Which Way Is Up?

“OK, Moire, the Attitude Control System’s reaction wheels swing James Webb Space Telescope through whatever angle changes it wants, but how does ACS know what direction JWST‘s at to begin with? Does it go searching through that million‑star catalog to find something that matches?”

“Hardly, Mr Feder, that’d be way too much work for a shipboard computer. No, ACS consults the orientations maintained by a set of gyroscopes that are mounted on JWST‘s framework. Each one points along an unvarying bearing relative to the Universe, no matter how the satellite’s situated.”

“Gyroscopes? Like the one I had as a kid? Winding the string around the axle was a pain and then however hard I pulled the string I couldn’t keep one going for more than half a minute. It always wobbled anyway. Bad choice.”

“Not the JWST choice, NASA mostly doesn’t do toys. Actually, the gyroscope you remember has a long and honorable history. Gimbals have been known and used in one form or another for centuries. A few researchers mounted a rotor inside a gimbal set for various purposes in the mid‑1800s, but it was Léon Foucault who named his gadget a gyroscope when he used one for a public demonstration of the Earth’s rotation. People used to go to lectures like we go to a show. Science was popular in those days.”

“Wait — Foucault? The pendulum guy?”
 ”Wait — Foucault? The knife‑edge test guy?”

“Our science museum used to have a big pendulum. I loved to watch it knock down those domino thingies one by one as it turned around its circle. Then they took it out to make room for another dinosaur or something.”

“Yup. A museum’s most precious resource is floorspace. That weight swinging on a long wire takes up a lot of square feet. Foucault’s pendulum was another of his Earth‑rotation demonstrations, just a year after the gyroscope show. Yeah, Al, same guy — Foucault invented that technique you use to check your telescope mirrors. He pioneered a lot of Physics. He showed that the absorption spectrum of a gas when a light shines through it matches the spectrum it emits when you heat it up. His lightspeed measurement came within one percent of our currently accepted value. ”

Astronomer Cathleen shakes her head. “Imagine, 200 years after Kepler and Newton, yet people in Foucault’s day still needed convincing that the Earth is a globe floating in space. A century and a half later some still do. <sigh> Funny, isn’t it, how Foucault was working at the same time on two such different phenomena.”

“Not so different, Cathleen. Both demonstrate the same underlying principle — inertia relates to the Universe and doesn’t care about local conditions. Foucault was really working on inertia. He made use of two different inertial effects for his demonstrations. By the way, Mr Feder, the pendulum doesn’t turn. The Earth turns beneath the pendulum to bring those domino thingies into target position.”

“That’s hard to believe.”

“Could be why his demonstrations used two different phenomena. Given 19th Century technology, those were probably his best options.”

“If only he’d had lasers, huh?”

“One kind of modern gyroscope is laser‑based. Uses photons going around a ring. Actually, photons or pulses of them going around the same ring in opposite directions. When the ring itself rotates, the photons or pulses going against the rotation encounter the Start point sooner than their opposites do. Time the difference and you can figure the rotation rate. Unfortunately, Foucault didn’t have lasers or the exquisite timing devices we have today. But that’s not the kind of gyroscope JWST carries, anyway.”

“OK, I’ll bite. What does it use?”

“The slickest one yet, Al. If you carefully tap the rim of a good wine glass it’ll vibrate like the red line here. The dotted blue circle’s the glass at rest. Under the right conditions inertia holds the planes of vibration steady even if the glass itself rotates. People have figured out how to use that principle to build extremely accurate. reliable and low‑maintenance gyroscopes for measuring and stabilizing rotations. JWST carries a set.”

“Nothing to lubricate, eh?”

Portrait of Léon Foucault from Wikimedia under Creative Commons Attribution 3.0 Unported license.

~~ Rich Olcott

Turn This Way to Turn That Way

“I don’t understand, Sy. I get that James Webb Space Telescope uses its reaction wheels like a ship uses a rudder to change direction by pushing against something outside. Except the rudder pushes against water but the reaction wheels push against … what, the Universe?”

“Maybe probably, Al. We simply don’t know how inertia works. Newton just took inertia as a given. His Laws of Motion say that things remain at rest or persist in linear motion unless acted upon by some force. He didn’t say why. Einstein’s General Relativity starts from his Equivalence Principle — gravitational inertia is identical to mechanical inertia. That’s held up to painstaking experimental tests, but why it works is still an open question. Einstein liked Mach’s explanation, that we experience these inertias because matter interacts somehow with the rest of the Universe. He didn’t speculate how that interaction works because he didn’t like Action At A Distance. The quantum field theory people say that everything’s part of the universal field structure, which sounds cool but doesn’t help much. String theory … ’nuff said.”

“Hey, Moire, what’s all that got to do with the reaction wheel thing? JWST can push against one all it wants but it won’t go anywhere ’cause the wheel’s inside it. What’s magic about the wheels?”

