How do you track the position of the James Webb space telescope? Does it have its own TLE, or...?

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    $\begingroup$ Seems like a TLE is available at N2YO , and they cite AFSPC as source. But considering the L2 distance, I'm not sure how useful that would be? In terms of propagation, I mean $\endgroup$
    – Rafa
    Jan 15, 2022 at 14:35
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    $\begingroup$ Considering that a TLE specifically describes an elliptical orbit around Earth, it seems at best of very limited use for JWST. You could express its position at any given time, but not its actual orbit. $\endgroup$ Jan 15, 2022 at 14:55
  • $\begingroup$ Different but related: Has a TLE ever been issued for a spacecraft trajectory not bound to Earth orbit? $\endgroup$
    – uhoh
    Jan 17, 2022 at 8:13
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    $\begingroup$ @uhoh ...that's what I said? You should be able to concoct a TLE with an elliptical orbit that correctly defines its position and velocity at the instant of the TLE's epoch, but it won't actually be following that orbit. $\endgroup$ Jan 17, 2022 at 23:36
  • 1
    $\begingroup$ Yeah, you only get both if it's actually slow enough wrt. Earth that it would be in an elliptical orbit in an otherwise-empty universe. Otherwise, you can still get position. $\endgroup$ Jan 18, 2022 at 0:17

2 Answers 2


@Rafa is right, there is at least one officially released TLE for the JWST mission and here's the latest one at n2yo.com:

1 50463U 21130A   21362.00000000  .00000000  00000-0  00000-0 0  9999
2 50463   4.6198  89.0659 9884983 192.3200  17.4027  0.01958082    27

But TLE's will not do a good job here.

@ChristopherJamesHuff is right that TLE's can't do the job here. They might reproduce the right position at epoch, but if you propagate them forward or backward they will no reproduce the trajectory or orbital motion.

Even a broken clock is right twice a day

But in this case, the TLE is only right at one moment in time, if ever.

The epoch 21362.00000000 is the 362nd day of 2021, which seems to be 2021-12-28 00:00 UTC.

JWST launched on 25 December 2021 at 12:20 UT so this is about two and a half days after launch; and therefore it would be in deep space already and not orbiting the Earth.

And yet, there was still a TLE issued!

Let's see what it says:

With a mean motion of 0.01958082 Earth orbits per day this TLE is trying to say that it's at least in a very high orbit. In reality it was no longer bound to Earth or in Earth orbit at this time, so it's not right.

$$T = 2 \pi \sqrt{\frac{a^3}{GM}}$$


$$a = \left( \frac{GM T^2}{4 \pi^2} \right)^{1/3}$$

and with GM = 3.986E+14 m^3/s^2 and a 51.07 day orbit that puts the semimajor axis at about 58,000 kilometers.

But with an eccentricity of 0.9884983 the predicted distance from Earth at epoch or another given moment could be almost twice $a$ or 1 percent of $a$. So we'd have to propagate it with SGP4 to find out what it's saying and how well it matches the real trajectory.

It turns out that I'm crazy enough to do just that!

From JLP's Horizons:

 date & time UTC        distance (km)   rate (km/s)
-----------------     ----------------  -----------
2021-Dec-26 00:00     1.3369842393E+05   2.2108515
2021-Dec-26 06:00     1.7778266287E+05   1.9034419
2021-Dec-26 12:00     2.1651707751E+05   1.6961163
2021-Dec-26 18:00     2.5157752082E+05   1.5470824
2021-Dec-27 00:00     2.8371359651E+05   1.4329090
2021-Dec-27 06:00     3.1363829802E+05   1.3408820
2021-Dec-27 12:00     3.4175065486E+05   1.2642573
2021-Dec-27 18:00     3.6833557041E+05   1.1988998
2021-Dec-28 00:00     3.9360502697E+05   1.1421383
2021-Dec-28 06:00     4.1777927226E+05   1.0948360
2021-Dec-28 12:00     4.4093726644E+05   1.0502208
2021-Dec-28 18:00     4.6318106793E+05   1.0100482
2021-Dec-29 00:00     4.8459829675E+05   0.9735797
2021-Dec-29 06:00     5.0526046491E+05   0.9402252
2021-Dec-29 12:00     5.2523381459E+05   0.9095617
2021-Dec-29 18:00     5.4457084676E+05   0.8812288
2021-Dec-30 00:00     5.6331803317E+05   0.8549237

From Skyfield: uhoh!

