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November 2011: IBEX Orbit - Raising Maneuver
November 14, 2011
We have a new "first" for the IBEX mission – the IBEX team has just carried out three long firings with our large aft thruster and inserted the spacecraft into a long term stable orbit that no spacecraft has ever flown in before! When it became clear that the IBEX mission could last longer than its original 2-year planned lifetime, and there was a lot of great additional science to be done, the team needed to think about ways to alter IBEX’s orbit to be able to maximize the spacecraft’s usefulness. We had hydrazine left in our fuel tanks, so it was a matter of finding the right orbit that would require the least amount of fuel to reach and maintain.
When IBEX launched in October 2008, we fired our large thrusters to move the spacecraft into an elliptical, or oval–shaped, orbit that goes out almost to the Moon’s orbit. In the rotating coordinate system shown below, the Earth is in the middle, the Moon’s orbit is the big white circle, and the Moon is fixed at the top of the circle. In this system, orbits look like the twisted Spirograph curves we made as kids. The plot on the left shows our old orbit where in some of the orbits the spacecraft passed relatively close to the Moon (straight up) – whenever this happened, the Moon’s gravity changed IBEX’s orbit, requiring us to make additional maneuvers to keep the spacecraft from burning up in the earth’s atmosphere or leaving Earth orbit entirely. In our new orbit shown on the right, the spacecraft makes three "petals" that are synchronized to always stay away from the Moon.
Notice how IBEX’s new orbit does not vary nearly as much as the old ones did. IBEX completes one orbital "petal" every 9.1 days, and 3 complete orbits in 27.3 days, matching the time it takes the Moon to orbit the Earth once. Orbit analysis indicates that IBEX should be stable in this orbit for decades to come. As an added bonus, in order to achieve this orbit, we raised the perigee, or closest point in the orbit to Earth, from about 6,000 miles to over 30,000 miles. We now have the orbital stability that will allow us to collect data and study the processes happening at the edge of our Solar System over the 11–year solar cycle. In fact, with over forty years of hydrazine for small repointing maneuvers left in the tanks, who knows how long the IBEX spacecraft could last!?!
Everyone involved in figuring out the new orbit and planning and carrying out the recent maneuvers did a fabulous job. I really want to thank all of these team members who worked so hard to bring IBEX to its new orbit!
In addition to all of the excitement and accomplishments of our orbit maneuver, I am also proud to announce that the IBEX Science Team recently received NASA’s 2011 Group Achievement Award. The certificate is signed by Administrator Bolden, and it reads as follows: "National Aeronautics and Space Administration Presents the Group Achievement Award to The IBEX Science Team For success in imaging the interaction of the Solar System with the local interstellar medium using the energetic neutral atom emission detectors of the IBEX Mission." While this award was given to a relatively limited group of IBEX scientists, I personally accepted this award from the NASA Administrator on behalf of the entire IBEX team and send my personal thanks and congratulations to everyone who has made the IBEX mission a success. Thanks also to all of you who follow and support us! Everyone should be very honored by this achievement!
Please read on for more information about IBEX’s amazing orbital maneuver success. We look forward to much more from IBEX!
Mission Update – IBEX Orbit–Raising Maneuver
Why was IBEX’s orbit altered?
There were a number of reasons to change IBEX’s orbit. First, the Moon affected IBEX’s distance from Earth and the tilt of IBEX’s orbit quite a bit. As Dave mentions in his introduction above, the team wanted to eliminate the orbital "chaos" caused by the varying gravitational effects of the Moon on the spacecraft's orbit. Next, the team wanted to raise IBEX’s orbit up above the harsh environment of the Van Allen radiation belts and get IBEX out of crossing through the busy geosynchronous orbit region, which is already home to many, many satellites. Third, we wanted to keep the point at which IBEX comes closest to Earth ("perigee") from being too close to Earth’s dangerous inner radiation belts. Opposite to that, in late 2014, gravitational effects would have affected the spacecraft’s farthest point from Earth ("apogee") – and we could have been flung out of the Earth–Moon system entirely! Finally, in September 2014, IBEX could have passed through the Earth’s shadow for a total "eclipse" of 7 hours in length. Our battery system was designed only to last for 4 hours. The new orbital configuration will keep us out of any extended eclipses for at least 20 years. The new orbit solves all of these issues.
