Crashing into the moon in search of frozen water
BY STEPHEN CLARK
Posted: April 23, 2008
Relying on heritage technology derived from parts on previous space missions, a spacecraft on a budget will guide a spent rocket stage on a collision course with the moon early next year to search for signs of permanently frozen water hidden inside craters near the lunar poles.
The Lunar Crater Observation and Sensing Satellite, largely built from flight-proven components used on a variety of earlier missions, is currently undergoing testing at a California factory.
"That means that from point of authority to proceed, meaning cash-in-hand, to launch, meaning the October launch date, we're looking at something like 29 months, which is incredibly fast," said Dan Andrews, LCROSS project manager at NASA's Ames Research Center.
"Then you further aggravate that speed by saying that you have absolutely no more than 1,000 kilograms (2,200 pounds) of up-mass, and that's a hard number because you're going to be topping what the Atlas 5 can lift given LRO's allocation," Andrews said. "Then you aggravate it further and say that you are a cost-capped mission."
The mission's total cost must not exceed $79 million due to funding constraints in NASA's robotic lunar exploration program.
"It's a well-constrained box for trying to pull off something fast," Andrews said.
The tight funding caused NASA to designate LCROSS as a Class D mission, the agency's most risk-tolerant category of missions. At the other end of the spectrum are Class A missions, encompassing NASA's human spaceflight programs and flagship planetary probes.
"What it means is you're allowed to do a whole lot less from the standpoint of overall mission assurance, your testing requirements are greatly reduced and tailorable, you can be a single-string spacecraft, meaning no redundancy at all, even though we have some," Andrews said.
Andrews said LCROSS is re-using several spacecraft bus components from LRO, including fuel tanks, avionics, sun sensors, and the transponder. Other off-the-shelf parts come from auto racing and military field equipment.
"The vendors love the idea that we, NASA, are going through the trouble to put their hardware through the paces," Andrews said. "I suspect the vendors will even be interested in throwing on their brochure being flown in space on a NASA mission. It's really actually very symbiotic."
The probe's design is built around an adapter originally developed to anchor smaller piggyback payloads during launch. Called the Evolved Expendable Launch Vehicle Secondary Payload Adapter, the ring was first used operationally during a military flight in early 2007.
The ESPA ring forms the foundation of the LCROSS spacecraft, housing the probe's propellant tank, solar array, science instruments, communications system, and avionics.
"The way I look at it is it's sort of like petals on a flower," Andrews said. "You have a central hub and then you have all those panels radially located around it. One of those panels is the Ames-built payload."
The instruments were transported in January from Ames to a Northrop Grumman Corp. facility in Redondo Beach, Calif., for integration on the spacecraft.
"That's been down there for a few months now and it's just completed its testing there verifying that it's operating properly with the rest of the systems," Andrews said.
The most significant component still remaining to be added to the spacecraft is the solar array, which will go on soon, according to Andrews.
Mechanical interface testing is already underway at the Northrop Grumman clean room, and environmental testing should begin in the next few weeks with acoustic vibration tests to simulate launch loads. Thermal vacuum testing to check the spacecraft's ability to survive in space will follow soon thereafter.
"Because we have positive schedule slack, we expect to have the spacecraft completed in advance of when we need to ship it, meaning we'll have it ready earlier than we need for shipping," Andrews said.
Officials could opt to continue further testing on the spacecraft at the factory, but that solution carries some risk.
"The more you interact with it, the more potential there is for something to go wrong," Andrews said.
Other options include storing the probe at Northrop Grumman, or shipping the spacecraft to the launch site for storage at the Astrotech clean room near Cape Canaveral.
"We're both going to be getting processed at Astrotech, LRO and LCROSS, which does cause for some competition of resources there. Astrotech believes that they can handle it just fine. They've processed dual manifests before, so this is nothing new for them. But we do have to plan around who's fueling when, what facility, what chamber are you in, what work area, who's going to need what resources, that type of thing," Andrews said.
The two spacecraft will be taken to the Atlas 5's seaside launch pad a few weeks before launch to await blastoff this fall.
The smashing mission
The 1,835-pound LCROSS spacecraft will remain attached to the Atlas 5 rocket's Centaur upper stage after LRO is released during the first hour-and-a-half of the mission. The stage will fire a final time to put LCROSS on a longer looping route to the moon, while simultaneously depleting its supply of propellant.
The Centaur, playing an unprecedented role in a space mission, will be used as projectile to dive into a crater shrouded in darkness near one of the moon's poles. An array of space-based telescopes and ground observatories will be used to analyze the material ejected from deep within the target crater in an effort to determine the extent of hypothesized water ice deposits there.
"Depleting propellants at the time of LCROSS orbital insertion and mission handoff will minimize the potential for tank venting during the subsequent mission phase, and will minimize the amount of non-lunar elements that Centaur contributes to the impact plume," United Launch Alliance said in a statement to Spaceflight Now.
Plans call for LCROSS to take control of the Centaur stage within five hours of liftoff.
United Launch Alliance, the joint company formed in 2006 by merging the rocket divisions of Boeing Co. and Lockheed Martin Corp., will provide the launch services for the LRO and LCROSS missions.
"The LCROSS mission is unique in many ways," a ULA spokesperson said. "One of these is the fact that, for the first time, Centaur and its payload will reverse roles several hours into the mission. After completing the LRO phase of the mission, then placing LCROSS into its required orbit several hours after launch, Centaur will hand over control of the mission to LCROSS and become an inert payload."
LCROSS and the Centaur will fly past the moon about five days after launch, using lunar gravity in a flyby to throw the spacecraft into an inclined high-altitude Earth orbit stretching past the distance of the moon's orbit.
