Is the James Webb telescope going to orbit around the earth?

  • If it’s going to orbit around the earth, what kind of orbit would it be (what distance)?
    Are there benefits to sending a telescope further away from the earth if it’s just to photograph galaxies and not objects that are in our solar system?


    Did you try to google 'JWST' and 'orbit'? First hit is a good one: https://jwst.nasa.gov/orbit.html

  • HDE 226868

    HDE 226868 Correct answer

    4 years ago

    Your first question - is JWST going to orbit Earth - is a little complicated. It will follow a mission profile that will send it to the Sun-Earth $L_2$ Lagrangian point. It will take the telescope about three months to achieve its orbit in $L_2$. Now, $L_2$ is unstable, and so some station-keeping - essentially, course corrections by thrusters - will be needed. It won't be orbiting Earth, but orbiting the Sun with Earth, in a nearby location.



    You're not quite right that JWST will only be observing objects outside the Solar System. Some of the most exciting targets - young, evolving galaxies and exoplanetary atmospheres - lie far beyond. However, the telescope's capabilities for atmospheric analysis (thank you, NIRSpec!), useful for determining the composition of exoplanetary atmospheres, will also be used for observations of the atmospheres of Mars and the giant planets, as well as compositional data for other bodies.



    Now, it's not really helpful to send JWST elsewhere in the Solar System, given that most of its observations will be of objects outside the Solar System. Okay, maybe you could get higher-quality data on Mars if you sent it to Mars orbit, but that would be quite costly, and not effective for a spacecraft designed to be used by many scientists for a number of purposes. Keeping it at $L_2$ simplifies the orbital trajectories immensely, although it's still much too far away to repair, unlike Hubble. You also lessen the communication time.



    Well, you might say, why not just launch JWST into Earth orbit? We have scientific instruments there; why do we go to the trouble of putting so many things at $L_2$? It turns out that debris high above Low Earth Orbit (LEO) would be too dangerous for the sensitive optics on the telescope. The best instruments are often also the most delicate, and JWST would be damaged if it was truly orbiting Earth, at LEO or elsewhere.


    I was wondering, is the *one and only reason* we're sending it to L2, the L.E.O. debris issue? What do you reckon?

    No, and in fact it's not even the most important reason. Much of the work of JWST will be in the infrared. Both the Earth and the Sun are bright in this part of the electromagnetic spectrum, so in order for JWST to observe faint and distant objects, it must be able to simultaneously shield the light from both the Earth and the Sun. This requires them to be in the same direction from the telescope, so the same sunshield can cover both of them at all times.

    Your link to the JWST webpage says that it will be at the Sun-Earth L2 point, and that it is in heliocentric orbit, but your answer says it is orbiting the Earth?

    @costrom "In a very broad sense" it will. It'll orbit the sun, and that orbit will follow the orbit of earth' but further out. He isn't saying it will orbit earth, but orbit **around** earth. There is a subtle difference there. Check Fatties picture of it to help.

    Consider that, from some overhead vantage point, L2 is on the left of Earth when Earth is left of the Sun, and right of Earth when Earth is right of the sun. L2 orbits the Earth once per year. To orbit the Sun at that distance would take somewhat longer than one year.

    @costrom Sorry for the confusion; I've edited to clarify.

    @costrom and others: You are assuming that there is a definitive answer to "Will the JWST orbit the Sun or the Earth?" There is a definitive non-pendantic answer, and it is "yes". The L1 and L2 points are right at the boundary that distinguishes geocentric from heliocentric orbits. It will be orbiting both the Earth and the Sun. The L1 and L2 points are markedly non-Keplerian. The pedantic answer to the question is "neither one." The JWST instead will be in a pseudo orbit about the L2 point.

    Indeed, the word "orbit" has no place in a pedantic discussion anyway. It's utterly commonplace that in English many words have more than one meaning, and it's totally commonplace for there to be a difference between the everyday nonscientific use of a term and the precise use in a given field. It's extremely naive - it's *not* precise, pedantic or rigorous - to not realize that in the OP's question, the first sentence means precisely: "Is it going to **go around** the Earth, and if it is going around the Earth at what distance?" Answer, no, it "goes around" the Sun. ....

    Popular science questions are *not* specialized technical questions. Certainly, after answering the question, it might be good to include a footnote, like "BTW you use the word 'orbit'. Technically really the moon doesn't even "orbit" the Earth! They both move around a common point. The Lagrange point is kind of unusual .. etc etc"

    @Fattie: JWST will still go around the Earth, at a distance of AU/100 and a period of 1 year, won't it?

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Content dated before 7/24/2021 11:53 AM