'L2' Will be the James Webb Space Telescope's Home in Space

Jun 23, 2010 by Michelle Thaller / Rob Gutro
The five Lagrangian points for the Sun-Earth system are shown in the diagram below. An object placed at any one of these 5 points will stay in place relative to the other two. Credit: NASA

(PhysOrg.com) -- When you ask an astronomer about the James Webb Space Telescope's orbit, they'll tell you something that sounds like it came from a science-fiction novel. The Webb won't be orbiting the Earth -instead we will send it almost a million miles out into space to a place called "L2."

L2 is short-hand for the second Lagrange Point, a wonderful accident of gravity and orbital mechanics, and the perfect place to park the Webb telescope in space. There are five so-called "Lagrange Points" - areas where gravity from the and balance the orbital motion of a satellite. Putting a spacecraft at any of these points allows it to stay in a fixed position relative to the Earth and sun with a minimal amount of energy needed for course correction.

The term L2 may sound futuristic and mysterious, but the name actually honors a Mathematician born in 1736. The Lagrange points were named after the Italian-born mathematician and Joseph-Louis Lagrange, who made important contributions to classical and celestial mechanics. Lagrange studied the "three-body problem" (so-called because three bodies are orbiting each other) for the Earth, sun, and in 1764, and by 1772 he had found the solution; there are five stable points at which you could put an object and have it stay fixed in place relative to the other two.

In the case of L2, this happens about 930,000 miles away from the Earth in the exact opposite direction from the sun. The Earth, as we know, orbits the sun once every year. Normally, an object almost a million miles farther out from the sun should move more slowly, taking more than a year to complete its orbit around the sun. However, at L2, exactly lined up with both the sun and Earth, the added gravity of the two large bodies pulling in the same direction gives a spacecraft an extra boost of energy, locking it into perfect unison with the Earth's yearly orbit. The Webb telescope will be placed slightly off the true balance point, in a gentle orbit around L2.

The James Webb Space Telescope (identified as "JWST" here) relative to the Hubble telescope's orbit around the Earth. Credit: NASA

Why send the Webb telescope all the way out to L2? When astronomers began to think about where the Webb telescope should be placed in space, there were several considerations to keep in mind. To begin with, the Webb telescope will view the universe entirely in infrared light, what we commonly think of as heat radiation. To give the telescope the best chance of detecting distant, dim objects in space, the coldest temperatures possible are needed.

"A huge advantage of deep space (like L2) when compared to Earth orbit is that we can radiate the heat away," said Jonathan P. Gardner, the Deputy Senior Project Scientist on the Webb Telescope mission and Chief of the Observational Cosmology Laboratory at NASA's Goddard Space Flight Center in Greenbelt, Md. "Webb works in the infrared, which is heat radiation. To see the infrared light from distant stars and galaxies, the telescope has to be cold. Webb's large sunshield will protect it from both Sunlight and Earthlight, allowing it to cool to 225 degrees below zero Celsius (minus 370 Fahrenheit)." For the sunshield to be effective, Webb will need to be an orbit where the sun and Earth are in about the same direction.

With the sun and the Earth in the same part of the sky, the Webb telescope will enjoy an open, unimpeded view of the universe. In comparison, the Hubble Space Telescope is in low-Earth orbit where it goes in and out of the Earth's shadow every 90 minutes. Hubble's view is blocked by the Earth for part of each orbit, limiting where the telescope can look at any given time.

The Spitzer Space Telescope, another infrared telescope, is in orbit around the sun and drifting away from the Earth. Spitzer is already more than 100 million kilometers (60 million miles) away from the Earth, and eventually its path will take it to the other side of the sun. Once we can no longer communicate with Spitzer that means it is at the end of its mission life.

In contrast, a major perk of parking at L2 is the ease of communications. Essentially, the Webb telescope will always be at the same point in space. "We can have continuous communications with Webb through the Deep Space Network (DSN)," Gardner said. "During routine operations, we will uplink command sequences and downlink data up to twice per day, through the DSN. The observatory can perform a sequence of commands (pointing and observations) autonomously. Typically, we will upload a full week's worth of commands at a time, and make updates daily as needed."

