From: NASA Blogs
Posted: Wednesday, July 22, 2009
Paul. D. Tompkins
The most frequent questions I get about LCROSS go something like, "You've been flying for 'x' days...aren't you at the moon already?", "Why does it take so long to reach the moon?", or "What are you doing with all the extra time between now and impact?" Admittedly, the answers are hard to communicate, because the LCROSS mission profile isn't as straightforward as one might guess.
Between Transfer Phase and Impact Phase is the longest phase of the LCROSS mission - Cruise Phase. Transfer and Impact Phases are the most intense and busy, and therefore get the most attention. Cruise Phase is when the Flight Team performs long-term preparations for Impact in every respect - we calibrate our science instruments, we better characterize our spacecraft, we finalize our lunar impact target and refine our impact trajectory. Many of us on the Flight Team envisioned Cruise to be a lot less busy than Transfer, but our team has been steadily busy since Lunar Swingby.
Over the next two posts, I'll introduce you to Cruise Phase, and get you all up to speed on what's happened since Lunar Swingby so far. Maybe in doing so, I'll answer those questions that you've wanted to ask about Cruise Phase that seems, at first glance, to be nothing more than a long delay between the really cool parts of the LCROSS mission.
The Real Reason for Cruise Phase: Impact Energy
Cruise Phase is first and foremost about trajectory and impact energy, so I'll start there. So, why does it take so long to get to the moon, and why are we still orbiting the Earth? Actually, it doesn't take very long to get to the moon. LCROSS passed by the moon on Day 5 of our mission, way back on the morning of June 22. We could have impacted the moon back then, but our objective was to impart as much energy into our impact as possible, and it just wasn't possible to do that on our first lunar encounter. I'll explain.
On our initial outbound trajectory from the Earth in the days after launch, we could have hit the moon's equatorial region very directly, but the only way we could have impacted the lunar south pole is at a very low, grazing angle. A low impact angle results in relatively little energy release. Our goal is to hit our south pole target as steeply as possible, transferring all of our kinetic energy into the impact site, thereby raising the greatest amount of lunar material from the surface for analysis. The only way to do that is to approach the moon roughly perpendicularly to its orbit plane, to smack the moon from "below", at the south pole. At Lunar Swingby, we used the moon like a slingshot to throw LCROSS from its Trans-Lunar Orbit, roughly in the same plane as the moon's orbit about the Earth, into a steeply-inclined Cruise Phase orbit that would allow us to do just that. So, we got to the moon in five days, but we never entered an orbit around the moon. Instead, we kept on orbiting the Earth, but in a much different orbit than we started in, thanks to the help of lunar gravity as the moon flew by us. That's right...you read this right: when you look at the animations of the LCROSS trajectory, the moon had the larger component of velocity (as compared to LCROSS) relative to the Earth, and actually flew by LCROSS rather than the other way around. LCROSS was at the right place at the right time to be thrown into its Cruise orbit!
This figure illustrates the Transfer Phase and Cruise Phase segments of the orbit, as seen for July 22, 2009. The moon's gravitational influence threw LCROSS into the Cruise Phase trajectory where it will remain, orbiting the Earth, until October 9. The moon has already made a full revolution around the Earth, and LCROSS is (at the time of posting) far below the moon's orbital plane, not quite 3/4 of the way around its first orbit around the Earth. At the time of Impact, LCROSS will rise and intersect the moon's orbit just at the time the moon crosses the same point. Courtesy of NASA Ames Research Center.
Now that we're in the Cruise Phase orbit, we'll orbit the Earth three times before we impact. As of today's post, we haven't even made it around the Earth once yet, though the moon has made one orbit around the Earth since Lunar Swingby. Among other things, Cruise Phase is a waiting period for when the orbits of the moon and LCROSS cross paths again on Impact day on October 9. Next time, the moon will be squarely in our sights!
Trajectory Correction Maneuvers: Fine-Tuning Impact
At special positions in the orbit during Cruise, LCROSS will perform additional TCM's (Trajectory Correction Maneuvers) to converge on the impact target parameters - position, timing, and impact angle. Originally we had 10 Cruise Phase TCM's planned: TCM 4a, 4b, 5a, 5b, 5c, 6, 7, 8, 9, 10. Some of these "maneuvers" are mandatory, others are optional. The mandatory ones cannot be skipped if we want to hit the moon. In the LCROSS mission, these are TCM 1 (executed during Transfer Phase) and TCM 5a. The others are optional, and are only performed if errors remain from previous burns, or stemming from other influences (e.g. solar radiation pressure, disturbances caused by our own attitude control thruster firings, etc). It'll be rare that we skip a burn opportunity, but it has happened already. We skipped the TCM 4a and 4b opportunities because we were so close to our target trajectory after Lunar Swingby that performing the burns wasn't worth the extra risk and effort involved in doing such maneuvers.
