Posted on by Ken Kremer

GRAIL Unveiled for Lunar Science Trek — Launch Reset to Sept. 10

Delta II Heavy rocket and GRAIL Lunar mappers unveiled at night at Launch Pad 17B. GRAIL liftoff was postponed to Sept. 10 at 8:29 a.m EDT after high levels winds scrubbed the Sept 8 launch attempt. Credit: Ken Kremer

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NASA’s Gravity Recovery and Interior Laboratory (GRAIL) moon mapping twins and the mighty Delta II rocket that will blast the high tech physics experiment to space on a lunar science trek were magnificently unveiled in the overnight darkness in anticipation of a liftoff that had originally been planned for the morning of Sept. 8.

Excessively high upper level winds ultimately thwarted Thursday’s launch attempt.

NASA late today has just announced a further postponement by another day to Saturday Sept. 10 to allow engineers additional time to review propulsion system data from Thursday’s detanking operation after the launch attempt was scrubbed to Friday. Additional time is needed by the launch team to review the pertinent data to ensure a safe blastoff of the $496 Million GRAIL mission.

There are two instantaneous launch opportunities at 8:29:45 a.m. and 9:08:52 a.m. EDT at Cape Canaveral, eight minutes earlier than was planned on Sept. 8. The weather forecast for Sept. 10 still shows a 60 percent chance of favorable conditions for a launch attempt.

GRAIL A and B enclosed in nose cone atop Delta II rocket at Cape Canaveral, Florida. Umbilical’s connect from Delta 2 to Fixed Umbilical Tower (FUT).
Credit: Ken Kremer (kenkremer.com)

Despite a rather poor weather prognosis, the heavy space coast cloud cover had almost completely cleared out in the final hours before launch, the surface winds were quite calm and we all expected to witness a thunderous liftoff. But measurements from weather balloons sent aloft indicated that the upper level winds were “red” and violated the launch criteria.

Mobile Service Tower is retracted from around Delta II rocket at Pad 17B. Credit: Ken Kremer

As the launch gantry was quickly retracted at Launch Complex 17B on Sept. 7, the Delta was bathed in xenon spotlights that provided a breathtaking light show as the service structure moved a few hundred feet along rails.

The cocoon like Mobile Service Tower (MST) provides platforms to access the rocket at multiple levels to prepare the vehicle and spacecraft for flight. The MST also protects the rocket from weather and impacts from foreign debris.

The GRAIL A and B mirror image twins ride side by side to space atop the Delta rocket. The washing machine spacecraft weigh about 677 pounds (307 kg) each.

The Delta II rocket stands 128 feet tall and is 8 feet in diameter. The first stage liquid and solid rocket fueled engines will generate about 1.3 million pounds of thrust.

During the Terminal Countdown, the first stage is fueled with cryogenic liquid oxygen and highly refined kerosene (RP-1).

GRAIL is an extraordinary first ever journey to the center of the moon that will — with its instruments from orbit — peer into the moons interior from crust to core and map its gravity field by 100 to 1000 times better than ever before. The mission employs two satellites flying in tandem formation some 50 km in near circular polar orbit above the lunar surface.

GRAIL A and B will perform high precision range-rate measurements between them using a Ka-band instrument. The mission will provide unprecedented insight into the formation and thermal evolution of the moon that can be applied to the other rocky planets in our solar system: Mercury, Venus, Earth and Mars.

After a 3.5 month journey to the moon, the probes will arrive about a day apart on New Year’s Eve and New Year’s Day 2012 for an 82 day science mapping phase as the moon rotates three times beneath the GRAIL orbit.

Photojournalists watch as Mobile Service Tower is retracted from around Delta II rocket at Pad 17B.
Credit: Ken Kremer

Xenon spotlights bathe Delta II rocket as Mobile Service Tower is retracted at Pad 17 and photojournalists watch from nearby at Pad 17B. Credit: Ken Kremer

Read Ken’s continuing features about GRAIL
Last Delta II Rocket to Launch Extraordinary Journey to the Center of the Moon on Sept. 8
NASAs Lunar Mapping Duo Encapsulated and Ready for Sept. 8 Liftoff
GRAIL Lunar Twins Mated to Delta Rocket at Launch Pad
GRAIL Twins ready for NASA Science Expedition to the Moon: Photo Gallery

Posted on by Nancy Atkinson

Winds Delay Launch For GRAIL

GRAIL and its Delta 2 rocket on the launchpad. Credit: Alan Walters (awaltersphoto.com) for Universe Today.

