Freitag, 31. Dezember 2021 - 07:06

Astronomie - NASA JamesWebb Telescope - Update-26

28.12.2021

James Webb Space Telescope successfully deploys antenna

The antenna will eventually beam Webb's cosmic images to Earth.

This NASA graphic shows the James Webb Space Telescope's antenna assembly (circled) after its deployment on Dec. 26, 2021. (Image credit: NASA)

NASA's new James Webb Space Telescope successfully deployed a critical antenna Sunday (Dec. 26) just one day after its Christmas launch into space.

The space telescope unfolded what scientists call a gimbaled antenna assembly that carries the high-rate data dish responsible for beaming Webb's observations of the early universe back to Earth. Webb launched into space Saturday atop an Ariane 5 rocket that lifted off from a spaceport in Kourou, French Guiana.

"This antenna will be used to send at least 28.6 Gbytes of science data down from the observatory, twice a day," NASA officials wrote in a mission update. "The team has now released and tested the motion of the antenna assembly — the entire process took about one hour."

The $10 billion Webb space telescope is designed to study the earliest stars in the universe and other cosmic mysteries. It is the largest and most powerful telescope ever launched into space.

Saturday's antenna deployment was the latest in a monthlong series of steps to prime the space telescope for its deep-space observing mission. Webb is currently on a 29-day journey to a stable point in space called Lagrange Point 2, or L2, which is nearly 1 million miles (1.6 million kilometers) from Earth. It is from there that the infrared space telescope will observe the universe.

In addition to the antenna deployment and testing, Webb flight controllers activated the telescope's temperature and strain gauges for the first time, according to NASA's update. Those gauges will allow flight controllers to monitor the Webb space telescope's temperature and structural health in the days to come.

Having an accurate reading of Webb's temperature and structural loads is key for mission scientists and engineers because one of the most harrowing periods for the space telescope is still ahead: deployment of its huge sunshield.

The James Webb Space Telescope is an infrared observatory designed to make the most detailed observations ever, but only if it can be kept cold enough. To do that, Webb will unfurl a huge multi-layered sunshield the size of a tennis court that will unfold in stages over the first eight days after launch. The first step in that deployment process is expected to begin Tuesday (Dec. 28), three days into the mission.

The next immediate step for the Webb space telescope will be a second mid-course correction burn, a maneuver designed to refine its course toward L2. That maneuver, the second of three planned burns, is expected to be completed later in the day on Monday.

Quelle: SC

+++

Webb telescope completes first course correction burn, deploys main antenna

A view of the circular high-gain antenna in its stowed position on the James Webb Space Telescope. Credit: Stephen Clark / Spaceflight Now

The James Webb Space Telescope fired its rocket thrusters for the first time late Saturday to line up for course toward an observing post nearly a million miles from Earth, then deployed a high-rate communications antenna Sunday to transmit science data to the ground.

The milestones were the first major events for Webb since its successful launch Christmas morning aboard a European Ariane 5 rocket from French Guiana. The Ariane 5 placed the 13,584-pound (6,161-kilogram) observatory right on target on a course into deep space, with Webb in stretched, or elliptical, orbit with an apogee, or high point, more than a million kilometers (600,000 miles) from Earth.

After deploying its solar array and priming its propulsion system following separation from the Ariane 5 upper stage, the spacecraft fired one of its main thrusters beginning at 7:50 p.m. EST Saturday (0050 GMT Sunday) for the mission’s Mid Course Correction 1a, or MCC 1a, maneuver.

The Ariane 5 rocket aimed to deploy the nearly $10 billion observatory at a velocity just shy of the speed required to reach the L2 Lagrange point, a gravitationally-neutral spot nearly a million miles (1.5 million kilometers) from Earth, four times farther than the moon’s orbit.

The launch trajectory was selected to ensure Webb could make up the speed required to enter orbit around the L2 Lagrange point, rather than needing to turn around to use its thrusters for a braking maneuver. A burn to slow down would force Webb to point its sensitive optics toward the sun, risking damage.

The Ariane 5 rocket placed Webb almost exactly on its planned course into deep space, according to officials from NASA and the European Space Agency, which supplied the launch vehicle as part of its contribution to the mission.