JWST doesn’t want to go anywhere else, Mr Feder. We’re happy with it being in its proper orbit, but it needs to be able to point to different angles. Reaction wheels and gyroscopes are all about angular momentum, not about the linear kind that’s involved with moving from place to place.”

“HAH! JWST is moving place to place, in that orbit! Ain’t it got linear momentum then?”

Newton’s Principia, Proposition II, Theorem II

“In a limited way, pun intended. Angular momentum is linear momentum plus a radial constraint. This goes back to Newton and his Principia book. I’ve got a copy of his basic arc‑splitting diagram here in Old Reliable. The ABCDEF line is a section of some curve around point S. He treated it as a succession of short line segments ABc, BCd, CDe and so on. If JWST is at point B, for instance, Newton would say that it’s traveling with a certain linear momentum along the BCd line. However, it’s constrained to move along the arc so it winds up at D instead d. To account for the constraint Newton invented centripetal force to pull along the Sd line. He then mentally made the steps smaller and smaller until the sequence of short lines matched the curve. At the limit, a sequence of little bits of linear momentum becomes angular momentum. By the way, this step‑reduction process is at the heart of calculus. Anyway, JWST uses its reaction wheels to swing itself around, not to propel itself.”

“And we’re back to my original question, Sy. What makes that swinging happen?”

“Oh, you mean the mechanical reality. Easy, Al. Like I said, three pairs of motorized wheels are mounted on JWST‘s frame near the center of mass. Their axles are at mutual right angles. Change a wheel’s angular momentum, you get an equal opposing change to the satellite’s. Suppose the Attitude Control System wants the satellite to swing to starboard. That’d be clockwise viewed from the cold side. ACS must tell a port/starboard motor to spin its wheel faster counterclockwise. If it’s already spinning clockwise, the command would be to put on the brakes, right? Either way, JWST swings clockwise. With the forward/aft motors and the hot‑side/cold‑side motors, the ACS is equipped to get to any orientation. See how that works?”

“Hang on.” <handwaving ensues> “Yeah, I guess so.”

“Hey, Moire. What if the wheel’s already spinning at top speed in the direction the ACS wants more of?”

“Ah, that calls for a momentum dump. JWST‘s equipped with eight small rocket engines called thrusters. They convert angular momentum back to linear momentum in rocket exhaust. Suppose we need a further turn to starboard but a port/starboard wheel is nearing threshold spin rate. ACS puts the brakes on that wheel, which by itself would turn the satellite to port. However, ACS simultaneously activates selected thrusters to oppose the portward slew. Cute, huh?”

~~ Rich Olcott

Attitude Adjustment

Mr Feder has a snarky grin on his face and a far‑away look in his eye. “Got another one. James Webb Space Telescope flies in this big circle crosswise to the Sun‑Earth line, right? But the Earth doesn’t stand still, it goes around the Sun, right? The circle keeps JWST the same distance from the Sun in maybe January, but it’ll fly towards the Sun three months later and get flung out of position.” <grabs a paper napkin> “Lemme show you. Like this and … like this.”

“Sorry, Mr Feder, that’s not how either JWST or L2 works. The satellite’s on a 6-month orbit around L2 — spiraling, not flinging. Your thinking would be correct for a solid gyroscope but it doesn’t apply to how JWST keeps station around L2. Show him, Sy.”

“Gimme a sec with Old Reliable, Cathleen.” <tapping> “OK, here’s an animation over a few months. What happens to JWST goes back to why L2 is a special point. The five Lagrange points are all about balance. Near L2 JWST will feel gravitational pulls towards the Sun and the Earth, but their combined attraction is opposed by the centrifugal force acting to move the satellite further out. L2 is where the three balance out radially. But JWST and anything else near the extended Sun‑Earth line are affected by an additional blended force pointing toward the line itself. If you’re close to it, sideways gravitational forces from the Sun and the Earth combine to attract you back towards the line where the sideways forces balance out. Doesn’t matter whether you’re north or south, spinward or widdershins, you’ll be drawn back to the line.”

Al’s on refill patrol, eavesdropping a little of course. He gets to our table, puts down the coffee pot and pulls up a chair. “You’re talking about the JWST. Can someone answer a question for me?”

“We can try.”
 ”What’s the question?”
  Mr Feder, not being the guy asking the question, pooches out his lower lip.

“OK, how do they get it to point in the right direction and stay there? My little backyard telescope gives me fits just centering on some star. That’s while the tripod’s standing on good, solid Earth. JWST‘s out there standing on nothing.”