How long did it take to plan this maneuver?
The IBEX team worked for a year to plan and carry out this orbit change. Many team members were involved, including staff from mission operations, science operations, flight dynamics, and the spacecraft bus systems and subsystems teams. The team looked for orbits that could be reached using the available fuel on board the spacecraft, the stability of those orbits over many years, orbits that would minimize the time that IBEX would be within the shadow of the Earth or the Moon, orbits that would raise perigee but keep apogee high, and orbits that would move the under sampled region in the sky maps due to the Earth’s magnetosphere so that this area could be filled in more quickly in the combined sky maps. Given all of these parameters, the team chose the best orbit that met all of the team’s requirements and goals: our three–lobed "petal" orbit.
What were the steps in the process to change IBEX’s orbit?
There were many steps to changing IBEX’s orbit, and each one happened on a specific date at a specific time. Below are the major steps and the date on which they occurred:
- June 4 — Turned off the science instruments in preparation for the orbit maneuver.
- June 5 — Repointed the spacecraft to achieve the most efficient maneuvers; spun spacecraft faster, from 4 revolutions per minute to 22 revolutions per minute for stability during the thruster burn.
- June 8 — First rocket firing for ten minutes; the rocket firing when IBEX was at apogee raised perigee to about 20,000 miles. Orbit determination data collected after this burn was used to calibrate the thruster and determine the optimal duration of the final burn.
- June 16 — 2nd thruster firing for ten minutes at apogee
- June 17 — 3rd final firing, which was the remainder of the thrust needed at this apogee to achieve the target orbit. The duration of this burn was set based on thruster calibration from the first thruster firing maneuver.
- June 22 — Slowed spacecraft spin back down to 4 revolutions per minute. Once telemetry showed that final orbit was achieved and that the spacecraft was safe & in good shape, and the team repointed the spacecraft back to its science configuration.
- June 23 — Science instruments were turned back on.
What was the team’s experience like "behind the scenes?"
From members of the IBEX team, in their own words:
Systems Engineer Bret Hautamaki: "I have been with the IBEX program since 2005. For this orbital maneuver, I was on the support team for the first two engine burns. I kept an eye on the overall state and health of the spacecraft before, during, and after the burns. I also was among the many people that had to approve the sets of commands that we had to send up to the spacecraft in order to perform the maneuvers. I was involved in technical discussions about the telemetry that we observed, and in some cases comparing what we saw to what we have seen from IBEX in past years or tests. After the maneuvers, I collected all of the relevant telemetry, saved it, and sent it out to a larger group of people so they could see for themselves how the maneuvers went. I also determined the 'success criteria' from a spacecraft perspective after the burn to answer two important questions: did IBEX do what it was supposed to do, and did it end up the way we wanted it to? I am happy to say that the answer is "yes!" All three of the burns were basically textbook–perfect! "The most stressful time for me was the start of the first burn, when we were using a thruster for the first time in nearly 3 years and for just the second time ever. The telemetry from IBEX is normally delayed about 20–30 seconds so we knew that the burn should have been occurring, but the telemetry hadn’t made its way down to the control center yet. When it did, within a few seconds we were able to confirm that the correct thruster was indeed firing because we saw it heating up and fuel being consumed from the tanks. After that first minute or so, it was a lot less stressful. But again, so much is happening and we had to keep on top of so many events happening in quick succession, there wasn't a lot of time to dwell on the stress."
Mission Planner/Flight Director/Flight Controller Sheral Wesley: What I found most interesting is that because of lessons learned during IBEX’s launch and early orbit, we were well prepared for this maintenance maneuver. Just one example is with our spacecraft communication strategies - thanks to our updated antenna models and new planning software, we are able to minimize the areas in our antenna radiation patterns where the signal drops to zero, and we have increased the number of good contacts with the spacecraft. The entire IBEX Team has worked very hard in ensuring 100% mission success. If there is ever another maneuver needed for IBEX in the future, then I say, "BRING IT ON!"
So, what is next for IBEX?
On June 23, we began collecting science data again, so the science team will be hard at work analyzing what comes from IBEX. More heliosphere maps will be released by the team in the future, and IBEX is in excellent shape for what could potentially be a very long life. The team looks forward to what could be many years of investigating the edge of our Solar System, the Moon, the Earth’s magnetosphere, and so much more!
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