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Halo orbits are a sub-class of Lissajous orbits. See this answer for (much) more on that.

DSCOVR's orbit will put it in it's Sun Exclusion Zone in about 2020 where the communications line of sight will be too close to the Sun, so there is a planned orbital correction there to handle the situation. You can see from the image the insertion point labeled LOI, and about a dozen cycles in five years. The horizontal and vertical periods are almost the same for this orbit. From Lissajous Orbit Control for the Deep Space Climate Observatory Sun-Earth L1 Libration Point Mission

After 2020, DSCOVR will then have to burn fuel every 3 or 6 months to stay on that exclusion zone-riding ellipse. The linked report predicts that fuel will run out around 2028.

DSCOVR's Lissajous orbit

Question: Why couldn't Triana have her halo orbit, and therefore a much longer continuous-coverage life? Why put DSCOVR in a Lissajous orbit instead of a halo orbit (to stay out of Sun exclusion zone)?

There is relevant discussion in answers and especially comments associated with Why would a mission to Sun-Earth L1 have an instantaneous launch window?

enter image description here

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    $\begingroup$ Larger fuel consumption needed for halo than Lissajous, I am guessing. Wind recently (~June, August, and November of 2020) had to do three maneuvers to put it into a halo instead of a Lissajous orbit to avoid the solar exclusion zone. But Wind was also sitting on ~120 years of remaining fuel, so fuel wasn't the big concern. DSCOVR is not in that same boat. $\endgroup$ Commented Feb 22, 2021 at 23:06
  • $\begingroup$ @honeste_vivere okay we're making progress. JWST's halo orbit has an expected delta-v budget of only 2.4 m/s per year, but it has an aggressive monitor/correction cadence of two weeks and a giant solar sail that can be used to advantage. 1, 2, 3, 4 $\endgroup$
    – uhoh
    Commented Feb 23, 2021 at 3:00
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    $\begingroup$ Oh sorry, no the maneuvers to maintain a halo orbit are not bad, it's the insertion that is problematic. The new Wind orbit is not significantly more expensive fuel-wise than the old Lissajous orbit. However, insertion into the halo orbit from the Lissajous cost us ~40+ m/s of delta-v (whereas typical station keeping maneuvers only cost ~4 cm/s). $\endgroup$ Commented Feb 23, 2021 at 14:18
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    $\begingroup$ @honeste_vivere related but not completely answered: Why would a mission to Sun-Earth L1 have an instantaneous launch window? $\endgroup$
    – uhoh
    Commented Feb 24, 2021 at 1:57
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    $\begingroup$ @Cornelis ya that should probably be edited, they've just copied "looping halo orbit" from Spaceflight Now's July 2015 article DSCOVR space weather sentinel reaches finish line "DSCOVR arrived Sunday and entered a looping halo orbit around L1." and maybe that's copied from somewhere else or the author didn't want to get into what a Lissajous orbit is. I'm not a Wikipedia editor, but if someone was interested they could raise the issue in the article's Talk page. $\endgroup$
    – uhoh
    Commented Feb 4, 2022 at 23:40

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Why couldn't Triana have her halo orbit, and therefore a much longer continuous-coverage life? Why put DSCOVR in a Lissajous orbit instead of a halo orbit (to stay out of Sun exclusion zone)?

This is mostly a "guess" based upon what we experienced while inserting Wind into a halo orbit from a Lissajous orbit. A Lissajous orbit is cheaper, fuel-wise, because the out-of-plane component of the orbit is disconnected from the two in-plane components. To change from a Lissajous to a halo orbit, three big maneuvers were required on June 26, 2020 (HT-1), August 31, 2020 (HT-2), and November 9, 2020 (HT-3). There was a small trim maneuver on September 24, 2020 (HT-2 Trim) too. The stats on the manuevers are as follows:

HT-1

  • Fuel used: ~5.518 kg
  • $\Delta v$ (radial): < 1.0 m/s (i.e., Sun-Earth direction)
  • $\Delta v$ (axial): >11 m/s (i.e., out of ecliptic plane)
  • $\Delta v$ (total): ~11.667 m/s
  • Fuel Remaining After: ~46.648 kg

HT-2

  • Fuel used: ~9.243 kg
  • $\Delta v$ (radial): ~0.923 m/s
  • $\Delta v$ (axial): ~17.794 m/s
  • $\Delta v$ (total): ~17.818 m/s
  • Fuel Remaining After: ~37.405 kg

HT-2 Trim

  • Fuel used: ~0.017 kg
  • $\Delta v$ (radial): ~0.036 m/s
  • $\Delta v$ (axial): ~0.0 m/s
  • $\Delta v$ (total): ~0.036 m/s
  • Fuel Remaining After: ~37.388 kg

HT-3

  • Fuel used: ~0.732 kg
  • $\Delta v$ (radial): ~1.265 m/s
  • $\Delta v$ (axial): ~0.273 m/s
  • $\Delta v$ (total): ~1.294 m/s
  • Fuel Remaining After: ~36.656 kg

Normal Station Keeping Maneuver (both orbit types)

  • Fuel used: ~0.100-0.150 kg
  • $\Delta v$ (radial): ~0.20-0.30 m/s
  • $\Delta v$ (axial): ~0.0 m/s
  • $\Delta v$ (total): ~0.20-0.30 m/s

You'll notice the biggest burns for Wind were an axial burns that used >14 kg of fuel or $>$28 m/s $\Delta v$, compared to the normal station keeping maneuvers of which are ~100 times smaller.

So I think the critical aspect here is that a halo orbit requires a large $\Delta v$ directed out of the ecliptic plane whereas with the Lissajous one can insert for less $\Delta v$. Given that DSCOVR was going to need to do a lot of station keeping and pointing-requirement maneuvers, fuel consumption was a big concern when it launched. That is, fuel was the primary mission lifetime constraint for the DSCOVR mission, so reducing insertion fuel costs was a big priority. Unfortunately, one of the laser gyros had an anomaly in June 2019 but was brought back to normal operations by March 2020.

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