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High upper level winds put a damper on hopes for launching the GRAIL mission on its first attempts on Thursday, September 8. While the weather looked perfect on the ground at Kennedy Space Center, weather balloons showed high winds in the region of the atmosphere where the Delta 2 launcher would normally experience the most turbulence.

NASA will try again on Friday, September 9 with two one-second launch windows available at 8:33 and 9:12 EDT (12:33 or 13:12 UT). There were two one-second launch windows for Thursday, and both were “red” because of the winds aloft.

The dynamic duo twin-spacecraft Gravity Recovery and Interior Laboratory (GRAIL) mission is designed to map the Moon’s gravity with extreme precision.

For more information on the mission, read our preview article by Ken Kremer.

GRAIL on the launchpad. Credit: Alan Walters (awaltersphoto.com) for Universe Today.
Posted on by Ken Kremer

Last Delta II Rocket to Launch Extraordinary Journey to the Center of the Moon on Sept. 8

Delta II Heavy rocket will blast GRAIL missions to the moon from Launch Pad 17B. Delta II rocket and twin GRAIL satellites are enclosed inside the Mobile Service Tower at Cape Canaveral Air Force Station. Credit: Ken Kremer

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Another American rocket Era is about to end. The venerable Delta II rocket, steeped in history, will fly what is almost certainly its final mission from Cape Canaveral. And it will do so quite fittingly by blasting twin satellites to the moon for NASA on a unique path for a truly challenging mission to do “extraordinary science”.

On Sept. 8, the most powerful version of the Delta II, dubbed the Delta II Heavy, is slated to launch NASA’s duo of GRAIL lunar mappers on an unprecedented science mission to unlock the mysteries of the moons deep interior. There are two instantaneous launch windows at 8:37:06 a.m. and 9:16:12 a.m. EDT lasting one second each.

“GRAIL simply put, is a journey to the center of the moon,” said Ed Weiler, NASA Associate Administrator of the Science Mission Directorate in Washington,DC at a pre-launch briefing for reporters on Sept. 6.

“It will probe the interior of the moon and map its gravity field by 100 to 1000 times better than ever before. We will learn more about the interior of the moon with GRAIL than all previous lunar missions combined.”

View of Delta II rocket looking out to Atlantic Ocean from upper level of Launch Complex 17. ULA and GRAIL logos painted on side of 8 ft diameter Delta rocket. Credit: Ken Kremer

GRAIL will depart Earth from Space Launch Complex 17B (SLC-17B) at Cape Canaveral Air Force Station, Florida, which is also the last scheduled use of Pad 17B.

GRAIL logo painted on the side of Delta II Rocket 1st Stage. Photo taken from inside upper level of launch gantry. GRAIL stands for Gravity Recovery and Interior Laboratory. Credit: Ken Kremer

“Trying to understand how the moon formed, and how it evolved over its history, is one of the things we’re trying to address with the GRAIL mission,” says Maria Zuber, principal investigator for GRAIL from the Massachusetts Institute of Technology. “But also, (we’re) trying to understand how the moon is an example of how terrestrial planets in general have formed.”

“GRAIL is a mission that will study the inside of the moon from crust to core,” Zuber says.

Delta II Heavy rocket is augmented by 9 wider diameter solid rocket motors providing more thrust. Credit: Ken Kremer

So far there have been 355 launches of the Delta II family, according to NASA’s Delta II Launch Manager Tim Dunn. The Delta II is built by United Launch Alliance.

“GRAIL is the last contracted Delta II mission to be launched from Complex 17. And it will be the 356th overall Delta to be launched. Complex 17 at the Cape has a proud heritage of hosting 258 of those 355 total Delta launches to date.

Hypergolic propellants have been loaded onto the 2nd stage after assessing all the preparations for the rocket, spacecraft, the range and facilities required for launch.

