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.”

<whew>

~~ Rich Olcott

Yardsticks

“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?”

“Lightspeed.”

“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