JWST‘s Attitude Control System has a whole set of functions to do that. It monitors JWST‘s current orientation. It accepts targeting orders for where to point the scope. It computes scope and satellite rotations to get from here to there. Then it revises as necessary in case the first‑draft rotations would swing JWST‘s cold side into the sunlight. It picks a convenient guide star from its million‑star catalog. Finally, ACS commands its attitude control motors to swing everything into the new position. Every few milliseconds it checks the guide star’s image in a separate sensor and issues tweak commands to keep the scope in proper orientation.”

“I get the sequence, Sy, but it doesn’t answer the how. They can’t use rockets for all that maneuvering or they’d run out of fuel real fast.”

“Not to mention cluttering up the view field with exhaust gases.”

“Good point, Cathleen. You’re right, Al, they don’t use rockets, they use reaction wheels, mostly.”

“Uh-oh, didn’t broken reaction wheels kill Kepler and a few other missions?”

“That sounds familiar, Mr Feder. What’s a reaction wheel, Sy, and don’t they put JWST in jeopardy?”

 Gyroscope, image by Lucas Vieira

“A reaction wheel is a massive doughnut that can spin at high speed, like a classical gyroscope but not on gimbals.”

“Hey, Moire, what’s a gimbal?”

“It’s a rotating frame with two pivots for something else that rotates. Two or three gimbals at mutual right angles let what’s inside orient independent of what’s outside. The difference between a classical gyroscope and a reaction wheel is that the gyroscope’s pivots rotate freely but the reaction wheel’s axis is fixed to a structure. Operationally, the difference is that you use a gyroscope’s angular inertia to detect change of orientation but you push against a reaction wheel’s angular inertia to create a change of orientation.”

“What about the jeopardy?”

Kepler‘s failing wheels used metal bearings. JWST‘s are hardened ceramic.”


~~ Rich Olcott

Pinks In Space

Mr Feder, of Fort Lee NJ, is outraged. “A pretty pink parasol? NASA spent taxpayer dollars to decorate the James Webb Space Telescope with froufrou like that?”

Astronomer Cathleen stays cool. “Certainly not, Mr Feder. This is no effete Victorian‑era parasol. It’s a big, muscular ‘defender against the Sun,’ which is what the word means when you break it down — para‑sol. Long and wide as a tennis court. Its job is to fight off the Sun’s radiation and keep JWST‘s cold side hundreds of degrees colder than the Sunfacing side. Five layers of highstrength Kapton film, the same kind that helped protect New Horizons against freezing and micrometeorites on its way to Pluto and beyond. Each layer carries a thin coat of aluminum, looks like a space blanket or those Mylar mirror balloons but this is a different kind of plastic.”

“Sounds like a lot of trouble for insulation. Why not just go with firebrick backed up with cinder blocks? That’s what my cousin used for her pottery kiln.”

I cut in, because Physics. “Two reasons, Mr Feder. First one is mass. Did you help your cousin build her kiln?”

“Nah, bad back, can’t do heavy lifting.”

“There you go. On a space mission, every gram and cubic centimeter costs big bucks. On a benefit/cost scale of 1 to 10, cinder blocks rate at, oh, about ½. But the more important reason is that cinder blocks don’t really address the problem.”

“They keep the heat in that kiln real good.”

“Sure they do, but on JWST‘s hot side the problem is getting rid of heat, not holding onto it. That’s the second reason your blocks fail the suitability test. Sunlight at JWST ‘s orbit will be powerful enough to heat the satellite by hundreds of degrees, your choice of Fahrenheit or centigrade. That’s a lot of heat energy to expel. Convection is a good way to shed heat but there’s no air in space so that’s not an option. Conduction isn’t either, because the only place to conduct the heat to is exactly where we don’t want it — the scope’s dish and instrument packages. Cinder blocks don’t conduct heat as well as metals do, but they do it a lot better than vacuum does.”

“So that leaves what, radiating it away?”


“Aluminum on the plastic makes it a good radiator, huh?”

“Sort of. The combo’s a good reflector, which is one kind of radiating.”

“So what’s the problem?”

“It’s not a perfect reflector. The challenge is 250 kilowatts of sunlight. Each layer blocks 99.9% but that still lets 0.1% through to heat up what’s behind it. The parasol has radiate away virtually all the incoming energy. That’s why there’s five layers and they’re not touching so they can’t conduct heat to each other.”

“Wait, they can still radiate to each other. Heat bounces back and forth like between two mirrors, builds up until the whole thing bursts into flames. Dumb design.”

“No flames, despite what the Space Wars movies show, because there’s no oxygen in space to support combustion. Besides, the designers were a lot smarter than that. The mirrors are at an angle to each other, just inches apart near the center, feet apart at the edges. Heat in the form of infrared light does indeed bounce between each pair of layers but it always bounces at an angle aimed outwards. The parasol’s edges will probably shine pretty brightly in the IR, but only from the sides and out of the telescope’s field of view.”

“OK, I can understand the aluminum shiny, but why make it pink?”