“The Launch Readiness Review was successfully completed and we can proceed with the countdown,” said Dunn.
The Delta II Heavy is augmented with nine larger diameter ATK solid rocket motors.

The Mobile Service Tower will be rolled back from the Delta II rocket tonight, starting at about 10:30 p.m. EDT depending on the weather.

The weather forecast for launch remains very iffy at a 60% percent chance of “NO GO” according to NASA and Air Force officials.

A launch decision will be made tomorrow morning Sept. 8 right after the weather briefing but before fueling begins at 6:30 a.m.

The weather forecast for rollback of the Mobile Service Tower tonight remains generally favorable. There is a 40% chance of a weather issue at 10:30 p.m. which drops to 30% after midnight. Tower rollback can be pushed back about 2 hours without impacting the countdown, says NASA.

Weather remains at 60% NO GO in case of a 24 hour delay but improves over the weekend. The team has about 42 days time in the launch window.

After entering lunar orbit, the two GRAIL spacecraft will fly in a tandem formation just 55 kilometers above the lunar surface with an average separation of 200 km during the three month science phase.

Stay tuned to Universe Today for updates overnight leading to liftoff at 8:37 a.m.

See my photo album from a recent tour of Launch Complex 17 and the Mobile Service Tower

GRAIL Flying in Formation. Using a precision formation-flying technique, the twin GRAIL spacecraft will map the moon's gravity field. The mission also will answer longstanding questions about Earth's moon, including the size of a possible inner core, and it should provide scientists with a better understanding of how Earth and other rocky planets in the solar system formed. GRAIL is a part of NASA's Discovery Program.

Read Ken’s continuing features about GRAIL

NASAs Lunar Mapping Duo Encapsulated and Ready for Sept. 8 Liftoff
GRAIL Lunar Twins Mated to Delta Rocket at Launch Pad
GRAIL Twins ready for NASA Science Expedition to the Moon: Photo Gallery

Posted on by Nancy Atkinson

NASA Releases Closer Looks at Apollo Landing Sites from the Lunar Reconnaissance Orbiter

Low periapsis Narrow Angle Camera image of the Apollo 17 Landing Site. Image is 150 meters wide, Credit: NASA/GSFC/Arizona State University.

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New images of the Apollo 12, 14 and 17 landing sites are the highest resolution pictures ever of human forays onto another world, as seen from a bird’s eye view — or in this case, a satellite’s eye view. The Lunar Reconnaissance Orbiter dipped to a lower altitude, just 21 kilometers (13 miles) over the lunar surface.

“We like to look at the Apollo landing site images because it’s fun,” said LRO principal investigator Mark Robinson at a media briefing today. “But LROC (Lunar Reconnaissance Orbiter Camera) is looking at the whole Moon, and we have now taken 1,500 of these very high resolution images from all around the Moon which will help scientists and engineers to plan where we want to go in the future.”

Apollo 17 landing site taken by LRO in its lower orbit, with 25 cm per pixel. Credit: NASA/Goddard/ASU

Apollo 17 landing site from the regular 50 km altitude and about 50 cm per pixel. Credit: NASA/ Goddard/ ASU

Compare in the images above the Apollo 17 landing site with 25 cm per pixel (top) and 50 cm per pixel (bottom).

Most notable are the tracks where the astronauts walked show up better, and details of the landers/descent stages can be resolved better.

Robinson said he was looking at the new images of the Apollo 17 landing site in Taurus Littrow Valley with Apollo 17 astronaut Jack Schmitt and Schmitt said “You need to image the whole valley at this resolution!”

This is the third resolution of Apollo sites that the LRO team has released — the first came from LRO’s commissioning phase where the altitude was about 100 km and the resolution was about 1 meter per pixel; next came the release of images from an altitude of about 50 km, with a resolution of about 50 cm per pixel; and now from about 21-22 km altitude with a resolution of 25 cm per pixel.

“These are the sharpest images of Apollo landing sites we’ll probably ever get with LRO,” said Rich Vondrak, LRO project scientist, “as we’ll never go as low in altitude as we were in the past month.”