Right now it looks like this TLE is so unphysical that it's outside of Skyfield version 1.41's ability to interpret. Stay tuned, since n2yo can do it, some newer version of Skyfield might be able to as well.

In the mean time I've asked Why can n2yo somehow interpret this TLE but Skyfield can't? Why does it return nans and zeros for position?

array([nan, nan, nan, nan, nan, nan, nan,  0.,  0.,  0.,  0.,  0.,  0.,
    0.,  0.,  0.,  0.])

failed plot of JWST distance to Earth based on a TLE

import numpy as np
import matplotlib.pyplot as plt
from skyfield.api import load, EarthSatellite, Loader, wgs84
import datetime

loaddata = Loader('~/Documents/fishing/SkyData')  # avoids multiple copies of large files
ts = loaddata.timescale() # include builtin=True if you want to use older files (you may miss some leap-seconds)
eph = loaddata('de421.bsp') # a small one, fine for this

L1, L2 = """1 50463U 21130A   21362.00000000  .00000000  00000-0  00000-0 0  9999
2 50463   4.6198  89.0659 9884983 192.3200  17.4027  0.01958082    27""".splitlines()

sat = EarthSatellite(L1, L2)

days = np.arange(17)/4. - 2.  # +/- 2 days in 6 hour increments

times = ts.tt_jd(sat.epoch.tt + days)

epoch_string = datetime.datetime(*sat.epoch.utc[:5]).isoformat().replace('T', ' ') + 'UTC'

print('+/- 2 days around epoch of: ', epoch_string)

g = sat.at(times) # geocentric position object

p = g.position.km  # geocentric positions (km)

d = g.distance().km

if True:
    fig, ax = plt.subplots(1, 1)
    ax.plot(days, d)
    ax.plot(days, d, 'ok')
    ax.set_xlim(days.min() - 0.1, days.max()+0.1)
    ax.set_xlabel('days relative to TLE epoch')
    ax.set_ylabel('distance to geocenter (km)')
    ax.set_xlabel('days relative to TLE epoch')
    plt.suptitle('JWST TLE epoch: ' + epoch_string)
  • $\begingroup$ Makes sense. Thank you. :-) p.s. it's nice to see you're still very active here! :) $\endgroup$ Jan 19, 2022 at 0:02
  • 1
    $\begingroup$ @lawndownunder python, math, coffeee and Stack Exchange are right up there with oxygen and water; couldn't live without 'em :-) $\endgroup$
    – uhoh
    Jan 19, 2022 at 0:07

Partial, and a bit simplistic, I compared it to Juno, currently orbiting Jupiter.

Like Juno, JWST is not orbiting Earth but rather the Sun (and or L2 itself).

So, like Juno, a TLE (list of orbital elements of an Earth-orbiting object for a given point in time) cannot exist for JWST (aside for the brief moment it was considered in Earth orbit, then the USSF assigned a TLE) as it does not orbit Earth.

JWST TLE briefly assigned:


SpaceForce TLEs for Webb showing it in a 316 x 1011924 km x 4.1 deg orbit. No TLE for the rocket stage so far.

TLEs for the ZY1-02E launch showing four objects, three in 761 x 766 km x 98.6 deg orbit and one (rocket stage) in 598 x 764 km

4:57 PM · Dec 26, 2021

Currently, US Air Force Space Command still has a TLE for JWST though..


Consider Juno, no TLE:


So, in answer to

Does the James Webb Telescope have a Two Line Element set?

Yes, but it shouldn't ..?

Also see:

Has a TLE ever been issued for a spacecraft trajectory not bound to Earth orbit?

  • 1
    $\begingroup$ Entirely understandable, I just put my blinkers on and said do I risk answering that specific question, well yes, but... (Maybe submit a TLE for Juno just for the sake of company..) $\endgroup$ Jan 18, 2022 at 0:54

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