“That’s a thin extra coat of a doped silicon preparation, just on the outermost two layers. It’s not so good at reflection but when it heats up it’s good at emitting infrared. Just another way to radiate.”

“But it’s pink?”

“The molecules happen to be that color.”

“Why’s it dopey?”

“Doped, not dopey. Pure silicon is an electrical insulator. Mixing in the right amount of the right other atoms makes the coating a conductor so it can bleed off charge coming in on the solar wind.”

“Geez, they musta thought of everything.”

“They tried hard to.”

~~ Rich Olcott


“Hi, Cathleen, meet Mr Richard Feder, of Fort Lee NJ. He’s got a question that’s more in your Astronomy bailiwick than mine. Have a strawberry scone.”

“Mmm, still warm from Al’s oven. Thanks, Sy. Hello and what’s your question, Mr Feder?”

“Hiya. So if the James Webb Space Telescope is gonna be a million miles behind the Moon, won’t the Moon block its signals to us?”

“Oh dear, he said ‘miles.’ Sy, you’d better get out Old Reliable to look up numbers and do unit conversions. Mr Feder, I don’t think in miles.”

“Huh? What do you use instead, like paces or something?”

“Depends on what objects I’m considering and why I’m thinking about them. There are so many useful ratios out there it’s often easier to use ratios than huge numbers one can’t wrap one’s head around. Jupiter’s radius, for instance, is eleven times Earth’s, and the Sun is ten times wider still. Diameter and circumference follow the same ratios, of course. Square those ratios for relative surface area, cube them for relative volume. Who needs miles or kilometers?”

“Those numbers right, Moire?”

“Mmm … 6371 kilometers or 3959 miles for Earth, 71492 kilometers or 42441 miles for Jupiter, 695700 kilometers or 432300 miles for the Sun. The Jupiter/Earth ratio’s 11.2, the Sun/Jupiter ratio’s 9.73. The lady knows what she’s talking about.”

“Here’s a few fun factoids. The Moon’s distance is 10 times Earth’s Equator which is 100 times the International Space Station’s altitude. For that matter, if you wrapped a string around Earth’s Equator, it’d be just long enough to reach up to a GPS satellite and back. But all those are near‑Earth measurements where it makes sense to think in miles or kilometers. That’s too cumbersome for the bigger picture.”

“What else you got?”

“Within the Solar System I generally use one or the other of two convenient yardsticks. They measure the same distances, of course, but they have different applications. One is the nominal radius of Earth’s orbit, about 150 million kilometers.’

“That’s 93 million miles, Mr Feder.”

“I knew that one, Moire.”

“Anyway, we call that distance an Astronomical Unit. It’s handy for locating bodies relative to the Sun. Parker Solar Probe has gotten within a tenth of an AU of the Sun, for instance, and Neptune’s about 30 AU out. The Oort Cloud begins near 2000 AU and may extend a hundred times as far.”

“I ain’t even gonna ask what the Oort‐thing is, but I’m glad it’s a long way away.”

“We think it’s where long‑period comets come from.”

“Far away is good then. So what’s your other yardstick?”


“186 thousand miles per second, Mr Feder.”

“Yeah, yeah.”

“It’s also 300 thousand kilometers per second, and one light‑second per second, and one light‑year per year. Within the Solar System my benchmarks are that Earth is 500 light-seconds from the Sun, and Pluto was 4½ light-hours away from us when New Horizons sent back those marvelous images. The Sun’s nearest star system, Alpha Centauri, is 4⅓ light‑years away, and when you compare hours to years that gives you an idea of how small we are on the interstellar scale.”

“Cathleen, when you mentioned New Horizons that reminded me of the JWST. We’ve gotten off the track from Mr Feder’s question. Why isn’t the Moon going to block those signals?”

“Because it’ll never be in the way.” <sketching on a paper napkin> “There’s a bunch of moving parts here so hold on. The Earth orbits the Sun and the Moon orbits the Earth once a month, right? The L2 point doesn’t orbit the Earth. It orbits the Sun, staying exactly behind Earth so yeah, once a month the Moon could maybe get between Earth and L2. But JWST won’t be at L2, it’ll be in a wide orbit around that point and mostly perpendicular to the orbits of the Earth and Moon.”

“How wide?”

“It’ll vary depending on what they need, but it’s big enough to keep the spacecraft’s solar panels in the sunlight.”

“Solar panels? I thought the IR sensors needed cold cold cold.”

“They do. JWST protects its cold side with a hot side featuring a pretty pink Kapton parasol.”

~~ Rich Olcott

Hyperbolas But Not Hyperbole

Minus? Where did that come from?”

<Gentle reader — If that question looks unfamiliar, please read the preceding post before this one.>

Jim’s still at the Open Mic. “A clever application of hyperbolic geometry.” Now several of Jeremy’s groupies are looking upset. “OK, I’ll step back a bit. Jeremy, suppose your telescope captures a side view of a 1000‑meter spaceship but it’s moving at 99% of lightspeed relative to you. The Lorentz factor for that velocity is 7.09. What will its length look like to you?”