Webb fired one of its four hydrazine-fueled Secondary Combustion Augmented Thrusters, or SCATs, for 65 minutes Saturday night, making up nearly all of the expected velocity shortfall needed to reach an orbit around the L2 Lagrange point.

Another burn, named MCC 1b, is penciled in the Webb flight plan Monday night. Keith Parrish, Webb’s commissioning manager at NASA, said before the launch that the MCC 1b burn might not be required, depending on the trajectory analysis following the MCC 1a maneuver.

Webb controllers at the Space Telescope Science Institute in Baltimore, Maryland, perform ranging measurements with the spacecraft using radio signals passed between Webb and ground stations around the world. The data gives engineers information about Webb’s location and speed, allowing navigators to plot future trajectory correction maneuvers. In some cases, they might not be needed, Parrish said.

Another mid-course correction burn, designated MCC 2, is planned 29 days after launch — around Jan. 23 — to inject Webb into its halo orbit around L2.

Webb has four main SCAT engines. One redundant pair is used for the MCC 1a and 1b burns, while the other redundant set is used the MCC 2 maneuver and all subsequent stationkeeping burns to keep Webb in its orbit around L2 during its science mission.

Sixteen smaller thrusters, divided into eight thruster modules each with two engines, are used for pointing control and to offload momentum from the observatory’s six spinning reaction wheels, which steer Webb toward its science targets. The reaction wheels were activated soon after Webb’s launch Saturday.

With the MCC 1a burn out of the way, Webb deployed its high-rate communications antenna shortly after 10 a.m. EST (1500 GMT) Sunday, according to NASA.

The Gimbaled Antenna Assembly, or GAA, includes Webb’s 2-foot-diameter (60-centimeter) high-gain dish antenna. The Ka-band antenna will be used to transmit at least 28.6 Gbytes of science data down from the observatory, twice per day, NASA said.

Ground teams released and tested the motion of the antenna during Sunday’s deployment activity.

Before the release of the Ka-band antenna, Webb communicated to ground teams using a broad-beam S-band antenna at lower data rates. The Ka-band high-gain antenna can downlink data at a speed of up to 3.5 Mbytes per second.

Other steps completed on Webb’s first full day in space included the switch-on of temperature sensors and strain gauges on the telescope, used for monitoring Webb’s thermal and structural parameters, NASA said.

The antenna release and first mid-course correction burn set the stage for the next step of Webb’s post-launch commissioning — the deployment and tensioning of the observatory’s tennis court-sized sunshield.

The five-layer thermal barrier will keep Webb’s telescope and instruments in permanent shade, allowing the optics and detectors to reach a temperature of minus 388 degrees Fahrenheit, or 40 degrees above absolute zero. The super-cold temperatures are necessary for Webb’s infrared instruments to detect faint infrared light from the distant universe.

Two pallets containing the sunshield membranes will fold down from their launch position as soon as Tuesday, if controllers stay on the pre-planned flight schedule. That will allow for the release and tensioning of the sunshield this weekend.

Two wings holding six of Webb’s 18 hexagonal primary mirror segments are expected to fold into place in early January, building the shape of the observatory’s 21.3-foot-diameter (6.5-meter) optical surface. Then comes additional steps to align the mirror and bring it into focus, all while engineers turn on and test each fo Webb’s four science instruments.

But Webb’s time-sensitive commissioning steps — the solar array deployment and first course correction burn — are now complete. Teams have more flexibility in scheduling the remaining commissioning steps over the next six months, culminating in the release of the first science images some time in mid-2022.

Quelle:SN

----

Update: 30.12.2021

James Webb Space Telescope sails beyond the orbit of the moon after 2nd course correction

Just one more burn — and a few major deployments — to go.

NASA's next-generation space observatory has sailed beyond the orbit of the moon after nailing the second of three required course-correction burns, agency officials said Monday.

The long-awaited James Webb Space Telescope launched on Saturday (Dec. 25), beginning a one-month deployment process. Over the course of that deployment, the observatory must execute three burns to adjust its trajectory, the first of which began 12.5 hours after launch and lasted 65 minutes; the second, much shorter burn, is now also complete.