LRO has now returned to its circular orbit of 50 km above the surface. This altitude requires monthly reboosts and since keeping LRO in that orbit would quickly exhaust the remaining fuel, in mid-December, LRO will move to an elliptical orbit, (30 km over south pole and 200 km over north pole). LRO will be able to stay in this orbit for several more years.

“This has been a highly productive mission, releasing a total of 245 terabytes of data — which would be a stack of 52,000 DVDs,” Vondrak said. Next week the science team will put out their 7th public release of data to the Planetary Data System, making all that data available to the public.

The paths left by astronauts Alan Shepard and Edgar Mitchell on both Apollo 14 moon walks are visible in this image. (At the end of the second moon walk, Shepard famously hit two golf balls.) The descent stage of the lunar module Antares is also visible. Credit: NASA's Goddard Space Flight Center/ASU

Robinson noted that the details of what pieces of equipment are in each location are verified by images taken from the surface by the astronauts. He was asked about the flags and if they are still standing: “All we can really see is the spots where the flag was planted because the astronauts tramped down the regolith. I’m not sure if the flags still exist, given the extreme heat and cold cycle and the harsh UV environment. The flags were made of nylon, and personally I would be surprised if anything was left of them since it has been over 40 years since they were left on the Moon and the flags we have here on Earth fade after they are left outside for one summer. If the flags are still there they are probably in pretty rough shape.”

The tracks made in 1969 by astronauts Pete Conrad and Alan Bean, the third and fourth humans to walk on the moon, can be seen in this LRO image of the Apollo 12 site. The location of the descent stage for Apollo 12's lunar module, Intrepid, also can be seen. Credit: NASA/Goddard/ASU

Since we can still see the tracks and equipment looking unchanged (at least from this vantage point) one reporter asked if these items will be on the Moon forever. “Forever is a long time, so no, they won’t be there forever,” Robinson replied. “The Moon is constantly bombarded by micrometeorites, and slowly over time the tracks will disappear, then the smaller pieces of equipment will disappear, and eventually the decent stages will probably get blasted by an a larger asteroid. The estimate is that rocks erode 1 mm per million years. In human terms it may seems like forever, but geologic terms, there will be no traces of Apollo exploration in 10 to 100 million years.”

This video shows more info and a “zoom in” of the sites:

Sources: Media briefing, NASA, LROC

Posted on by Nancy Atkinson

September is Moon Month!

Jane Houston Jones from JPL provides information on what’s up for September, focusing on the Moon. The next few days will be a good time to look for the Apollo landing sites — and no, you won’t be able to see any details from Earth, even with a good telescope, but it is fun to try and locate the areas humans have walked on the Moon. Jane shows you how. And of course, the GRAIL mission to the Moon is scheduled to launch on Sept. 8. Learn more about the mission here.

And as a heads up, look for new images of the Apollo landing sites from the Lunar Reconnaissance Orbiter that will be released next week. LRO recently moved closer to the Moon to take new and improved images of these historic sites. We’ll share them as soon as they are available.

Posted on by Ken Kremer

NASAs Lunar Mapping Duo Encapsulated and Ready for Sept. 8 Liftoff

Twin GRAIL Lunar Mappers being enclosed with payload fairing atop Delta II rocket. Spacecraft technicians monitor the movement of a section of the clamshell-shaped Delta payload fairing as it encloses NASA's twin Gravity Recovery and Interior Laboratory spacecraft at Space Launch Complex 17B on Cape Canaveral Air Force Station in Florida. Liftoff is slated for Sept. 8 at 8:37 a.m. EDT. Credit: NASA/Jim Grossmann

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NASA’s powerful lunar mapping duo of GRAIL spacecraft are now poised for liftoff in just one weeks time on Thursday, Sept. 8.

Mission managers held a Flight Readiness Review on Wednesday (Aug.31) and gave a tentative approval to begin fueling the Delta II rockets second stage on Sept. 2 and 3 after evaluating all issues related to the rocket, launch pad and payloads.

Launch preparations are proceeding on schedule towards an early morning lift off from the seaside Space Launch Complex 17B (SLC-17B) at Cape Canaveral Air Force Station, Florida. There are two instantaneous launch windows at 8:37:06 a.m. and 9:16:12 a.m. EDT lasting one second each.