“Lorentz contracts lengths so the ship’s kilometer appears to be shorter by that 7.09 factor so from here it’d look about … 140 meters long.”

“Nice, How about the clocks on that spaceship?”

“I’d see their seconds appear to lengthen by that same 7.09 factor.”

“So if I multiplied the space contraction by the time dilation to get a spacetime hypervolume—”

“You’d get what you would have gotten with the spaceship standing still. The contraction and dilation factors cancel out.”

“How about if the spaceship went even faster, say 99.999% of lightspeed?”

“The Lorentz factor gets bigger but the arithmetic for contraction and dilation still cancels. The hypervolume you defined is always gonna be just the product of the ship’s rest length and rest clock rate.”

His groupies go “Oooo.”

One of the groupies pipes up. “Wait, the product of x and y is a constant — that’s a hyperbola!”

“Bingo. Do you remember any other equations associated with hyperbolas?”

“Umm… Yes, x2–y2 equals a constant. That’s the same shape as the other one, of course, just rotated down so it cuts the x-axis vertically.”

Jeremy goes “Oooo.”

Jim draws hyperbolas and a circle on the whiteboard. That sets thoughts popping out all through the crowd. Maybe‑an‑Art‑major blurts into the general rumble. “Oh, ‘plus‘ locks x and y inside the constant so you get a circle boundary, but ‘minus‘ lets x get as big as it wants so long as y lags behind!”

Another conversation – “Wait, can xy=constant and x2–y2=constant both be right?”
  ”Sure, they’re different constants. Both equations are true where the red and blue lines cross.”

A physics student gets quizzical. “Jim, was this Minkowski’s idea, or Einstein’s?”

“That’s a darned good question, Paul. Minkowski was sole author of the paper that introduced spacetime and defined the interval, but he published it a year after Einstein’s 1905 Special Relativity paper highlighted the Lorentz transformations. I haven’t researched the history, but my money would be on Einstein intuitively connecting constant hypervolumes to hyperbolic geometry. He’d probably check his ideas with his mentor Minkowski, who was on the same trail but graciously framed his detailed write‑up to be in support of Einstein’s work.”

One of the astronomy students sniffs. “Wait, different observers see the same s2=(ct)2d2 interval between two events? I suppose there’s algebra to prove that.”

“There is.”

“That’s all very nice in a geometric sort of way, but what does s2 mean and why should we care whether or not it’s constant?”

“Fair questions, Vera. Mmm … you probably care that intervals set limits on what astronomers see. Here’s a Minkowski map of the Universe. We’re in the center because naturally. Time runs upwards, space runs outwards and if you can imagine that as a hypersphere, go for it. Light can’t get to us from the gray areas. The red lines, they’re really a hypercone, mark where s2=0.”

From the back of the room — “A zero interval?”

“Sure. A zero interval means that the distance between two events exactly equals lightspeed times light’s travel time between those events. Which means if you’re surfing a lightwave between two events, you’re on an interval with zero measure. Let’s label Vera’s telescope session tonight as event A and her target event is B. If the A–B interval’s ct difference is greater then its d difference then she can see Bif the event is in our past but not beyond the Cosmic Microwave Background. But if a Dominion fleet battle is approaching us through subspace from that black dot, we’ll have no possible warning before they’re on us.”

Everyone goes “Oooo.”

~~ Rich Olcott

Earth’s Closed Eye

Question in the chat box, Maria, and I paraphrase to preserve anonymity — ‘So the Arecibo telescope won’t work any more. Why should we care? There’s lots of other telescopes that could so the same job.‘”

“But profesora, there aren’t. Arecibo is special in many ways. First, it is a very good telescope. That means it has high sensitivity and high resolution. Compare two radio telescopes with different‑size dishes and the same kinds of antennas and everything else. The one with the bigger dish is more sensitive because it can capture more photons. Arecibo’s 300‑meter dish used to be the largest in the world. China activated their FAST instrument five years ago. Its 500‑meter dish should make it more than 200 times as sensitive as Arecibo, but it doesn’t because neither telescope is designed to use the entire dish surface at once except for looking straight up. Their active areas are about the same.”

Is FAST another one of those goofy acronyms?

“Of course. It stands for ‘Five‑hundred‑meter Aperture Spherical radio Telescope‘ but in Chinese its name is Tianyan, which means ‘Heaven Eye.’ I think that is more pretty. FAST and Arecibo overlap their wavelength ranges, although FAST can receive some longer wavelengths and Arecibo can receive some shorter ones. Oh, there is also a big Russian radio telescope, RATAN‑600, with an even bigger diameter. But it is a ring, not a disk, so not as sensitive as Arecibo or FAST.”