"At 7:20 pm EST — 60 hours after liftoff — Webb's second mid-course correction burn began," NASA officials wrote in a statement released on Monday (Dec. 27). "It lasted 9 minutes and 27 seconds and is now complete."

The spacecraft has also crossed the moon's orbit of Earth, according to a mission tweet. As of Tuesday, Webb was more than 293,000 miles (471,000 kilometers) from Earth. The average distance from Earth to the moon is about 240,000 miles (384,000 km).

"It's been a busy evening! Not only did we just complete our second burn, but #NASAWebb also passed the altitude of the Moon as it keeps cruising on to the second Lagrange point to #UnfoldTheUniverse," Webb mission officials wrote. "Bye, @NASAMoon!"

("Bye, moon," is not literal here; the moon was elsewhere in its orbit and there was no close flyby. In fact, according to astronomer Jonathan McDowell, the observatory was closest to the moon while on the launch pad, since the spacecraft launched in the opposite direction.)

The spacecraft will execute its third and final course-correction burn about 29 days after launch, according to a NASA deployment timeline. This burn will be the mission's orbital insertion burn, establishing the spacecraft in a halo orbit around a point in space known as L2, or Earth-sun Lagrange point 2.

This location is nearly 1 million miles (1.5 million kilometers) away from Earth on the side opposite the sun. The remote location will help Webb detect infrared light precisely with minimal interference from the sun.

For now, however, the focus turns to the observatory's massive sunshield, which was carefully folded up for launch. Now, the spacecraft must unfold the shield, which stretches as long as a tennis court, and separate its five ultrathin layers. The sunshield, like Webb's location, is integral to the mission's goal of observing infrared light.

Sunshield deployment is tentatively scheduled to begin on Tuesday (Dec. 28), three days after launch, although each stage of deployment is controlled by personnel on the ground and the timeline is flexible.

First, Webb must deploy the sunshield's pallet and tower, then release a flap and the sunshield covers. That sequence of events is estimated to take about two days. Then, the sunshield's booms will deploy, unveiling the sunshield at its full width. By eight days after launch, the mission team hopes that the sunshield will be fully deployed.

Quelle: SC

+++

High-stakes sunshield deployment begins on Webb telescope

Artist’s illustration of the Webb telescope’s sunshield pallet opening. Credit: NASA

Mission controllers started the risky process Tuesday to unfurl the James Webb Space Telescope’s sunshield, a five-layer thermal barrier necessary to give the observatory infrared vision into the distant universe.

Two large pallets containing the sunshield membranes were folded up on each side of Webb’s primary mirror for launch. Now, with Webb on course toward an observing post nearly a million miles (1.5 million kilometers) from Earth, ground teams are ready to open up the sunshield to its full dimension.

Made of five fragile kapton membranes, each as thin as a human hair, the sunshield will keep Webb’s mirrors, instruments, and detectors in constant shadow, allowing their operating temperature to reach near minus 400 degrees Fahrenheit. Such cold conditions are required to allow Webb to see the faint infrared light from the first galaxies in the universe more than 13.5 billion light years away.

Webb, the largest space telescope ever launched, had to fold up to fit inside the payload fairing of its Ariane 5 rocket for liftoff Christmas morning.

“We built a world-class infrared telescope,” said Mike Menzel, Webb’s mission systems engineer at NASA’s Goddard Space Flight Center. “We built it, we’ve aligned it, we’ve tested it, and proved it worked.”

“Now, we’re going to have to break it up, fold it up, and actually rebuild it on orbit — rebuild it, realign it, retune it and get it to work robotically on orbit. That’s never been done before.”

Most NASA managers and astronomers waiting to use the nearly $10 billion Webb telescope give the same answer about the most stressful moment of the mission: Sunshield deployment.

“The sunshield is one of these things that is almost inherently indeterministic,” Menzel said. “NASA is used to deploying rigid beams on hinges, because they’re deterministic, you can determine how they move.”

Once the sunshield is opened to its full size — with the approximate dimensions of a tennis court — NASA will send commands to open wings on each side of the telescope’s primary mirror, giving the aperture its final diameter of 21.3 feet (6.5 meters). A tripod-like structure with Webb’s secondary mirror must also deploy.