“Launch vehicle and spacecraft closeouts will begin on Tuesday, and the Launch Readiness Review is also scheduled for Tuesday morning,” NASA KSC spokesman George Diller told Universe Today.

“This morning’s launch countdown dress rehearsal went fine.”

“Delta II 2nd stage fueling has been rescheduled for Friday and Saturday [Sept. 2 and 3]. Last evening a software error was found in the launch processing system data base. ULA (United Launch Alliance) decided they would like to look for any additional errors before the fueling begins. There is no impact to the launch date and currently no work is scheduled on Sunday or on Labor Day,” said Diller.

The launch period extends through Oct. 19, with liftoff occurring approximately four minutes earlier each day in case of a delay. The flight plan was designed to avoid a pair of lunar eclipses occurring on December 10th, 2011 and June 4th 2012 which would interfere with the missions operations and science.

The team is keeping a close watch on the weather as the season’s next Atlantic Ocean storm heads westwards. Katia has just been upgraded to Hurricane status and follows closely on the heels of the continuing vast destruction, misery and deaths caused by Hurricane Irene earlier this week.

“The preliminary weather forecast is favorable for launch day as long as the wind remains out of the west as is currently forecast for that time of the morning,” Diller told me.

Twin GRAIL Lunar Mappers are secured atop Delta II rocket and await enclose in the Delta payload fairing. The fairing will protect the spacecraft from the impact of aerodynamic pressure and heating during ascent and will be jettisoned once the spacecraft is outside the Earth's atmosphere. Credit: NASA/Jim Grossmann

The twin probes known as GRAIL-A and GRAIL-B (Gravity Recovery and Interior Laboratory) were encapsulated inside the clamshell like payload fairing on Aug. 23 The nearly identical spacecraft are mounted side by side and sit atop the Centaur upper stage.

The fairing shields the spacecraft from aerodynamic pressures, friction and extreme heating for the first few minutes of flight during ascent through the Earth atmosphere.

This Delta II Heavy booster rocket is the most powerful version of the Delta II family built by ULA. The booster’s first stage is augmented with larger diameter solid rocket motors.

GRAIL was processed for launch inside at the Astrotech payload processing facility in Titusville, Fla. See my GRAIL spacecraft photos from inside the Astrotech clean room facilities here.

“The GRAIL spacecraft inside the handling can departed Astrotech and arrived at the launch pad, SLC-17B on Aug. 18” said Tim Dunn, NASA’s Delta II Launch Director in an interview with Universe Today. “The spacecraft was then hoisted by crane onto the Delta II launch vehicle and the spacecraft mate operation was flawlessly executed by the combined ULA and NASA Delta II Team.”

An Integrated Systems Test (IST) of the mated booster and payload was completed on Aug. 22

Technicians prepare twin GRAIL spacecraft for enclosure in the Delta payload fairing. Credit: NASA/Jim Grossmann

The dynamic duo will orbit the moon in a tandam formation just 50 kilometers above the lunar surface with an average separation of 200 km. During the 90 day science phase the goal is to determine the structure of the lunar interior from crust to core and to advance understanding of the thermal evolution of the moon.

GRAIL-A & GRAIL-B will measure the lunar gravity field with unprecedented resolution up to 100 times improvement on the near side and 1000 times improvement for the far side.

NASA’s twin GRAIL Science Probes ready for Lunar Expedition
GRAIL B (left) and GRAIL A (right) spacecraft are mounted side by side on top of a payload adapter inside the clean room at Astrotech Space Operations facility. The spacecraft await lunar launch on Sept. 8, 2011. Credit: Ken Kremer

Read Ken’s continuing features about GRAIL

GRAIL Lunar Twins Mated to Delta Rocket at Launch Pad
GRAIL Twins ready for NASA Science Expedition to the Moon: Photo Gallery

Posted on by Ken Kremer

GRAIL Twins ready for NASA Science Expedition to the Moon: Photo Gallery

NASA’s twin GRAIL Science Probes ready for Lunar Expedition. GRAIL B (left) and GRAIL A (right) spacecraft are mounted side by side on top of a payload adapter inside the clean room at Astrotech Space Operations facility. The spacecraft await lunar launch on Sept. 8, 2011. Credit: Ken Kremer

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NASA’s GRAIL twins – dubbed GRAIL-A & GRAIL-B – are ready to embark on America’s next science expedition to the moon in less than 1 month’s time from Cape Canaveral Air Force Station, Fla.

