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Puppies and Pringles Get Webb Oriented at Lagrange Point 2

James Webb,The Universe,Puppies
Nancy Atkinson
Matthew Giordano
May 17, 202210:00 AM UTC (UTC +0)

For the next 20 years — hopefully longer — the James Webb Space Telescope will cast its infrared gaze outward on our universe.

JWST launched on Christmas morning in 2021 and spent a month unfolding like an Autobot into its final (awesome) form. All the while, Webb was traveling to its ultimate orbit in space, a location called Lagrange point 2 (L2), about 1.5 million kilometers (1 million miles) from Earth.

Lagrange points have been described as stable points in space, but that doesn’t fully explain them, or define why L2 is such a good location for our newest telescope in space. 

To help clarify, we’ll rely on puppies and Pringles potato chips.

Lagrange points are “wonderful accidents of gravity and orbital mechanics,” says astronomer Michelle Thaller, assistant director for Science Communication at the Goddard Spaceflight Center. These are points in space between any two massive, gravitationally significant objects — such as the Sun and Earth — where gravity from those two large objects balance out the orbital motion of another smaller body — such as a satellite. 

Placing a spacecraft at a Lagrange point allows it to stay in a fixed position relative to the Earth and Sun, which creates a stable location from which the spacecraft can make observations. The added benefit is that once a spacecraft is placed at a Lagrange point, it tends to stay there. This reduces fuel consumption, which in turn lengthens the lifetime of a mission. 

“Putting a spacecraft at any of these points allows it to stay in a fixed position with a minimal amount of energy needed for course correction,” Thaller explained.

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But since everything in space is always in motion, Lagrange points are not actually fixed locations in space, either. While the positions are fixed relative to the Earth and Sun, Lagrange points are constantly moving spots that follow Earth around the Sun. Additionally, a spacecraft isn’t just “at” a Lagrange point, the spacecraft actually orbits it.

From Earth, JWST looks like it is circling an empty point in space.

“A way to describe the orbit is that the Lagrange points always stay aligned with Earth as it orbits the Sun,” Dr. Stefanie Milam, Webb Deputy Project Scientist for Planetary Science told Supercluster. “Therefore, JWST’s orbit around L2 will go around the Sun with the Earth. It’s like we have a small puppy doing circles at the end of a leash as we walk around the Sun.”  

JWST orbits L2 in what’s called a halo orbit, but its orbit isn’t exactly halo-shaped. 

"The way I see it in my head is like a Pringles potato chip," said Jane Rigby, JWST Operations Project Scientist, at a press briefing earlier this year. “We’re just kind of inching up one side and then gently falling back down for the life of the mission.”  

Webb orbits L2 about once every six months and will need to use its thrusters to perform station-keeping maneuvers about every 25 days, in order to stay in the correct orbit and attitude. In maintaining the Pringle-shaped, puppy-on-a-leash orbit, Webb will always be located in the direction away from the Sun. This vantage point allows JWST to keep its giant sunshield positioned to block light and heat from the Sun, Earth, and Moon.  With no background light to interfere, Webb’s instruments can look out unhampered to incredible distances in deep space, seeing with great clarity.

The sunshield also keeps the spacecraft and the instruments at the frigid temperatures required for infrared observing, less than 40 kelvins, (-223 degrees Celsius, -370 degrees Fahrenheit). 

Any concerns about JWST’s location in regards to accessibility for repair or maintenance and even refueling are really not warranted, as Milam says “the design and operational plan for JWST has mitigated most concerns at this position.” Additionally, Webb’s fuel supply is estimated to last for upwards of 20 years of maneuvers. And the ability to frequently realign the mirror segments means there should be no worries for any mirror problems, like the ones Hubble experienced. 

L2 is ideal for astronomy because a spacecraft is close enough to Earth for easy communications, and, as in the case of JWST, can capture sunlight for solar power, with proper shielding. JWST is not the first spacecraft to call L2 home, and two other spacecraft are also currently there: the Planck spacecraft (which is studying the Cosmic Microwave Background, the radiation left over from the Big Bang) and Gaia, which measures the positions, distances and motions of stars with unprecedented precision.

Unlike Webb, however, Planck and Gaia orbit L2 in a rectangular-shaped orbit called a Lissajous orbit. Don’t worry, there’s no chance of a collision. The three telescopes are always between 400,000 and 1,100,000 km apart, (25,000 and 680,000 miles) depending on where they are in their respective orbits.

Lagrangian or ‘L’ points were named after 18th-century Italian astronomer and mathematician Joseph-Louis Lagrange. In total, there are five Lagrange points, and they can be found around the other planets in our solar system, as well. 

The L1 point of the Earth-Sun system allows for an uninterrupted view of the Sun; the Solar and Heliospheric Observatory Satellite SOHO is located there. At L3, a spacecraft would remain hidden behind the Sun at all times, and so is not a practical location for a satellite. But, the concept of a hidden planet has been popular in science fiction. 

The L4 and L5 points are home to stable orbits only for larger objects, such as asteroids. Any asteroids captured in these orbits are called Trojan asteroids, and Earth is known to have two Trojans. But there are hundreds in the solar system. Most orbit with Jupiter, but Mars has some, along with more which act as companions to Saturn's moons.  

And so, L2 is the perfect place to park the Webb telescope in space. It was designed and built to live there. And with a flawless execution so far of the six-month-long commissioning period for the telescope, the anticipation grows for Webb’s ambitious mission to understand the early days of our universe, to examine distant exoplanet atmospheres for signs of life, zoom in on objects in our own solar system, and much more.

Nancy Atkinson
Matthew Giordano
May 17, 202210:00 AM UTC (UTC +0)