just the Radio Telescope, other facilities like a LIDAR center were damaged but expected to continue to be funded with a consortium of universities like NSF and UCF, who decided to intervene back in 2018 to keep funding going for the observatory that the current administration in the USA did not see as being important.
As the radio telescope was also involved more recently in the search of asteroids that could destroy cities or make life miserable to all on earth, I’m sure for the current administration it made a lot of sense to not fund things that are truly involved in our security, while choosing to fund the expansion of a less needed and controversial wall at the US border.
Besides its size, Arecibo was unique in one important way: it could generate signals too. Virtually all other radio telescopes are only passive receivers of signals. Arecibo was originally meant to study the ionosphere and it had to generate EM waves to do that. It quickly was repurposed to do regular radio astronomy, but also to do some active observatons. For instance, one of the early (mid-60s) results was to do radar studies of Venus that determined that planet’s rotation rate. Venus’ atmosphere is featureless in visible light, so we had no idea what its length of day was. Sending radio waves to Venus and receiving the return established that it rotates slowly in a retrograde direction.
Probably not. In these highly tribal political times, statehood depends more on politics in Washington DC than in PR itself.
The more I think about it, the more it seems you must have hit on the answer. What else would we expect them to do? It’d be pretty weird looking if they encoded the perpendicular-to-screen dimension with grayscale, more like something translucent under the microscope.
LSLGuy, good though, but isn’t it rotating? The bright area in one frame would be someplace else in any other frame.
If tumbling I’d expect the bright area would be in a random spot from image to image. If the object has a single clear axis of rotation they may have rectified that to “up” in the pix and then we’d be looking at the “north polar” region. I’m not suggesting that pole would be icier than anywhere else, but rather that if that polar area was smooth, it would look pretty similar regardless of which longitude we were sighting down.
In all, I’m conflicted. My original interpretation seems increasingly far-fetched. The rest of the images seem to show a tumbling motion of the obvioulsy potato-shaped asteroid.
But why would somebody trying to generate images with actual science content in them apply such an artificial illumination scheme? Although I suppose most of these projects also know PR is an important function and will create some images mostly for that purpose. Which these might be.
Then again, compare the 2nd image in the 2nd row with the e.g. 2nd image in the third row. The tilt of the long axis is opposite from our POV. As if we’re seeing the object from more or less opposite longitudes. And we see a very different distribution of the light area. It’s large and facing us in row 2 and small and just barely peeking out from behind the right side limb in row 3.
IOW, if it’s “artificial” lighting, the one in row 2 is lit from above and straight ahead while the one in row three is lit from above and behind; well behind.
Such fun, but darn hard to give any kind of conclusive answer or even opinion.
Napier’s idea that they did it to encode depth in a way that would be naturally understood by someone outside the field makes sense to me.
But, if that is the case, I think such conventions should be specified on the image somewhere. It can be a footnote or something, same as it should be when the image uses false color. Then we woudln’t need to wonder how real it is.
No, the towers that anchored it are either down or structurally damaged. As are some of the buildings hit by falling debris.
I think putting a new radio telescope there would require nearly razing the entire area then rebuilding from scratch. That’s do-able, but it will take a huge amount of money.
The question is whether Arecibo or whatever would replace it is worth spending the money on in light of today’s other science priorities. The current high-priority projects in astronomy are the new generation of giant telescopes like Magellan, the TMT and the LSST. The James Webb telescope is way over budget and eating into other science, and WFIRST is on the cancellation bubble constantly.
It remains to be seen if Arecibo is worth the reinvestment. Maybe one of the billionaires will save it as a pet project.
Radar images of Comet 209P/LINEAR taken from May 23 through May 27, 2014. The Earth is at the bottom of these images: the “side view” is a result of the radar imaging method. Several features are visible on the comet, perhaps ridges or cliffs. This is only the fifth comet nucleus imaged by Arecibo in the last 16 years, and the most detailed. Resolution in the vertical direction is 7.5 meters (25 feet) per pixel. Image credit: Arecibo Observatory/NASA/Ellen Howell
So, as you expected, brightness is almost certainly radar reflectivity. But what none of us got is that the image isn’t facing us! Instead it’s presented such that the transmitter (Earth) is coming from above or below. That also explains why the illumination is exactly perpendicular–if it were totally artificial, one would expect them to pick a light source that’s a little more toward us, so that we could see inside craters that were exactly facing us. But that’s not possible with this method. We’re seeing it at exactly a 90 degree angle.