These burns vary dramatically in magnitude. Earlier burns change the orbit more significantly. By TCM 10, we'll really be polishing things. To give you a feel for the difference, TCM 5a was designed to alter the LCROSS velocity by 21.1 meters/second (47 miles per hour). By comparison, our final burn will be changing the velocity by less than 10 cm/s (0.22 miles per hour), perhaps even less. However, when you think about it, this kind of refinement is necessary. Imagine the effect of 1 cm/s of velocity error over 10 hours of drift time prior to Impact. The distance adds up fast: 0.01 m/s x 3600 s/hr x 10 hr = 360 meters of position error!
The fewer burns we have remaining in the mission, the more "locked in" we'll be to a very specific impact target position and timing. We started the mission with quite a bit of flexibility in both, but that will soon change. In fact, the LCROSS science team has to make its final impact target selection by 60 days prior to Impact. Any later, and the propellant cost to change to a different target becomes prohibitive.
Science Instrument Calibrations
Cruise Phase provides some nice opportunities to continue calibrating our science instruments before Impact. Our most valuable calibration opportunity was Lunar Swingby, executed on Day 5 as the close of Transfer Phase. The close range to the moon meant that the moon filled a high percentage of the fields of view of our instruments, thereby improving our resolution, etc. As I mentioned, I'll have the Science/Payload team provide a guest posting sometime soon to discuss those results so far.
Cruise Phase never gets another opportunity like Lunar Swingby. We only get that close to the moon once more - on our final approach before Impact. It's far too late to be worrying about calibrations by then. However, there are times on each orbit when LCROSS gets closest to the moon and Earth (making them look bigger in the instruments), and these are valuable opportunities for follow-on calibrations.
Our mission has three nominal calibrations planned: two "Earth Look" calibrations, and one "Moon Look" calibration. As the names indicate, we'll train the instruments on the Earth and moon, and perform a series of sweeps back and forth across their distant shapes to improve our knowledge of instrument pointing accuracy, radiometric response and spectral response to known inputs. The Science team's lessons from Lunar Swingby will guide the specific measurements and focus of these activities.
Omni Pitch Maneuvers: Staying in Contact
The Cruise Phase orbit takes LCROSS high above and far below the moon's orbital plane on each revolution. Our standard spacecraft attitude for Cruise places our long axis (the one common to LCROSS and the Centaur) perpendicular to the ecliptic plane - the plane of the Earth's orbit around the sun. There's nothing particularly special about this orientation, but it makes things a lot easier to think about operationally, and that's worth a lot. The catch is that our primary low-gain (omni-directional) antenna points in one direction off one side of the spacecraft, and it's not always pointed toward the Earth. It enables communications over about a full hemisphere of angle, but as we orbit the Earth without changing orientation, the Earth will move into and out of the antenna pattern. When Earth is out of the pattern, we can't communicate with LCROSS (note that we have a secondary omni antenna on the opposite side of the spacecraft, but it's less effective, so we try to stay on the primary whenever possible).
This figure depicts the LCROSS shepherding spacecraft and the Centaur, shortly after separation. The spacecraft depiction shows the location of the primary omni-directional antenna and the solar array, relative to the standard body reference frame axes. Omni Pitch Maneuvers rotate the spacecraft about the Pitch axis to re-orient the primary omni antenna. The Cold Side Bakeout maneuvers rotate the spacecraft 180 degrees about the Roll axis to orient the cold side of the spacecraft (and Centaur) towards the sun. Artwork courtesy of Northrop Grumman.
The remedy is to flip the spacecraft 180 degrees, keeping our solar array pointed at the sun, but turning the omni antenna pattern to the opposite direction. At the time of the flip, neither the starting nor the ending orientation is very good for communications. However, if left in the starting orientation, the Earth would move out of the antenna pattern. By flipping the antenna (and the spacecraft) over, the Earth will tend to move deeper into the pattern over time. These flips are rotations about the spacecraft "pitch" axis, and so are officially called "omni pitch maneuvers". During Cruise Phase, we have to perform these roughly every two weeks, though the timing depends on the specific orbit geometry.
Cold Side Bakeout: Getting the Water (and Other Stuff) Out of the Centaur
The Centaur upper stage (our impactor) is covered with material that is naturally absorbent and was expected to absorb some water from the atmosphere when sitting on the pad in the humid conditions in Florida. In fact, if you watch the LCROSS launch video closely (the Centaur-mounted camera view), you can see ice accumulated in ridges on the vehicle surface, frozen by the extremely cold temperatures caused by the cryogenic contents (liquid oxygen and hydrogen) of the Centaur propellant tanks.
