How the James Webb Telescope traversed 932,000 miles to Lagrange Point 2

Even now, after everything, Webb is due for course corrections.
Brad Bergan
The James Webb Space Telescope (left), and Webb's trajectory for L2 insertion (right).1, 2

Space travel is tricky.

Even for a state-of-the-art spacecraft like the James Webb Space Telescope.

Its journey from Earth to a parallel orbit outside of our planet's called for ultra-precise calculations, decades of planning, and near-perfect execution, every step of the way. But even now, with NASA's flagship space telescope in orbit of the Lagrange 2 point (L2), Webb has more work to do than astronomy, alone — with maneuvering thrusters needed to counteract the Sun's constant push on the spacecraft, to keep its relative proximity to Earth, according to a NASA blog post.

This is the story of Webb's journey from Europe's Arianespace's ELA-3 launch complex near Kourou, French Guiana to L2, and the challenges to come.

The James Webb Telescope's million-mile journey

Webb's initial launch on December 25, 2021, gave it most of the kinetic energy required to take it on its million-mile trip to L2. This velocity was provided by the colossal Ariane 5 rocket. But there was still much work to be done. Following the launch, Webb was released from its launcher, and readied to use its own, onboard propellants.

"Webb will use its own system of small rocket thrusters to fine-tune its approach to its final halo orbit around the L2 point," reads a NASA blog shared several weeks before the launch. "Webb will use its own system of small rocket thrusters to fine-tune its approach to its final halo orbit around the L2 point (illustrated below), where the telescope and instruments will cool in the shade of the enormous sunshield, protected from the heat of the Sun, Earth, and Moon."

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But additional maneuvers were necessary en route to Webb's final trajectory, three in total: "Mid-Course Correction maneuvers for refining the trajectory are planned (nominally) for 12.5 hours and 2.5 days after launch, with a third one month later, to ease Webb into its L2 orbit," reads the NASA blog post.

Webb to L2
A tactical view of Webb's journey from Earth to its L2 orbit. Source: Steve Sabia / NASA Goddard

A Havard astronomer wonders when Webb will perform station-keeping maneuvers

And, now positioned in its orbit around L2, these thrusters will continue to fire periodically to "maintain that orbit, with small maneuvers called 'station keeping,'" reads the NASA post. These station-keeping maneuvers were initially going to be once every 21 days. These maneuvers are necessary because the pressure of solar radiation builds up on the large surface area of the James Webb Space Telescope's sunshield.

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"Although Webb is designed to keep that pressure well balanced, angular momentum builds up as the telescope points at different targets, so occasional, small momentum-unloading maneuvers are required to keep the observatory's reaction wheels within their proper operating ranges," says the NASA post. The reaction wheels are flywheels designed to help Webb keep its payload in the correct orientation.

But, bizarrely, updates on the station keeping thruster maneuvers were not immediately forthcoming. Astronomer at Harvard's Center for Astrophysics Jonathan McDowell noted this slight hiccup in information with a tweet: "I am puzzled we haven't heard anything about the L2 stationkeeping burns - shouldn't there have been at least one by now?"

Webb will perform station-keeping maneuvers every 42 days

As an expert and authority on space, McDowell's question came at an interesting time. Webb had been in orbit of L2 since late January, but there appeared to be no word on its station-keeping maneuvers. In theory, this could become a big problem — if no thrusters were fired to correct for the Sun's immense push, the James Webb Space Telescope might be pushed out of its ideal orbit around L2, and veer into some planetary body in our solar system, or out of the system altogether.

But a paper from NASA reveals that the idea of having Webb do station keeping maneuvers every 21 days was scrapped because missing one window for station-keeping maneuvers carried with it the risk of not having another window for far too long — long enough for the James Webb Space Telescope to drift out of position, and lose its primary objective (LPO). "If we skipped one maneuver, and then for some reason we could not perform the next maneuver 21 days later, we could end up waiting 63 days between [station keeping] maneuvers, presenting a potential risk for an LPO mission," reads the NASA paper.

Station Keeping
Webb's station-keeping maneuvers are crucial for its mission. Source: NASA

In other words, a short period between station-keeping maneuvers might ironically endanger Webb. In "Most cases a [station keeping] maneuver would be performed every 42 days, not every 21 days," says the NASA paper. This means there's no cause for immediate alarm — as of late March 2022, Webb is due for its first station-keeping maneuver, but could still conceivably not happen until sometime in April.

The end of the beginning - The James Webb Space Telescope is a marvel of engineering — incorporating four dynamic primary instruments that will not only gaze at the very early universe, revealing the beginning of stars and galaxies, but also work in parallel to reveal the statistical features of the entire universe, giving a new and colossal body of data from which we can finally attempt to solve the greatest mysteries in physical science. And, of course, it might find signs of alien life.

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