“Given that there are 40 different major deployments, and hundreds of pulleys and wires, the whole thing makes me nervous and will until its fully deployed,” said John Grunsfeld, an astrophysicist, former astronaut, and head of NASA’s science mission directorate from 2012 until 2016, a key period in Webb’s development.

But it’s the sunshield that got the biggest share of Menzel’s attention during the design and testing of Webb.

Menzel compares predicting the behavior of the sunshield layers to guessing what a string will do when you push it on a table top.

“So it is with the membranes of the sunshield,” he said. “So we can’t really predict their shape, but we can constrain it. “We can try to prevent it from going in places that we don’t want it to go, places where it could snag or tear, or maybe impede the deployment of other members.”

If everything goes according to plan, the timeline to deploy and tension the sunshield will take five days. But mission planners have flexibility in that schedule, and could decide to postpone or defer some steps to look at data in the event of a problem.

The first day of sunshield deployment apparently went off without a hitch. NASA said the two sunshield pallets lowered into position in front of and behind the telescope Tuesday. The two pallets extend 69.5 feet (21.2 meters) tip to tip.

The pallets, called Unitized Pallet Structures, contain the five kapton sunshield layers carefully folded and stowed by engineers at Northrop Grumman, NASA’s prime contractor for the Webb mission.

“Webb is beginning to resemble the form it will take when it is fully deployed – now that the mission operations team has successfully deployed and latched into place the observatory’s forward and aft Unitized Pallet Structures,” NASA said in an update late Tuesday.

The forward pallet lowered first, with its unfolding finished at around 1:21 p.m. EST (1821 GMT) Tuesday, NASA said. The next step was the unfolding of the aft pallet, which was confirmed fixed in its deployed position at around 7:27 p.m. EST (0027 GMT).

It took around 20 minutes to deploy the forward pallet, and 18 minutes to move the aft pallet, NASA said. But the process included additional steps spanning several hours, allowing ground teams to monitor structural temperatures and maneuver the observatory with respect to the sun to provide optimal thermal conditions for the deployments, the agency said.

Ground teams also turned on heaters, activated release mechanisms, and verified software configurations before latching the pallets into place, NASA said. Mission control for Webb is at the Space Telescope Science Institute in Baltimore, Maryland.

“Each deployment usually involves a bunch of motor checks, small moves to verify motor performance, heating of anything that is too cold, etc,” said Keith Parrish, Webb’s commissioning manager at NASA. “We also have to power on and configure our deployment electronics, which controls the whole show. “

The deployment of the sunshield pallets followed a smooth first three days in space for Webb.

The three-story-tall observatory launched from French Guiana on Christmas morning atop a European Ariane 5 rocket. The launcher delivered Webb to a bullseye point in space, and the spacecraft unfurled its solar array, completed two mid-course correction burns, and opened its high-gain antenna over the last few days.

Webb has passed the orbit of the moon en route to an operating orbit around the L2 Lagrange point, a gravitational balance point nearly a million miles beyond the night side of the Earth. The spacecraft is scheduled to arrive in its halo orbit around L2 in late January.

Next up is the raising of Webb’s Deployable Tower Assembly, which moves the telescope away from the spacecraft. The telescope structure contains Webb’s 18 hexagonal primary mirror segments and four infrared instruments mounted in a carbon composite module built to withstand the observatory’s cryogenic operating temperature.

That is scheduled to occur Wednesday.

“All of the material for the sunshield is actually stowed in that area, or some of it is, so we need to get the telescope up away, so that will allow us to pull that material out,” Parrish said.

The next big event, tentatively set for Thursday, will be the opening of protective covers over the sunshield membranes. The five layers took a month to carefully, and manually, stow inside the pallets, akin to the way a parachute is packed before a skydive.

“It needs to be folded perfectly so that it unfolds and deploys perfectly, without snags, without any tangles,” said Krystal Puga, Webb’s lead spacecraft systems engineer at Northrop Grumman.

