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...
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
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.
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:
- Is Herschel no longer in a halo orbit, but still in a Lissajous orbit?
- 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?
- Why the change?