A ring? Why did they build it that way?

“Because of the other thing you need in a good telescope, resolution. If you have good resolution in an image, you can see points that are very close together. The how‑close limit angle comes from dividing the light wavelength by the dish diameter. The diameter of RATAN’s ring is 600 meters, so RATAN’s resolving power is twice as good as Arecibo’s 300‑meter disk. RATAN doesn’t need to be sensitive, though, because it is used mostly for looking close at the Sun, not at stars and galaxies. That is OK because RATAN is so far north.”

What difference does that make?

“No telescope can see what is below its horizon. RATAN is at 43° north, almost 1400 miles north of Arecibo. It has a good view of the northern sky but cannot see down to the Equator where many asteroids and all the planets are.”

Sorry, Maria, that’s not quite correct. Earth is tilted relative to the orbital plane by 23° so even Arecibo only sees the northern portion of planetary orbits. While I’ve got the mic I’ll add some background on RATAN‑600. RATAN is the acronym for ‘Academy of Sciences Radio Telescope’ in Russian. It was built in the Cold War era when that part of the world was the USSR. Although I don’t believe it’s ever been publicly confirmed, many people think that RATAN‑600‘s original purpose was detection of ICBMs coming in over the North Pole. However, over the decades it has been a productive source of information for the solar physics community. Back to you, Maria.

“That is good to know, profesora. Thank you. So, Arecibo is — was —special because of its sensitivity and its resolution. It is also about 500 miles further south than FAST. But Arecibo has one additional feature that FAST cannot have — radar. Arecibo has high-powered transmitters that can send out terawatt pulses to things in the Solar System that are closer than Saturn. The dish gathers echoes that give us detailed knowledge of those objects. For instance, Arecibo’s radar echoes from Mercury showed us that the planet is not tidally locked to the Sun. We used to think Mercury’s day was 88 days long, like its year, but now we know it rotates in only 59 days.”

Why can’t the Chinese just add transmitters to FAST?

“The Chinese designers gave FAST a light‑weight antenna carriage to hang over its dish. Arecibo’s 900‑ton carriage can handle massive transmitters, but FAST’s cannot. There is one other radio telescope with radar, at Goldstone, California, but it has less than one‑millionth the power of Arecibo’s transmitters. Without Arecibo’s sensitivity, resolution, location and high-powered radar capability we cannot find near‑Earth asteroids on track to hit us.”

~~ Rich Olcott

Author’s note — Early in the morning of 1 December, after I completed last week’s and this post, the National Science Foundation reported that Arecibo’s central instrument platform has fallen onto the dish as a result of further cable failures.
“Vale, nostri servi boni et fidelis”
Farewell, our good and faithful servant.

Arecibo ¡que lástima!

Hello, Astronomy video class. I’ve made room in the syllabus schedule for a quick talk from someone with a personal connection to a timely topic. You may know we’ve lost one of Astronomy’s premier radio telescopes, Puerto Rico’s Arecibo Observatory. I’ve asked Maria to fill us in on the what and the why. If you have a question, type it into your chat window and I’ll relay it to her. Maria, you’re on.”

“Thank you, profesora. Yes, I do know Arecibo because I have worked there. I grew up in Hatillo, a small city on the north coast about half an hour away from the Observatory. My teacher of science in high school, somehow he got me a summer job there. Sometimes I worked in the gift shop, sometimes I helped the guided tours, but my best thing was running errands because then I could visit the science offices and chat with people about what they were doing. There I fell in love with Astronomy and that is why I came here to study.

“When people think of Arecibo they think of the big 300 meter dish, about 1000 feet across. Sharing my screen for you… there. This picture I got from Wikipedia:

The Arecibo Observatory
photo by JidoBG, licensed under the Creative Commons Attribution-Share Alike 4.0 International

“The installation sits in very rough mountains. They are so rough because they are mostly limestone that slowly dissolves in water. The water seeps in through cracks to attack the rock and make cliffs and holes and caves. The Arecibo observatory is where it is because water eroded a cavern close to the surface. The topmost material fell into the empty space to make a huge round sinkhole like very few other places in the world.”

Question from the chat, Maria. Did the rock actually dissolve into that convenient smooth reflector shape?

“¡Por Dios no! The circular shape, yes, but the sinkhole floor is nearly flat. The dish itself is many aluminum panels fixed to a floating steel grid. Here is a picture Mr Phil Perillat took from beneath the dish. I don’t know Mr Perillat’s title but he is always very busy keeping things running.

“Above you see the grid, five meters or more above the ground. The grid is supported by concrete all around the edges. Coming down from the grid you see cables leading to those round concrete piers. These cables pull the grid down into its curved shape which is actually a piece of a sphere.”

A sphere, not a parabola?