In space, this accumulated water might melt or sublimate (a straight conversion from ice to vapor) if exposed to the extreme heat of the sun. This is what we expected the water to do on the side of Centaur aligned with our spacecraft's solar array, since that side is almost always pointed toward the sun. After several days, we expected all of the water on the sun side of the Centaur to be removed naturally.
Water trapped within the very cold foam on the opposite side of the Centaur, however, might remain for a lot longer, perhaps through the end of the mission. In fact, Lunar Swingby spectrometer measurements show a significant water signal (along with other exotic hydrocarbons) - given the dry mid-latitude targets we were measuring, the water must have come from the spacecraft, and the strongest possibility is that it was from ice debris floating around the spacecraft. There are two problems with water trapped on the Centaur. The more obvious of these, but actually the less problematic, is that LCROSS is trying to find water on the moon. Bringing water to the moon from the Earth will only confuse the issue! Our Science team is actually not so worried about this, because we're talking about only 10 kg of water or so, and they wouldn't expect to "see" such a small amount relative to the hundreds of tons of material LCROSS is expected to loft and the percentages (albeit small) of that material expected to be water. Still, it's a concern.
Interestingly, the larger problem is that water escaping from the Centaur is predicted to cause a disturbance to its trajectory (via conservation of momentum). The worry is that in the hours before Impact, as the Centaur is descending alone to the lunar surface, its shadowed side will be exposed to sunlight, causing a release of water and a notable trajectory disturbance. A significant release of water could turn a perfectly-aimed impactor into a poorly-targeted dud. We're talking potentially 100's of meters error or more! It's hard to believe that water escaping the surface could cause that much harm, but welcome to space physics.
To prevent this from happening, we've scheduled a Cold Side Bakeout maneuver that rotates the spacecraft about its long axis to point the solar array directly away from the sun, and bakes the water right out of the Centaur in full sunlight. Over enough exposure time, the water should vent out of the Centaur. The challenge is that LCROSS is designed to spend all but short periods of time pointing its solar array toward the sun. Thermal and Power subsystem flight rules prevent long dwells in this opposite orientation.
Other Engineering Stuff
Aside from giving us time to conduct all of the above activities, Cruise Phase is a chance to get more familiar with our spacecraft so that we can totally predict how it will behave at Impact. Recall that we came out of Transfer Phase with a few "anomalies", or unexpected problems. The Flight Team needs time to sort through all of the issues, starting with the most severe ones, and gradually moving down to the more benign ones.
Our goal is to figure out why each of the problems is occurring, and then to come up with ways to work around the problems, or to show that their behaviors are completely benign and not worth worrying about. Each must be considered in terms of risks against achieving our mission objectives. Our team can only bring an anomaly investigation to closure by proving to our mission stakeholders (NASA Ames, LPRP and NASA Headquarters management) that our mitigating strategies maintain our risk of failure to acceptable levels.
Flight Team Planning, Training and Rest
As Flight Team Lead, I'm responsible for creating and modifying the short and long-term plans for the mission, including Cruise Phase. Before launch, most of our time was spent preparing for the first week of flight. Once Lunar Swingby came and went, I had to drop into overdrive to get our Cruise Phase planning in order. This meant (and continues to mean) nailing down times for TCM's and other maneuvers (courtesy of our MMDS team - see the post on "LCROSS Flight Team Breakdown"), nailing down DSN contact times (via our CLASS team at Ames and JPL), putting together staffing schedules, etc.
Then there's planning for Impact. Again, our team spent the 2-3 months prior to launch working out final details on Transfer Phase operations. We last practiced Separation and Impact in February. We have things worked out pretty well, but now we need to practice some more so that we nail our Impact in October. As Flight Team Lead, I am responsible for setting down the rehearsal schedule, working with the CLASS team to schedule DSN readiness tests, and refining our procedures as needed.
When we're not planning for our next activity, coordinating on anomaly resolution, working out schedules, or training for Impact, the Flight Team has a little extra time to rest than it did before launch, and during Transfer Phase. We're certainly not avoiding work - it's what you as a taxpayer are paying us to do. But the Flight Team has been working full steam ahead for six months or more, and now that the schedule is a little more relaxed, we can afford to take some hours and days off here and there. It's not much, but it sure helps!
This is it for now. In the next post, I'll present a brief diary to catch everyone up on all the activities we've performed since Lunar Swingby!
More at the LCROSS Flight Director's Blog
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