5
$\begingroup$

The Herschel space telescope wins as largest optical telescope mirror in space when "optical" includes far infrared (~55 µm). The Wikipedia article Herschel Space Observatory lists the orbit as

Reference system: L2 point (1,500,000 km / 930,000 mi)

Regime: Lissajous

The article also includes an animated GIF of Herschel's orbit with the caption:

Animation of Herschel Space Observatory's trajectory from 14 May 2009 to 31 August 2013...

Animation of Herschel Space Observatory trajectory

According to the animation's page the GIF is an:

Animation of Herschel Space Observatory's trajectory from 14 May 2009 to 31 August 2013

Own work

Data source: HORIZONS System, JPL, NASA

Herschel's orbit around Sun-Earth L2 as shown in the GIF looks exactly like what one would call a Halo orbit. According to this answer to What is the difference between halo orbits and Lissajous orbits? a Halo orbit is a sub-class of Lissajous orbit where the in-plane and out-of-plane periods are in a 1:1 ratio.

But in the last frame of the GIF suggests something changed in 2013.

Last frame of Animation of Herschel Space Observatory trajectory

I've added the question mark annotation.

Rather than the circa 1.5 million km from Earth of a Sun-Earth Lagrange point halo orbit, the last frame shows a distance of about 6.2 million kilometers; four times farther from Earth than the halo orbit!

This then leads to the Questions:

  1. Is Herschel no longer in a halo orbit, but still in a Lissajous orbit?
  2. Where is Herschel now? What does it's orbit look like now, and how does it differ from the halo orbit shown in this 2009 to 2013 animation?
  3. Why the change?
$\endgroup$
5
$\begingroup$

The telescope was placed into a heliocentric orbit when its helium supply was depleted

On 29 April 2013, ESA announced that Herschel's supply of liquid helium, used to cool the instruments and detectors on board, had been depleted, thus ending its mission. At the time of the announcement, Herschel was approximately 1.5 million km from Earth. Because Herschel's orbit at the L2 point is unstable, ESA wanted to guide the craft on a known trajectory. ESA managers considered two options:

  • Place Herschel into a Heliocentric orbit where it would not encounter Earth for at least several hundred years.
  • Guide Herschel on a course toward the Moon for a destructive high-speed collision that would help in the search for water at a lunar pole. Herschel would take about 100 days to reach the Moon.[50]

The managers chose the first option because it was less costly.

They have a nifty animated gif of the orbit (sorry, no embed through Imgur or SE)

$\endgroup$
  • $\begingroup$ So the orbital parameters section is historical, representing the operational orbit, not the current one i.stack.imgur.com/zNM8W.png In other words, it's not wrong as much as I'm reading more into the information than I should? $\endgroup$ – uhoh Jun 4 at 0:50
  • $\begingroup$ I think you're correct. It can't be in a Lissajous orbit because those orbits are unstable. $\endgroup$ – Machavity Jun 4 at 0:56
  • $\begingroup$ that's not really the correct way to think about it; DSCOVR is in a Lissajous orbit for example. Some halo orbits are unstable and some or all Lissajous orbits are, but with 2-4 m/s station keeping per year many spacecraft can enjoy long lives in "unstable" orbits. SOHO, JWST, etc. 1, 2, 3. $\endgroup$ – uhoh Jun 4 at 1:03
  • $\begingroup$ Less... costly? How is that one less costly? Just the logistics of having a control crew remap the trajectory/perform the maneuver and the salaries of those individuals? $\endgroup$ – Magic Octopus Urn Jun 4 at 14:50
  • 4
    $\begingroup$ @MagicOctopusUrn Fair enough. With the graveyard solar obit, it's basically burn, verify, and close up shop. With the south polar Lunar impact, you need to keep at least part of your control team on to do course corrections on the way in to ensure that the impact happens on schedule so that whatever moon-orbiting spacecraft will be watching will be in position for a good look.So your team has to work with their team, shift around their science priorities,and get the spectroscopy and planetary science specialists in for the event (and possibly the media), that's where I see the expense. $\endgroup$ – notovny Jun 5 at 0:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.