Launch restraints will disengage to allow the covers to roll off the top of the sunshade. Another deployment on tap Thursday, if schedules hold, is the release of Webb’s aft momentum tab, a rear flap that balances the pressure from solar light pressure on the sunshield, helping keep the observatory stable as it goes around the sun in lock-step with Earth.

Then, as soon as Friday, two booms will extend from each side of Webb. With the assistance of deployment motors, the structural support booms will pull the five sunshade membranes out into their distinctive diamond shape.

It all happens slowly, with sensors across the observatory tracking how the sunshield opens. Ground controllers can pause in between steps to ensure everything is working as designed.

Each layer of the sunshield is slightly different in size and shape, created using thermally bonded sections of kapton with around 10,000 seams, according to Puga. There are reinforcement strips, or rip stops, to contain any tears or holes, and metallic ribbons giving the kapton some structural support.

The sunshield membranes are coated with aluminum, and two of the outermost layers are treated with silicon, giving the skin-like material a purple hue.

Webb has 344 devices that must work exactly as intended. Of those, 107 are membrane release devices, non-explosive actuators that pin the sunshield in place for launch.

In total, the mission’s deployment sequence relies on 140 release mechanisms, 70 hinge assemblies, eight deployment motors, 400 pulleys, and 90 cables running a quarter-mile in length. There are also an array of bearings, springs, and gears to transform Webb from its launch to operational configuration.

With the sunshield in its diamond shape, covering an area the size of a tennis court, Webb controllers will send commands for the observatory to tension each of the five layers over two days — currently planned on Jan. 1 and Jan. 2.

“Once we get the sunshield out, that’s great, but then we have to sort of tighten it up,” Parrish said. “All five layers have different points around them where they’re connected up, and then we’ll pull on cables in each one of those corners to actually tighten up the sunshield.”

“The very last step is super important,” Puga said. “We need to tension all of the membranes using a series of pulleys and cables to create the separation between each of the five layers.”

The tensioning will separate each of the five ultra-thin kapton membranes, spacing them a few inches at the center and a few feet at the outermost edges. The tapered spacing helps allow heat from the sun to reflect between the layers, and eventually radiate back into space.

If there’s a minor tear or snag that only affects a small part of the sunshield, Webb could still accomplish many of its science goals. Three of the telescope’s instruments — the U.S.-made Near-Infrared Camera, the European Near-Infrared Spectrograph, and the Canadian Near Infrared Imager and Slitless Spectrograph — are tuned to observe the universe in near-infrared wavelengths closer to visible light.

They’re designed to work at temperatures around minus 388 degrees Fahrenheit, or 40 degrees above absolute zero. Those instruments could still be sensitive to faint infrared light if they’re a few degrees warmer, Menzel said.

Webb’s Eurpoean-built Mid-Infrared Instrument, designed to detect longer light waves, needs to be chilled 33 degrees colder than the others. A cryocooler pumping cold helium gas into the instrument will get the mid-infrared instrument to its prescribed temperature, but any deficiencies in the observatory’s sunshield might jeopardize MIRI’s science goals, which are aimed at detecting light from the universe’s oldest galaxies, and peering into star-forming dust clouds opaque at shorter wavelengths.

But any large-scale malfunction would be difficult for Webb to overcome.

“All of these single-point failures, all of these deployment mechanisms, all of the things the have to go right to make a telescope, there’s no halfway stage,” said Mark McCaughrean, a senior science advisor at ESA and an interdisciplinary scientist with observing time on Webb in its first year of operations.

“If the sunshield only half-deploys, the telescope never gets cold, the instruments won’t turn on,” McCaughrean said. “So that’s the nerve-wracking time — that first month — and after that the five months of cool down and commissioning, and then we can start doing the science.

This illustration shows the major elements of the James Webb Space Telescope. The Integrated Science Instrument Module, or ISIM, contains the mission’s four science instruments. Credit: NASA

“There’s never 100% certainty on anything like this,” McCaughrean said before the launch. “And if it fails, I think it would be an absolute catastrophe for astronomy, for space science. There’s been a lot of talk about this mission for many years, and if it does fail somewhere along the way, there will be a reckoning.

“But that’s why we feel so much pressure to have done the right job before we launched it,” he said.

