IBEX Lunar Synchronous Orbit

For the whole story, check out the June 24th, 2011 mission update by Dave McComas on the Official IBEX Web Site (ibex.swri.edu) .

The real challenge was to find an orbit that we could control for a long time. The original orbit could only be predicted for about 3 years. Cislunar space is a crazy place to try and fly a long-term trajectory; the slightest change in initial conditions can cause completely different trajectories years later. Because IBEX’s original orbit was often closer to the Moon than to the Earth, it was performing gravity assists regularly. The uncertainty in our initial conditions from orbit estimation knowledge and attitude re-pointing maneuvers would cause nearly identical trajectory predictions to eventually experience different gravity assist geometries, and the slightest change in that geometry during the encounter causes huge differences in the subsequent trajectory. Some predictions would hit the Earth (not a good thing for most space missions, by the way) and others would escape the Earth-Moon system (great for science, but not so good for communicating with a spacecraft designed to stay close to Earth.) Our new orbit keeps the apogee away from the Moon and has reduced the sensitivities. We can now predict this trajectory well beyond 11 years now so we can get the full Solar Cycle.

Here is our original trajectory in an Earth-centered Inertial (Mean Earth Equator and Mean Equinox of J2000.0) Coordinate system:

Original IBEX orbit

Original IBEX orbit at the beginning of nominal operations

The Earth is hard to see, but it is in the center, and the white circle is the Moon’s orbit. Now here it is in the Earth-Moon Rotating coordinate system; note the apogees that come close to the Moon:

The Original IBEX orbit shown in the Earth-Moon Rotating System

The Original IBEX orbit shown in the Earth-Moon Rotating System

In this rotating system the Earth and the Moon are both pretty much fixed (although the Moon moves up and down a bit because of its orbit’s eccentricity). The Earth is still in the center, and the Moon at the top of the white circle. The white line connects the Earth to the Moon (Don’t look for that with your telescope).
Here is our new orbit in Earth Inertial – not really all that different from the original (just a slightly larger semi-major axis and higher radius of perigee):

The new IBEX Orbit in Earth-centered Inertial coordinates

The new IBEX Orbit in Earth-centered Inertial coordinates

And here it is in the Earth-Moon rotating frame:

The new IBEX orbit in Earth-Moon Rotating Coordaintes

The new IBEX orbit in Earth-Moon Rotating Coordaintes

Note that the apogees stay far away from the Moon, so the gravity assists are much weaker. The spacecraft is always very near perigee when it crosses the Earth-Moon line. (And, no, the spacecraft trajectory is not perturbed by that big white line.)

For more information on the IBEX Flight Dynamics, check out the white papers at the Applied Defense Solutions website.

New IBEX 3:1 Lunar Resonant Orbit

IBEX Earth-Moon Rotating Frame 10-year Trajectory

John and I tried to get the IBEX folks to go for this in 2007, but it was too late in the game to change the science orbit. After 2.5 years of nominal ops (and a whole lot of analysis by John and his team at Applied Defense Solutions) the IBEX science team decided to go for it and use a 3:1 Lunar resonant orbit. A 3:1 Resonance, of course, means simply that IBEX orbits 3 times for every Lunar orbit. There are some very nice things about this, the first being that you can keep apogee away from the Moon, and prevent Lunar perturbations from driving your perigee into the Earth. You can also keep the geometry of the orbit constant with respect to the ecliptic and keep Earth shadows off the orbit for most of the time. John will come back later with pictures and some more details. It really is a neat orbit. My good friend Chauncey Uphoff used to love to talk about resonant orbits and all their uses. He’d love this!

This animation shows the IBEX orbit in an Earth-Moon rotating frame. The Moon is at the top, Earth in the Middle. Note that the Apogees are away from the Earth-Moon line.