The twin Gravity Recovery and Interior Laboratory (GRAIL) spacecraft have been exhaustively tested, fueled for flight and mounted side-by-side on a specially designed payload adapter inside the controlled environment of a clean room at the Astrotech Space Operations facility in nearby Titusville, Fla.

The next processing step is to encapsulate the lunar probes inside their protective payload fairing. The duo are set to be shipped from Astrotech to their Cape Canaveral launch pad next week on Aug. 16, where they will be mated to an already assembled Delta II booster.

Liftoff of the GRAIL twins is slated for Sept. 8 at 8:37 a.m. EDT by a Delta II Heavy rocket from Launch Complex 17 at Cape Canaveral for a nearly four month voyage to the moon.

After entering lunar orbit, the two GRAIL spacecraft will fly in a tandam formation just 50 kilometers above the lunar surface with an average separation of 200 km during the 90 day science phase.

Side view of twin GRAIL probes
The GRAIL spacecraft are mounted to a 3 inch high Launch Vehicle Adapter Assembly and 20 inch Payload Adapter spacer ring on top of a 30-inch high GSE stand. Credit: Ken Kremer (kenkremer.com)

GRAIL’s mission goal is to map the moon’s gravity field to high precision and thereby deduce the structure of the lunar interior from crust to core. This will also lead to a better understanding of the composition of the moon’s interior, according to Sami Asmar, GRAIL co-investigator from NASA’s Jet Propulsion Laboratory in Pasasdena, Calif., during an interview inside the Astrotech clean room at a photo opportunity for the media. A gravity experiment is also aboard the just launched Jupiter bound Juno spacecraft.

GRAIL Photo Album special taken from inside the Astrotech cleanroom facility.

Twin GRAIL lunar probes inside clean room at Astrotech. Credit: Ken Kremer
Close up of twin lunar probes, GRAIL- B (left) & GRAIL- A (right). Credit: Ken Kremer
GRAIL-B solar panels. Credit: Ken Kremer
GRAIL Science and Launch team inside clean room at Astrotech. Credit: Ken Kremer
GRAIL Co-Investigator Sami Asmar (left) from JPL and Ken Kremer discuss science objectives inside clean room at Astrotech.
Posted on by Nancy Atkinson

LRO to Move in For Closer Look at the Apollo Landing Sites

Artist concept of LRO in lunar orbit. Credit: NASA

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NASA’s Lunar Reconnaissance Orbiter (LRO) is changing our view of the Moon by literally bringing it into sharper focus with its three high resolution cameras. But now, things are about to get even sharper. Today, LRO fired its thrusters to begin dipping down from its usual orbit about 50 km above the surface and moving to an orbit that will allow the spacecraft’s cameras me to image the Apollo sites from about 20 km away.

“This will allow me to obtain images of the Apollo sites that are about 4 times sharper than my current best images,” said the LRO spacecraft on Twitter.


This is just a temporary orbit and the spacecraft will take images of and around the Apollo sites between August 14 and 19, 2011. After that, the spacecraft will return to the 50-km-orbit until December.

LRO has two narrow angle cameras (NACs) and one wide angle camera (WAC).

According to Mark Robinson, LROC Principal Investigator, who spoke at the Lunar Forum at Ames Research Center last month, as of the end of July, 2011 the amount of data returned by LRO has been about 400 gigabits of data every day, which includes 371,027 high resolution images. The WAC has taken about 160,000 images, with about 90,000 in color. In total, the spacecraft has imaged the entire Moon about 20 times with the WAC, and has imaged 20 per cent of the moon with NACs, which provides a narrower but higher resolution view.

“We want to map the whole moon at 50 cm/pixel to 200 cm/pixel, and that would be LROC’s legacy for the next 100 years of lunar exploration and science,” Robinson said.