Assuming no delays with the sunshield deployment, Webb’s ground team will turn their attention to moving elements of the telescope into place next week.

First will come the deployment of the secondary mirror, currently planned for Jan. 4, which focuses light from the primary mirror into the telescope’s internal focal plane, ultimately leading to Webb’s instrument detector arrays.

Then the two wings of the primary mirror will fold into place Jan. 6 and Jan. 7, according to Webb’s preliminary flight plan.

Each wing holds three of Webb’s six-sided mirror segments, with 12 mirrors fixed to the primary mirror’s central structure.

All along the way, we’re doing a lot of releases and (actuating) launch locks,” Parrish said. “We release our cryogenic cooler assembly, we have some additional radiators that we have to deploy … I don’t want to call them small deployments, but there are other deployments that don’t quite make the headlines like the sunshield does.”

Once the mirrors are in place, optical engineers will begin unlocking each segment from its launch restraint. Tiny motors behind each mirror can move it back and forth, up and down, left and right, and even slightly bend the mirror segment to bring the entire telescope into sharp focus.

“That’s a very tedious process,” Parrish sad. “They only move them a really, really tiny bit each time.”

Once Webb is in orbit around L2 next month, ground controllers will continue aligning the telescope, switch on and calibrate the instruments, and ready the observatory for science operations. All this work will occur in parallel to the natural cool-down of the telescope to cryogenic temperatures.

Engineers devised numerous emergency procedures to deal with a problem during Webb’s intricate deployment sequence. They range from options as simple as switching to a backup drive, where possible, to backing up a motor and re-attempting a deployment.

Engineers work with Webb’s sunshield inside a Northrop Grumman clean room in California. Credit: Northrop Grumman

The last-ditch options? Teams can shimmy or twirl Webb in space, or subject the observatory to fire and ice.

“A shimmy is our ability to shake Webb a little bit using its on-board propulsion system,” Parrish said. “We can actually impose some shaking into that system.

“We can also twirl the observatory, which means we can spin it at a rate to where we can put some static forces and help put some loading on an are that may be a little stubborn.

“We also have the ability to use the attitude control system, specifically the propulsion system, to be able to move Webb back and forth rapidly, so we can heat and cool a specific area that may be giving us some trouble … We call the back and forth fire and ice.

“If we’re using those tools in our toolbox, we’re not having the best of days,” Parrish said.

With Webb heading four times farther than the moon, there’s no way for astronauts to visit for repairs, like space shuttle crews serviced the Hubble Space Telescope. It also wasn’t designed for robotic servicing.

Grunsfeld, who flew to Hubble on three shuttle repair missions, said he tried to implement a late design change on Webb to make it more amenable to servicing.

“When I took over management of the science mission directorate, and by extension the James Webb Space Telescope, one of my first responses was let’s put a little grapple fixture — really small — and some targets on the James Webb, just in case we have to run it down and give it a little shake to try and deploy it.

“And I got enormous push back,” Grunsfeld said. “The design for James Webb was incredibly mature, basically finished, and mass was a concern. Even though what I proposed was half a kilogram, they said, ‘No, we can’t add any mass.'”

And there are no cameras on-board to give ground teams a visual perspective on how things are going. But any camera in a position to observe the telescope’s deployment would need to be on the “cold side” off the observatory, requiring them to be armored to withstand the super-cold, shadowed conditions.

Warmth put out by the camera electronics could interfere with Webb’s science observations.

“Anything that’s hot within the field of view of the telescope will ruin it,” Grunsfeld said.

“We have lots of little micro-switches to tell us what the configuration is exactly, so we don’t necessary need cameras. We can image it on a computer,” Grunsfeld said. “While I agree, totally, that that’s true, being visual animals, we like to see things, but we will have those animations.”

Quelle: SN

----

Update: 31.12.2021

JWST begins sunshield deployment

Two structures on either side of JWST lowered into position Dec. 28, starting a delays-long process of deploying the spacecraft's sunshield. Credit: NASA

WASHINGTON — NASA’s James Webb Space Telescope has started the process of deploying the sunshield required to keep the spacecraft cold enough to operate, a process that is one of the riskiest aspects of the mission.

