Webb set to arrive at its final destination, L2

Diagram: Sun, Earth, Earth's orbit, straight lines from Earth to points labeled L1 to L5.
View larger. | A body’s distance from the sun, and the speed it maintains to keep that distance, are correlated. There are 5 points in the Earth-sun system where a spacecraft can move at such a speed that the craft stays put relative to the Earth and sun. These are the 5 Lagrange points, shown here. Webb is headed to L2. Image via NASA.

The James Webb Space Telescope’s dramatic unfolding is now complete. On Wednesday (January 19, 2022), engineers finished deploying the individual mirror segments. On Monday, ground teams plan to fire Webb’s thrusters, placing the telescope at its final destination, Lagrange point 2, or L2, some million miles (1.5 million km) away from Earth. NASA TV is scheduled to begin its coverage at mission control at 19 UTC (2 p.m. Eastern) on Monday. There’s also going to be a live event, where experts will be answering questions. That’ll begin at 20 UTC (3 p.m. Eastern) Monday.

The 2 p.m. Eastern broadcast will air live on NASA TV.

The 3 p.m. Eastern live online panel discussion will air on the NASA Science Live website, as well as YouTube, Facebook, and Twitter. Audio of the teleconference will stream live on the agency’s website.

Before this past month, space fans were jokingly referring to the month between Webb’s December 25 launch – and its January 24 placement at L2 – as 30 days of terror. That’s because, overall, Webb has 344 single-point failure items. About 80% of those items followed launch and were part of the deployment process. But wow! Things have gone smoothly so far. Webb’s engineers must be secretly breathing sighs of relief and feeling very proud! As they should. Webb is a marvelous machine.

Once Webb is safely at L2, the team will work on telescope alignment, with first images from the telescope due to arrive this summer.

EarthSky lunar calendars are back in stock! We’re guaranteed to sell out – get one while you can.

Why L2, and where is it?

So, why is Webb going to a point so far away? Basically, this point in space – the second Lagrangian point – is where, in the Earth-sun system, gravitational forces and a body’s orbital motion balance each other. So, in effect, a spacecraft can “hover” relatively easily at L2. It can stay near Earth, while both Earth and the spacecraft orbit the sun. In fact, the European Space Agency (ESA) has called L2:

… a preeminent location for advanced space probes.

Other notable space observatories orbit, or will orbit, the sun at L2, including ESA’s redoubtable Gaia spacecraft, which has made so many fascinating discoveries about our Milky Way galaxy. And Gaia and Webb aren’t the only spacecraft at L2. Click here for a list of past, present and planned space probes at L2.

Keeping Webb safe from the heat

L2 has another advantage. It’s a step farther away from the heat of the sun and Earth. Satellites in Earth orbit – for example Hubble and the space station – undergo temperature changes about every 90 minutes, depending on whether the satellite is in shadow or sun. At L2, Webb won’t undergo this same temperature-shifting effect that would otherwise create distortions in the telescope’s ability to view the universe.

Webb will observe primarily infrared light coming from faint and very distant objects. To be able to detect those faint signals, the telescope itself must be kept extremely cold: -370 F (about -220 C) or lower. That’s why Webb has a five-layer, tennis-court-sized sunshield, to protect the telescope from the heat of the sun and keep its instruments cold.

Being at L2, farther from the sun than the Earth or moon, will help, too.

Webb’s unfolding, step by step

Webb successfully launched on an Ariane 5 rocket on December 25, 2021. Next, it successfully deployed its solar array to stop draining battery power. It then successfully completed two mid-course correction burns. Then, Webb released its antenna. It lowered the forward and aft pallet structures that support the sunshield. On January 4, it completed the deployment and tensioning of all five layers of the sunshield.

On January 5, 2022, Webb’s team deployed the secondary mirror, the step that gave mission scientist Heidi Hammel the most trepidation. She’d been quoted earlier as saying:

For me, personally, that’s the scariest part of the whole deployment sequence, the secondary mirror … If we don’t have a secondary mirror, we don’t get any light from space into our cameras and spectrographs. There’s nothing.

On January 7, Webb unfolded the first of two primary mirror “wings.” On January 8, it unfolded the second mirror “wing,” concluding major deployments. Webb’s primary mirror is six times larger than Hubble’s. It’s made of 18 individual mirror segments, which were folded for launch. Now that they’ve been successfully deployed – and after Webb is safe at L2 – those 18 segments will all need to be recalibrated (aligned), which will take about 10 days.

Ten days of mirror alignment, followed by weeks of testing, are the reasons the telescope won’t be ready for scientific operations for about six months.

Webb: Large sunshield with big gold mirror attached at right angles with long struts to smaller mirror.
Artist’s concept of Webb Telescope in space, with its sunshield deployed and its secondary mirror (small mirror at the end of long booms) in place, before the complete deployment of the large mirror. “We are 600,000 miles from Earth and we have a telescope,” announced Bill Ochs, Webb’s program manager, after the secondary mirror successfully extended and securely latched into place. Image via ESA.

Earlier NASA Webb Telescope tweet updates

On Twitter, NASA has been providing near real-time updates when Webb successfully completes steps of its mission. You can follow along what’s going on with the telescope, too, at NASA Webb Telescope.

Webb vs. Hubble

Hubble provided images in visible light. But Webb is an infrared telescope. The advantage of infrared is that Webb will be able to look farther into the universe than ever before. NASA gave a great visualization of the power of Webb to look into the past during a Q&A session on Reddit:

Imagine all of time, from the beginning of the universe until now, is represented on a year-long calendar. If right now is December 31 at 11:55 pm, Webb will be able to see all the way back to January 6th.

NASA also explained why scientists want to see in the infrared and use spectroscopy. Spectroscopy allows astronomers to:

understand what is there, not just how it looks.

Looking deeper

When a Reddit user asked if Webb was going to reproduce the famous Hubble Ultra Deep Field photo, NASA said:

Webb will observe the Ultra Deep Field in our first year of science operations. It’ll take Webb less than a day to see deeper than Hubble saw in two weeks of staring. Webb is going to go much deeper, finding tens of thousand of galaxies that are too red and too faint for Hubble to detect.

Of course, we’ve all been wowed and moved for decades by what the Hubble Space Telescope has shown us of the universe. Now, we expect to be further blown away by the images and revelations of Hubble’s successor, the James Webb Space Telescope.

Sun at left, Earth's orbit, and long arrow from sun through Earth to point labeled L2.
For the past month, the James Webb Space Telescope has been traveling to Lagrangian point 2, aka L2, some 4 times the moon’s distance away. L2 is a gravitationally stable point in the Earth-sun system. A spacecraft at that point orbits the sun while “hovering” in place with respect to Earth. Image via ESA.

Bottom line: The James Webb Space Telescope has been unfolding its major components on its way to L2. After it’s safely in orbit, the telescope will undergo alignment in preparation for its science mission.

Via Inverse



January 22, 2022

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