If one wanted to do something to support the PR economy, something that is small, very local, very expensive, and is composed almost exclusively (by tonnage or by dollar) of materials, equipment, and skilled personnel sourced outside PR would be a spectacularly bad (or at least inefficient) way to go about it.
The same money invested in roadways, upgraded electrical infrastructure still reeling from Hurricane Maria 3 years ago, increasing spending on schooling or college scholarships, etc., would deliver vastly more long-term economic bang for the buck.
I’m all for radio astronomy IF the Arecibo site can be made useful given the current state of the art in radio astronomy and trends in astronomy in general.
But rebuilding Arecibo just because it was groundbreakingly useful 50 years ago and only recently collapsed is not a good use of the US astronomy budget nor the US PR aid budget.
I know I don’t know enough to judge the astronomy merits of any rebuild. But the folks who do know are almost certainly going to be involved in the decision.
Excellent find. Which explains it exactly. Thank you!
The pix in your link made it obvious to me and the captions, as you cited, totally confirm that. The images there, where the radar signal is coming up from below look exactly like ordinary ground mapping radar images of rough ridgy terrain. Which of course it mostly is. Do’h! It was obvious once I saw that.
Those images are either what we call a narrow segment PPI or a B-scope. Although at long (!!) range the difference is immaterial for our purposes.
In either case our transmit source is at bottom of the screen/image, and our POV into the image is looking “down” on the fan of radar energy flowing up from the bottom, hitting the object, reflecting back down, and being received. This registers azimuth as left-right, distance as up-down with up = farther, and reflectivity as brightness. It’s an XY plot that you’re viewing from high up on the Z axis looking back down towards the Z=0 plane where the energy is.
The first link to the montage of “top-lit” tumbling potatoes buffaloed me because nobody but nobody displays a B-scope or PPI with the source = “you” at the top. They inverted the images to give us the familiar lit from above experience. Which, at least for me, created more confusion that it solved.
I love sleuthing out a problem like this. It’s like stone soup where each of us brings our tidbit to add to the shared pot.
The capital repairs funding that was in the bank, originally appropriated during FY18 for hurricane damages, was around $15 million and some of it was already being used, but that was for what was the hurricane damages and regular deferred maintenance, there was no provision for major critical structural changes.
After the cable failures that destabilized the structure, numbers (almost surely back-of-envelope) were thrown around informally that proper dismantling/removal of the at-risk structure and repurposing and refurbishing of what parts of the observatory would be retained, would be in the neighborhood of $51 million, vs. over $300 million to “save” the RT (taking it down, refurbishing, upgrading and replacing everything that needed to and putting it back up). And that was before it was ruled that it would be too dangerous to have people doing the work on the structure. That and how with newer technologies you could perform much of the science with other types of installation for less cost, were part of what had led NSF to write it off to begin with.
Now if you wanted to still have the single-aperture large-dish instrument, as you said they’d really be facing a situation of “clear the site and build a new one”. Surely significantly costlier than any of those early spec estimates.
God, how I hate that the different leaders in the Island keep counting on this.
Quite so. Truth is, NSF has been looking around for someone to take the cost of operations off their hands for years, especially since the operation consortium with Cornell ended in 2011. The latest search for an academic partner, that placed it under U of CF in 2018, was predicated explicitly on no expectation of increased funding (UCF would get 4 million a year for 5 years).
Indeed! Your original idea was closer to the mark than mine was–which was, after all, more or less a situation of “if all you have is a hammer,” since my background is in computer graphics. This visualization, as it turned out, was very much more in the analog domain. But despite my idea being vaguely plausible, little questions you raised kept niggling at me. So I searched around for how Arecibo does their radar processing and eventually came across this.
In fact, I was initially searching for the actual radar data sets, hoping they’d be public information. Didn’t have any luck there, but the searches led to other things.
One question, since their visualization has now clicked for you and you can apply your significant experience–how do they distinguish between the front and back sides of the target? Or do they, and what we’re seeing is actually the merged view, without any removal of the far side? Maybe some trick with polarization?
" The world’s largest single-dish radio observatory is preparing to open to astronomers around the world, ushering in an era of exquisitely sensitive observations that could help in the hunt for gravitational waves and probe the mysterious fleeting blasts of radiation known as fast radio bursts. The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) in southern China has just passed a series of technical and performance assessments, and the Chinese government is expected to give the observatory the final green light to begin full operations at a review meeting scheduled for next month… The complex project has not been without challenges — it has a radical design and initially struggled to attract staff, in part because of its remote location. But the pay-off for science will be immense. FAST will collect radio waves from an area twice the size of the next-largest single-dish telescope, the Arecibo Observatory in Puerto Rico."