NASA announced Dec. 28 the spacecraft controllers had started the multiday process of deploying the sunshield by lowering two components called Unitized Pallet Structures, one on the front side of the spacecraft and the other on the back. The front structure locked into position at 1:21 p.m. Eastern, followed by the rear one about six hours later.

The structures contain the five-layer aluminum-coated Kapton sunshield. Once deployed, the sunshield will block sunlight from reaching the telescope and its instruments, allowing them to cool to their operating temperature of –233 degrees Celsius. Those cold temperatures are required for the telescope to function effectively for infrared observations.

The lowering of the structures is only the start of a complex process to deploy the sunshield. After the tower that holds the telescope is extended, a process scheduled for Dec. 29, the sunshield covers will release the next day. Booms on the left and right sides of the spacecraft will extend a day later, after which cables will shape the five layers into their final form.

The projected timeline for the mission calls for the sunshield deployment process to be completed Jan. 2. However, project engineers cautioned before the launch that deploying the sunshield was perhaps the most difficult process of the overall commissioning of JWST.

“The sunshield is one of those things that is almost inherently indeterministic,” said Mike Menzel, lead mission systems engineer for JWST at NASA’s Goddard Space Flight Center, during a November briefing. “The sunshield is one that has some risk to it.”

At that same briefing, Krystal Puga, JWST spacecraft systems engineer at Northrop Grumman, compared the sunshield deployment process to a “Rube Goldberg machine” in that each step of the deployment triggers the next, using a combination of cables, pulleys and motors.

She added, though, that she was confident that the sunshield will deploy as designed despite that complexity. “We performed multiple deployment testing over several years on both small and full-sized models,” she said. “This gives us the confidence that Webb is going to deploy successfully.”

Quelle: SN

+++

Telescope tower extends on Webb observatory

The James Webb Space Telescope extended a four-foot tower Wednesday to give the observatory’s mirrors and instruments, designed to function at cryogenic temperatures, enough separation from the hot side of the spacecraft after the mission’s sunshield deploys over the next few days.

Webb’s Deployable Tower Assembly moved the telescope’s mirrors, still folded up in their launch configuration, away from the spacecraft bus. The telescope structure contains Webb’s 18 gold-coated hexagonal primary mirror segments and four infrared instruments mounted in a carbon composite module.

In an update posted online, NASA said the motor-driven telescoping tower extended about 48 inches (1.22 meters). The deployment took more than six-and-a-half hours, beginning at around 9:45 a.m. EST (1445 GMT) and ending at approximately 4:24 p.m. EST (2124 GMT), according to NASA.

Ground teams at the Space Telescope Science Institute in Baltimore, Maryland, methodically sent commands to Webb throughout the day. The human-in-the-loop control plan allows engineers to track each step of Webb’s critical deployments, from launch through commissioning and instrument calibration, culminating in the first science observations around six months into the mission.

But the mechanisms on Webb are robotic and outside the reach of engineers on Earth, or astronaut servicing. There are 344 critical devices that must function as intended for Webb to accomplish its mission observing the universe.

Webb is the largest space telescope in history, 100 times more powerful than the Hubble Space Telescope. Its primary mirror will span 21.3 feet, or 6.5 meters, in diameter once fully deployed. Hubble’s monolithic primary mirror has a diameter of 7.9 feet, or 2.4 meters.

The Deployable Tower Assembly provides separation between the telescope and the spacecraft. Webb’s sunshield, which will unfurl to the size of a tennis court, will put the telescope’s mirrors and instruments in permanent shadow, allowing them to cool down nearly minus 400 degrees Fahrenheit.

Webb’s detectors need to be cold to sense the faint infrared light emitted from galaxies more than 13.5 billion years ago, within 100 million to 200 million years after the Big Bang. The mission will also study the atmospheres of planets around other stars, and reveal new insights into our own solar system.

Webb’s spacecraft bus, which must point its solar array toward the sun to generate electricity, will see temperatures of nearly 200 degrees Fahrenheit.

The sunshield enables Webb to achieve the nearly 600-degree temperature differential, and the tower extension Wednesday also helped clear the way for the observatory to start unfolding the five-layer thermal barrier in the next few days.