He noted that all three cameras are performing way better than he had hoped.

“We are very excited about the quality of the data,” Robinson said.

So get ready for a little more quality views of the Apollo landing sites!

Update: as commenter MoonOrBust noted, the LRO Twitter feed had an addendum later in the day, adding that there are several technical challenges associated with getting improved resolution images at the lower altitude orbit. For example, the spacecraft will not slow from its orbital speed of about 1.6 km/s (about 3,500 mph) when it gets closer to the Moon’s surface, which might cause some image blurring, particularly for the LROC Narrow Angle Camera images. “However, it will certainly be fun to compare the images from the different orbits!” the spacecraft Tweeted.

Posted on by Nancy Atkinson

Second Moon May Have Orbited Earth Billions of Years Ago

Four snapshots from the computer simulation of a collision between the moon and a smaller companion moon show most of the companion moon is accreted as a pancake-shaped layer, forming a mountainous region on one side of the moon. Credit: M. Jutzi and E. Asphaug, Nature.

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It’s a view science fiction fans could only hope for: twin moons in the night sky above Earth. But it might have been reality about 4 billion years ago. A new model suggests the lunar farside highlands could have been created from a collision with a smaller companion moon in what scientists from the University of California, Santa Cruz are calling “the big splat.”

Why the near and far sides of the Moon are so different has long puzzled planetary scientists. The near side is relatively low and flat, while the topography of the far side is high and mountainous, with a much thicker crust.

We actually have a somewhat lopsided Moon.

The new study, published in the August 4 issue of Nature, builds on the “giant impact” model for the origin of the moon, in which a Mars-sized object collided with Earth early in the history of the solar system and ejected debris that coalesced to form the moon.

According to the new computer model, the second moon around Earth would have been about 1,200 kilometers (750 miles) wide and could have formed from the same collision. Later, the smaller moon fell back onto the bigger Moon and coated one side with an extra layer of solid crust tens of kilometers thick.

“Our model works well with models of the Moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the Moon itself,” said Erik Asphaug, professor of Earth and planetary sciences at UC Santa Cruz. “It agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the moon, and the ages of lunar rocks.”

Other computer models have suggested a companion moon, said Asphaug, who coauthored the paper with UCSC postdoctoral researcher Martin Jutzi.

A previous collision with a smaller companion could explain why the Moon's two sides look so different. Credit: Martin Jutzi and Erik Asphaug

Asphaug and Jutzi used computer simulations to study the dynamics of the collision between the Moon and a smaller companion, which was about one-thirtieth the mass of the “main” moon. They tracked the evolution and distribution of lunar material in its aftermath.

The impact between the two bodies would have been relatively slow, at about 8,000 kph (5,000 mph) which is slow enough for rocks not to melt and no impact crater to form. Instead, the rocks and crust from the smaller moon would have spread over and around the bigger moon.

“Of course, impact modelers try to explain everything with collisions. In this case, it requires an odd collision: being slow, it does not form a crater, but splats material onto one side,” Asphaug said. “It is something new to think about.”

He and Jutzi hypothesize that the companion moon was initially trapped at one of the gravitationally stable “Trojan points” sharing the Moon’s orbit, and became destabilized after the moon’s orbit had expanded far from Earth. “The collision could have happened anywhere on the Moon,” Jutzi said. “The final body is lopsided and would reorient so that one side faces Earth.”

The model may also explain variations in the composition of the moon’s crust, which is dominated on the near side by terrain comparatively rich in potassium, rare-earth elements, and phosphorus (KREEP). These elements, as well as uranium and thorium, are believed to have been concentrated in the magma ocean that remained as molten rock solidified under the moon’s thickening crust. In the simulations, the collision squishes this KREEP-rich layer onto the opposite hemisphere, setting the stage for the geology now seen on the near side of the moon.

While the model explains many things, the jury is still out among planetary scientists as to the full history of the Moon and what really happened. Scientists say the best way to figure out the Moon’s history is to get more data from lunar orbiting spacecraft and – even better – sample return missions or human missions to study the Moon.

Sources: Nature, UC Santa Cruz