DSCOVR Satellite L1 Lissajous Orbit - Questions

NASA’s DSCOVR earth & solar observation satellite Deep Space Climate Observatory - Wikipedia is floating about at the Earth-Sun L1 point. Producing among other things a really cool slide-show-of-the-day visible at http://epic.gsfc.nasa.gov/

The L1 “point” isn’t a point. But rather a region of very very low net gravity where a vehicle can “hover” with very little expenditure of fuel. Sitting isn’t long-term stable, so the vehicle orbits in what’s called a Lissajous orbit. Lissajous orbit - Wikipedia

There’s a couple things about DSCOVR’s orbit that I don’t understand and I haven’t been able to come up with a search that got deep enough into the details to explain it. I can find breathless fact-lite PR and also a bit of stuff that assumes more context than I have. And nothing in between. Nor the deeper details I’d struggle to understand but that hold the answer. I don’t know enough terminology to find the needles in the haystack.

Perhaps our resident orbital mechanics and rocket surgeons can help?
I’ve been watching the EPIC website linked above pretty much daily ever since it came online about 18 moths ago. One factor the website shows is the distance and angle between Sun, Earth, and DSCOVR. This whole time the satellite has been hovering between about 5 and 10 degrees “above” = North of the Earth’s orbital plane vs. the Sun. The angle has decreased, increased, and decreased again many times. So, as I’d expect, it’s periodically nodding “up” and “down” versus the Earth orbit plane as DSCOVR moves in its orbit around L1. But so far it’s remained above the plane.

Which mystifies me. I did some more digging and found this 2.6MB pdf of a PowerPoint deck: http://www.swpc.noaa.gov/sites/default/files/images/u33/DSCOVR-Halfway-to-L1.pdf . Which on page 4 describes some aspects of the orbit.

It says:
A) The orbit nods North and South over the course of a couple months.
B) The amplitude of the nod expands and contracts over the course of 6ish years
C) The vehicle remains “above” = North of the Earth orbit plane the whole time.

A & B make reasonable sense. I don’t know whether that’s a necessity of orbital mechanics or a result of deliberate burns, but at least it makes sense.

But what about C? The one thing I’d have naively assumed from the git-go is that each and every pass around the orbit would be symmetrical about the Earth orbital plane. ISTM you’d have to burn a lot of station-keeping fuel to hold the orbit offset on one side of the Earth-Sun plane of symmetry.

It also seems less than ideal for mission purposes. Although I suppose it’s more important for the US to see into the far North latitudes and to detect solar wind headed that way than it is to equally cover the far South latitudes. So that might be the motivation.
With all that background:

Can anyone shed light on:

  1. Why is the long term orbit asymmetrically located “above” = North of the Earth orbital plane?
  2. As a matter of orbital mechanics, how is that doable on a reasonable fuel budget?
  3. Is the 6-year cycle of Z-axis amplitude a natural artifact of physics or is it induced by burns? If the former, what’s driving it? If the latter, why?
  4. Any other cool info about these sorts of orbits in general or DSCOVR’s in particular you think worthy of mention.

Slide 4 was confusing at first to me too, since I also think things have to be symmetrical about the Z axis (North/South). To understand where the spacecraft is, you have to look at the plot on the bottom of slide 4, which is nicely symmetrical about the origin, which is directly along the Sun/Earth line. I believe the plot on the top shows the total Sun-Earth-Vehicle angle, i.e. the radial distance in the plot below. By definition, this angle is always positive, it only reaches zero in the rare occurrence that the Lissajous orbit goes directly across the Sun-Earth line.

So you were correct to question conclusion C, it is not always North and as you expected is equally often North and South. There is a more detailed discussion of the orbits in a PDF on the NASA site, including plots showing the orbit with and without control burns.

Perfect. Thank you. That pdf ref was about 90% obvious and I can look up the rest of the mysterious terminology.

I now see that two-and-a-half things conspired to fool me.

  1. The graphic on the EPOD website is constructed to always display the SEV angle as if it was in the +Z region of the YZ plane regardless of the angle’s actual orientation vs. the YZ plane. I had assumed the graphic would invert for -Z but it doesn’t.

1a. Driven by the same graphic I’d also implicitly assumed the orbit had a negligible X component so Z would be the only direction that mattered. Not so; rather the opposite.

Assuming (there he goes again :smack:) that the graphics on e.g. page 4 of your pdf are scaled correctly in X vs. Z we see the orbit is about 2x as “wide” in X as it is “tall” in Z. With the effect that even a maximal 12 degree SEV has only a 6 degree projection into the Z plane.

  1. In watching the EPIC pix evolve over almost 2 years now I lost the fairly small <6 degree Z axis delta in the seasonal 23 degree delta.
    Thanks again for the good info and the good ref. This is the SDMB at its finest.

Bumping my own thread …

In an interesting development, the EPIC pic-of-day website has just been updated with a new and better look. One of the changes was to remove the graphic of SEV angle and display only the numeric value.

It’s almost as if they listened to my confusion in this thread and fixed it. Thanks NASA!! :smiley:

And what did you implicitly assume be the graphic of a bearded man and a smiling little girl?

nm