“All of the material for the sunshield is actually stowed in that area, or some of it is, so we need to get the telescope up away, so that will allow us to pull that material out,” said Keith Parrish, NASA’s commissioning manager for Webb, in an interview before the mission’s Christmas Day launch.

Since launch on a European Ariane 5 rocket, Webb has extended its solar panel, completed two mid-course correction burns, unfurled its high-gain antenna, and unfolded two pallet structures Tuesday containing the ultra-thin sunshield material.

The next big event, tentatively set for Thursday, will be the opening of protective covers over the sunshield membranes. The five layers, each made of an insulating material known as kapton, took a month to carefully, and manually, stow inside the pallets, akin to the way a parachute is packed before a skydive.

An aft momentum flag will also deploy Thursday to help balance Webb against the tenuous, but constant, bombardment by pressure from sunlight. Photons zipping by Webb at the speed of light will push against the sunshield, and the rear flap will help stabilize the observatory and minimize fuel usage to counteract the pressure.

As soon as Friday, mission control will uplink commands for Webb to extend two booms from each side of the spacecraft. The booms will help pull the sunshield layers into their distinctive kite-like diamond shape.

A system of cables and pulleys will tension each of the five layers this weekend, creating space between each membrane to help radiate heat from the sun back into space. The sunshield deployment is widely cited by astronomers and engineers as the riskiest moment in the life of the nearly $10 billion Webb telescope.

Then engineers will move on to position Webb’s secondary mirror and the telescope’s port and starboard mirror wings into place on each side of the fixed central mount. The wings each hold three of Webb’s 18 primary mirror segments, while 12 segments are on the center section.

Quelle: SN

+++

Precise Ariane 5 launch likely to extend Webb's expected lifetime

After a successful launch of the NASA/ESA/CSA James Webb Space Telescope on 25 December, and completion of two mid-course correction manoeuvres, the Webb team has analysed its initial trajectory and determined the observatory should have enough propellant to allow support of science operations in orbit for significantly more than a 10-year lifetime (the minimum baseline for the mission is five years).

Webb’s precise launch on an ESA-provided Ariane 5 rocket was performed by Arianespace on behalf of ESA from Europe’s Spaceport in French Guiana.

The analysis shows that less propellant than originally planned is needed to correct Webb’s trajectory toward its final orbit around the second Lagrange point known as L2, a point of gravitational balance on the far side of Earth away from the Sun. Consequently, Webb will have much more than the baseline estimate of propellant – though many factors could ultimately affect Webb’s duration of operation.

Webb has rocket propellant on board not only for midcourse correction and insertion into orbit around L2, but also for three necessary functions during the life of the mission: ‘station-keeping’ manoeuvres – small thruster burns to adjust Webb's orbit; pointing the telescope toward science targets; and what’s known as momentum management, which maintains Webb’s orientation in space.

The extra propellant is largely due to the precision of the Arianespace Ariane 5 launch, which exceeded the requirements needed to put Webb on the right path, as well as the precision of the first mid-course correction manoeuvre – a relatively small, 65-minute burn after launch that added approximately 20 m/s to the observatory’s speed. A second correction manoeuvre occurred on 27 December, adding around 2.8 m/s to the speed.

The accuracy of the launch trajectory had another result: the timing of the solar array deployment. That deployment was executed automatically after separation from the Ariane 5 based on a stored command to deploy either when Webb reached a certain attitude toward the Sun, ideal for capturing sunlight to power the observatory, or automatically at 33 minutes after launch. Because Webb was already in the correct attitude after separation from the Ariane second stage, the solar array was able to deploy about a minute and a half after separation, approximately 29 minutes after launch.

From here on, all deployments are human-controlled so deployment timing – or even their order -- may change.

Webb is the largest, most powerful telescope ever launched into space. As part of an international collaboration agreement, ESA has provided the telescope’s launch service using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the development and qualification of Ariane 5 adaptations for the Webb mission and for the procurement of the launch service by Arianespace.

Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).

Quelle: ESA

Astronomie - NASA JamesWebb Telescope - Update-26

« Zum vorherigen Eintrag Zum nächsten Eintrag »

909 Views