Most launch dates support a lunar impact about three or four months after launch, but the mission's exact duration and target crater will depend on when liftoff occurs, according to LCROSS officials.
See our chart showing launch dates and times here.
Andrews said the October and November launch opportunities are geared toward a target crater on the moon's south pole, but specific craters will selected based on the mission's exact launch date.
"We have these working groups, and we talked about what the community thought was the best ranking of target craters on both poles. It looked at things like crater age, it looked at things like crater size, it looked at the known hydrogen measured levels within the craters, it looked at relative elevation off of the pole, and a bunch of other things," Andrews said.
"So for any given launch scenario, we can find an ideal or a best-case scenario crater to hit," Andrews said. "We do have sort of a big menu option that we're putting together right now that's really a big multiple (variable) problem. Mission duration is part of it, launch date is part of it, crater rank, meaning the ideal crater location, is part of it."
To begin the impact sequence, the LCROSS spacecraft will perform a final targeting maneuver to line up for the selected crater. LCROSS will command the Centaur to separate a few hours before the scheduled impact, followed by a thruster firing to increase the distance between the spacecraft and the deactivated rocket stage.
The Centaur will hit the moon about ten minutes before LCROSS, allowing a suite of instruments aboard the probe to relay science data to the ground before it plunges into the moon.
LCROSS carries five cameras, three spectrometers and a photometer to capture imagery of the impact's flash and the resulting plume of dust propelled high above the lunar surface. The science payload also includes a color video camera provided by Ecliptic Enterprises Corp., the company that builds RocketCam units to beam back live video from rocket launches.
Because of strict limits on the bandwidth of communications from the spacecraft, the video will be downlinked back to Earth at about two frames per second, Andrews said.
"It will be a little choppy video. We would like to pipe this out and make it consumable by the public as soon as possible, if not a direct feed, but those details are being worked right now," Andrews said.
According to Andrews, all of the LCROSS sensors will provide key information on the nature of the expelled material. Scientists predict the Centaur's impact will excavate more than two million pounds of dust from moon, carving its own crater nearly 100 feet in diameter and more than 16 feet deep.
"We would have loved to have put a (high) frame-rate HD camera on here and send back some really nice video, but the fact is for weight, power and a bunch of other reasons - bandwidth and cost - it wasn't feasible. So what we have in our suite of instruments are all ones related to confirming, or not, the presence of water ice on the moon," Andrews said.
The demise of the LCROSS spacecraft a few minutes later will uproot additional material, providing space- and ground-based telescopes a second chance to study an impact plume. The Hubble Space Telescope, Chandra X-ray Observatory, and other orbital spacecraft will be part of a coordinated effort to turn a range of sensors toward the moon.
"The idea is that we get a whole bunch of ejecta out into the sunlight that's both visible and we can see it sublimating if there's any water ice there," Andrews said.
Scientists' expectation of finding evidence of water ice in the LCROSS plume is based on chemical traces discovered deep inside craters permanently encased in darkness. Three smaller spacecraft, the Pentagon's Clementine mission, NASA's Lunar Prospector, and Europe's SMART 1, were also driven into the moon at the end of their lives to throw up dust for analysis by telescopes.
"This is the first mission that is actually designed with this purpose specifically in mind, as opposed to being an opportunity at the end of a mission," Andrews said. "Lunar Prospector was never intended to be an impactor."
Andrews said Lunar Prospector collided with the moon at a grazing angle and at a low relative velocity, and the spacecraft's low mass also limited the energy of the impact.
"All of those things are things that have been optimized by LCROSS," Andrews said. "The Centaur impactor is going to be something like 2,200 kilograms (4,850 pounds), and it's going to be going in at 2.5 kilometers per second (5,592 miles per hour), and it's going to be going in at a pretty steep impact angle, which means ejecta is going to be kicked up maybe pretty close to normal out of craters."
Using the Centaur to plummet into the lunar surface effectively tripled the usable mass for LCROSS, which has a restricted launch weight of about 2,200 pounds, or 1,000 kilograms
"Since we are only given 1,000 kilograms to work with, if the impactor was part of that 1,000 kilograms, you can imagine the mass that we could actually use to impact the moon would be very small because you have to keep some back for your spacecraft. Well, we've got the Centaur anyhow. That in effect grew our effective usable mass from 1,000 kilograms to over 3,000 kilograms (6,614 pounds) without costing anything extra," Andrews said.
"What a clever idea to use that 2,200-kilogram impactor and then spend the 1,000 kilograms in the equipment and the follow-on shepherding spacecraft to watch it right there, where you're going to get less signal noise compared to Earth (telescopes), or even Hubble."
ULA had to extensively study the Centaur system to ensure it would behave as expected during its lengthy mission.
Much of the testing focused on the Centaur's separation system and associated electrical connectors to make sure they will still work after more than three months in space.
"ULA is performing a detailed assessment of the thermal environment Centaur will experience during the time that LCROSS is in control," a company spokesperson said. "This will allow characterization of the out-gassing environment and the potential for tank venting, and verify that Centaur components will remain stable during their ride to the moon."
Engineers will paint the sidewall of the Centaur white to help control the effects of the thermal environment on the rocket body. Changes were also made to paint and coatings on the adapter connecting the rocket to the LCROSS spacecraft, according to ULA.
ULA said no other major changes are planned for the Centaur, which has flown in space nearly 200 times during almost 46 years of service. Testing and analyses confirmed the stage was otherwise ready to support the LCROSS mission.
"This mission will significantly expand Centaur's operating envelope and demonstrate unique capabilities that will be of value to future missions. Examples of missions that could benefit are those that require long-duration support from Centaur, or that require placement of multiple spacecraft in orbits that are different, but not different enough to require a standard engine burn between them."