Even before the Webb telescope, L2 has been known to astronomers as a good spot for space-based observatories. There are already several satellites in the L2 orbit, including the Wilkinson Microwave Anisotropy Probe, and the Herschel and Planck space observatories. But there's plenty of room for another neighbor, and the Webb telescope will be heading out to L2 in the near future.

The Webb is a joint project of NASA, the European Space Agency and the Canadian Space Agency.

Explore further: Image: ESRO-4 readied for solar simulation testing

More information: James Webb Space Telescope - www.jwst.nasa.gov/

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fmfbrestel
5 / 5 (2) Jun 23, 2010
ok, I seem to understand Lagrange points, and how they are gravitationally stable. What i dont seem to understand is how an object can "orbit" an empty piece of space? To my mind this would take continuous course corrections, can someone explain how it is possible to "orbit" L2?
yyz
5 / 5 (1) Jun 23, 2010
Recently an Italian physicist testified before the United Nations Committee on the Peaceful Uses of Outer Space "proposing a radio-quiet zone on the farside that will guarantee radio astronomy and SETI a defined area in which human radio interference is impossible." Banning telcom satellites from L2 is at the heart of the proposal. Science satellites either planned or already there would not cause major disruptions because the radio frequencies they use are well known and mostly narrowband, making it easier to filter them out by lunar farside radio telescopes. JWST should pose no problem here, but an interesting topic for future commercial development of L2. The story here: http://www.centau...?p=13011
yyz
not rated yet Jun 23, 2010
Oops, should read "proposing a radio-quiet zone on the [lunar] farside...."

Additionally, L4 and L5 would be open to commercial development.
joefarah
3.8 / 5 (4) Jun 23, 2010
I believe how it works is as follows:
When JWST is a bit closer to earth than L2, it will move faster than the L2 point in orbit and will move ahead of the earth/sun alignment. At the same time, this will increase the orbit distance from earth to beyond L2 (i.e. the speed will boost the orbit) This will reduce the aligned gravity force on JWST so it will start to slow down wrt L2 as it increases its orbit distance. When it is moving slower than L2 it will begin to re-align with earth/sun (and then overshoot) and the gravity will cause it to start "falling" toward earth for it's slower speed. It will pick up speed and will be traveling closer than L2 as it reaches alignment, when the cycle restarts.

Now this could be wrong as I've assumed an orbit plane parallel to the Sun/L2 point rather than perpendicular.
fmfbrestel
not rated yet Jun 23, 2010
Thanks Joe, that clears things up considerably. So if i understand you right, its really being regularly jostled around L2 by the combination of the Earths and Suns gravity? I have been waiting for one of these JWST articles that come out quite regularly to explain that to me for a while now.
CreepyD
not rated yet Jun 24, 2010
What about gravity from other bodies in the solar system like Jupiter? Surely that would cause a variation in gravity?
lengould100
not rated yet Jun 24, 2010
Hmmm... 930,000 km out?

The radius of the earth's orbit is 1.0 AU.
The radius of Mars' orbit is approximately 1.5 AU.
The period of the earth's orbit is 1 year.
The period of Mars' orbit is approximately 2 years.

So Mars orbit is about 150,000,000 / 2 = 75,000,000 km from earth's orbit. Ok, no problem, nearly 100 times further out. Just checking.
fmfbrestel
not rated yet Jun 24, 2010
Well, it certainly would have 'some' effect, which is why the lagrange points are not totally stable, but they are very much more stable then any other place in the vicinity of earth. The moon probably creates much more instability in the Earth-Sun L2 then anything else. As far as i understand it, any two body system has lagrange points, so you could 'fairly' easily put a satellite into orbit around the earth in the the Earth-Moon L2 and it would stay on the opposite side of the moon from our vantage point on earth. However the suns influence would probably have enough of an effect on the Earth-Moon L2 that would not be very stable.
bottomlesssoul
5 / 5 (1) Jun 27, 2010
@fmfbrestel Lagrange points are unstable as they represent saddle points in the gravity fields of the combined bodies. Imagine resting a ball on the top of a hill, if you can get it very still and as close to the center of the top it will just sit there. However if a small impulse moves the ball it will keep rolling by itself away (down) from the top of the hill and here is the magic.

If you can detect the motion in time and give a small impulse in the opposite direction it remains in place. The faster you can respond to external forces the less fuel you need as long as you can stay near the top of the hill.