“No, profesora, and that is important. A fixed dish with a parabola shape like most telescope mirrors always would aim straight up. It would see targets at the top of the sky but for only a few minutes as the Earth turns through the day. With a sphere‑shaped dish and the antennas mounted where the center of the sphere would be, then the whole sky is in focus. The scientists aim the telescope by moving the antennas to point at different parts of the dish like you look at different parts of one of those funny mirrors in, sorry I don’t know the word, una casa de la diversión.”

A funhouse.”

“Thank you. The antenna carriage is so complicated because it must look at different parts of the dish. Here you see the carriage:

The Arecibo receiver mounting and dome
Photo by Phil Perillat, National Astronomy and Ionosphere Center

“The antennas point downward from inside that dome. When motors swing the dome along that crescent‑shaped arc, the antennas scan along an arc of the dish. More motors can rotate the arc around that circular track. By swinging and rotating together, the antennas can follow the reflection of any object that moves through the sky.”

All those motors and tracks and antennas must be heavy.”

“Yes, 900 tons hanging 500 feet above the grid. Eighteen cables hold it up. Each is many strands of steel braided together. Compressed air blows through the braids to prevent corrosion, but the storms won out in the end. Three cables have failed and it is too dangerous for repair. So sad.”

~~ Rich Olcott

Author’s note — Early in the morning of 1 December, after I completed this and next week’s posts, the National Science Foundation reported that Arecibo’s central instrument platform had fallen onto the dish as a result of further cable failures.
“Vale, nostri servi boni et fidelis”
Farewell, our good and faithful servant.

Zeroing In on Water

<chirp, chirp> “Moire here.”

“Hi, Sy, it’s me, Vinnie. I just heard this news story about finding water on the Moon. I thought we did that ten years ago. You even wrote about it.”

“The internet never forgets, does it? That post wasn’t quite right but it wasn’t wrong, either.”

“How can it be both?”

“There’s an old line in Science — ‘Your data’s fine but your conclusions are … nuts.’ They use a different word in private. Suppose you land on a desert island and find a pirate’s treasure chest. Should the headlines say you’d found a treasure?”

“Naw, the chest might be empty or full of rocks or something.”

“Mm-hm. So, going back to that post… I was working from some reports on NASA’s Lunar Reconnaissance Orbiter. Its LAMP instrument mapped how strongly different Moon features reflected a particular frequency of ultraviolet light. That frequency’s called ‘Lyman‑alpha.’ Astronomers care about it because it’s part of starlight, it’s reflected by rock, and it’s specifically absorbed by hydrogen atoms. Sure enough, LAMP found some places, typically in deepshadow craters, that absorbed a lot more Lymanalpha than other places.”

“And you wrote about how hydrogen atoms are in water molecules and the Moon’s deep crater floors near the poles are sheltered from sunlight that’d break up water molecules so LAMP’s dark spots are where there’s water. And you liked how using starlight to find water on the Moon was poetical.”

“Uhh… right. All that made a lot of sense at the time and it still might be true. Scientists leapt to the same hopeful conclusion when interpreting data from the MESSENGER mission to Mercury. That one used a neutron spectrometer to map emissions from hydrogen atoms interacting with incoming cosmic rays. There again, the instrument identified hydrogen collected in shaded craters at the planet’s poles. Two different detection methods giving the same positive indication at the same type of sheltered location. The agreement seemed to settle the matter. The problem is that water isn’t geology’s only way or even its primary way to accumulate hydrogen atoms.”

“What else could it be? Hydrogen ions in the solar wind grab oxide ions from Moon rock and you’ve got water, right?”

“But the hydrogens arrive one at a time, not in pairs. Any conversion would have to be at least a two‑step process. The Moon’s surface rocks are mostly silicate minerals. They’re a lattice of negative oxide ions that’s decorated inside with an assortment of positive metal ions. The first step in the conversion would be for one hydrogen ion to link up with a surface oxide to make a hydroxide ion. That species has a minus‑one charge instead of oxide’s minus‑two so it’s a bit less tightly bound to its neighboring metal ions. Got that?”

“Gimme a sec … OK, keep going.”

“Some time later, maybe a century maybe an eon, another hydrogen ion comes close enough to attack our surface hydroxide if it hasn’t been blasted apart by solar UV light. Then you get a water molecule. On balance and looking back, we’d expect most of the surface hydrogen to be hydroxide ions, not water, but both kinds would persist better in shadowed areas.”

“OK, two kinds of hydrogen. But how do we tell the difference?”

“We evaluate processes at lower‑energies. Lyman‑alpha photons pack over 10 electronvolts of energy, enough to seriously disturb an atom and blow a molecule apart. O‑H and H‑O‑H interact differently with light in the infra‑red range that just jiggles molecules instead of bopping them. For instance, atom pairs can stretch in‑out. Different kinds of atom bind together more‑or‑less tightly. That means each kind of atom pair resonates at its own stretch energy, generally around 6 microns or 0.41 electronvolts. NASA’s Cassini mission had a mapping spectrometer that could see down into that range. It found O‑H stretching activity all over the Moon’s surface.”

“But that could be either hydroxyls or water.”

“Exactly. The new news is that sensors aboard NASA’s airborne SOFIA mission map light even deeper into the infra‑red. It found the 3‑micron, 0.21‑electronvolt signal for water’s V‑shape scissors motion. That’s the water that everybody’s excited about.”

“Lots of it?”

“Thinly spread, probably, but stay tuned.”

~~ Rich Olcott

Presbyopic Astronomy

Her phone call done, Cathleen returns to the Spitzer Memorial Symposium microphone with her face all happiness. “Good news! Jim, the grant came through. Your computer time and telescope access are funded. Woo-hoo!!”

<applause across the audience and Jim grins and blushes>

Cathleen still owns the mic. “So I need to finish up this overview of Spitzer highlights. Where was I?”

Maybe-an-Art-major tries to help. “The middle ground of our Universe.”

“Ah yes, thanks. So we’ve looked at close-by stars but Spitzer showed us a few more surprises lurking in the Milky Way. This, for instance — most of the image is colorized from the infra‑red, but if you look close you can see Chandra‘s X‑ray view, colorized purple to highlight young stars.”

The Cepheus-B molecular cloud
X-ray: NASA/CXC/PSU/K. Getman et al.; IRL NASA/JPL-Caltech/CfA/J. Wang et al

<hushed general “oooo” from the audience>

“Giant molecular clouds like this are scattered throughout the Milky Way, mostly in the galaxy’s spiral arms. As you see, this cloud’s not uniform, it has clumps and voids. By Earth standards the cloud is still a pretty good vacuum. The clumps are about 10-15 of our atmosphere’s density, but that’s still a million times more dense than our Solar System’s interplanetary space. The clumps appear to be where new stars are born. The photons and other particles from a newly-lit star drive the surrounding dust away. My arrow points to one star with a particularly nice example of that — see the C-shape around the star?”

The maybe-an-Art-major pipes up. “How about that one just a little below center?”

“Uh-huh. There’s so much activity in that dense region that the separate shockwaves collide to create hot spots that’ll generate even more stars in the future. The clouds are mostly held together by their own gravity. They last for tens of millions of years, so we think of them as huge roiling stellar nurseries.”

“Like my kid’s day care center but bigger.”

“Mm-mm, but let’s turn to the Milky Way’s center, home of that famous black hole with the mass of four million Suns and this remarkable structure, a double-helix of warm dust.”

False-color infra-red image of the Double-Helix Nebula
The double helix nebula.
Credit: NASA/JPL-Caltech/M. Morris (UCLA)

Vinnie blurts out, “That’s a jet from a black hole! One of Newt’s babies.”

Newt can’t resist breaking into Cathleen’s pitch. “Maybe it’s a jet, Vinnie. Yes, it’s above the central galactic plane and perpendicular to it, but the helix doesn’t quite point to the central black hole.”

“So take another picture that follows it down.”

“We’d love to, but we can’t. Yet. That image came from a long-wavelength instrument that only operated during Spitzer‘s initial 5-year cold period. Believe me, there are bunches of astronomers who can’t wait for the James Webb Space Telescope‘s far-IR instruments to get into position and start doing science. Meanwhile, we’ve got just the one image and a few earlier ones from an even less-capable spacecraft. This thing may be a lit-up part of a longer structure that twists down to the black hole or at least its accretion disk. We just don’t know.”

Cathleen takes control again. “The next image comes from outside our galaxy — far outside.”

Spitzer visualization of Galaxy MACS 1149-JD1
Credit: NASA/ESA/STScI/W. Zheng (JHU), and the CLASH team

The maybe-an-Art-major snorts, “Pointillism derivative!”

“No, it’s pixels from a starfield image with a very low signal-to-noise ratio. That red blotch in the center is one of the most distant objects ever observed, gracefully named MACS 1149-JD1. It’s a galaxy 13.2 billion lightyears away. That’s so far away that the expansion of the Universe has stretched the galaxy’s emitted photons by a factor of 10.2. Spectrum-wise, 1149-JD1’s ultra-violet light skipped right past the visible range and down into the near infra-red. Intensity-wise, that galaxy’s about 5200 times further away than the Andromeda galaxy. Assuming the two are about the same overall brightness, 1149-JD1 would be about 27 million times fainter than Andromeda.”

“How can we even see anything that dim?”

“We couldn’t, except for a fortunate coincidence. Right in line between us and 1149-JD1 there’s a massive galaxy cluster whose gravity acts like a lens to focus 1149-JD1’s light.”

The seminar’s final words, from maybe-an-Art-major — “A distant light, indeed.”

~~ Rich Olcott