Typical meteorological applications for the Polar Communications and Weather (PCW) mission. Credit: CSA.
OTTAWA, CANADA — A new Canadian satellite — should it launch — might carry a sort of magnetized force field on board to keep charged particles away from vital electronics.
The Polar Communications and Weather Satellite (PCW), depending on its orbit, could skim through the radiation-filled Van Allen belts on its mission to deliver reliable weather reports and communications to northern communities.
Its polar orbit will likely take it through clouds of charged particles high above Earth. If the particles hit crucial components on the spacecraft, it can short out electronics and cause brownouts or complete failure. This has happened several times before, such as to the Japanese ADEOS-II satellite after a large solar storm in 2003.
A concept being explored by Winnipeg’s Magellan Aerospace, one of the companies working on the early phase studies, would make a plasma field around PCW, a sort of “mini magnetosphere” that would use large dipole magnets to deflect charged particles.
It may also be useful for human missions in the future, said Paul Harrison, a satellite control systems engineer at Magellan Aerospace, although he acknowledged the technology is still in an early stage and that they would like a demonstrator mission to fly first.
“It’s still very much in the development phase. We want to develop for satellites before we start sticking people in them,” Harrison said in a presentation at the Canadian Space Society annual summit in Ottawa, Canada, today (Nov. 14.)
He also said it is not clear if the technology would be useful for cosmic rays that originate from outside the solar system, as well as charged particles that flow from the sun and are present near the Earth.
PCW has not been assigned a launch date yet and is still in the early stages of development. Other issues being explored include how to keep track of it without constant access to near-equator-orbiting GPS satellites, and how to maintain temperature control as it plunges from day to night to day again during its journey.
Its orbit could be a 12-hour Molniya orbit or perhaps a 16-hour or 24-hour highly eccentric orbit, depending on what designers feel is best.
CORRECTION: This article has been changed to say “near-equator-orbiting” GPS satellites.
Mike Hankey of Monkton, Maryland took this photo of Comet ISON in outburst this morning Nov. 14. The tail now shows multiple streamers. Click to enlarge. Credit: Mike Hankey
After a sleepy week, Comet ISON is suddenly coming alive. Several amateur astronomers and at least one professional astronomers are reporting today that the comet has brightened at least a full magnitude overnight. Two days ago it glowed at around magnitude 7.5 and was visible weakly in 10×50 binoculars from a dark sky. Now it’s surged to around magnitude 5.5 – just above the naked eye limit – and continues to brighten. Several amateur astronomers have even seen it without optical aid.
Comet ISON on Nov. 10 before the recent outburst with well-developed dust (upper) and gas tails. Click ot enlarge. Credit: Damian Peach
ISON’s appearance has radically changed too. A week ago the comet developed a second gas or ion tail streaming alongside the wider, brighter dust tail. That new appendage has since grown like Pinocchio’s nose to nearly equal the length of the dust tail. I spotted it with averted vision Tuesday morning Nov. 12 through a 15-inch (37 cm) telescope. More exciting, the ISON’s head has been much brighter and more compact. Astronomers rate a comet’s degree of condensation or “DC” on a scale of 0 to 9 from extremely diffuse with no brightening in the center to disk-like or stellar. In recent days, Comet ISON has been packing it in at DC=6 or moderately compact and bright. Now amateurs are reporting that the comet’s head has brightened and become much more compact with a DC of 8.
Comet ISON in outburst with a completely changed tail appearance and bright, very compact coma shot this morning. Gonzalez reports the comet at magnitude 6.4. Click to enlarge. Credit: Juanjo GonzalezYou can watch Comet ISON evolve right before your eyes in this panel of photos taken by Juanjo Gonzalez. Top row left-right: Nov. 3 and Nov. 9. Bottom row left right: Nov. 12 and Nov. 14. The tail structure changes are dramatic. Click to enlarge. Credit: Juanjo Gonzalez
Backing up reports of the outburst, astronomer Emmanuel Jehin of the TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) team, noted a tenfold increase in dust production around the comet’s nucleus on Nov. 11 and 12 plus additional jets of material blasting into the coma. Jehin reports that the inner coma near the nucleus is still very sharp and shows no sign of disruption – so far, ISON’s hanging in there.
If you haven’t seen the comet yet, you can use this map to track it through the weekend as it zips quickly through Virgo. The map shows the sky facing southeast just before the start of morning twilight or about 100 minutes before sunrise. ISON should be plainly visible in binoculars in a dark sky. Created with Chris Marriott’s SkyMap program
This is all great news for comet observers. The intense heat of the sun is beginning to boil away the comet’s ice with greater fury. The heat may also be exposing new cracks or breaks in ISON’s crust. Fresh ice means even more material becomes available for the sun to vaporize and likely additional jumps in brightness in the next day or two.
Trouble finding Virgo? Use this wide-view map to get oriented. Slide from Mars toward Spica near the southeastern horizon. ISON is about halfway between Spica and Gamma Virginis. The map shows the sky around 5-5:30 a.m. CST. Stellarium
Hail damage is visible on the external tank attached to Space Shuttle Atlantis on the launchpad during a strong thunderstorm that passed through Kennedy Space Center on Feb. 26, 2007. Credit: NASA/KSC.
The STS-117 mission was supposed to carry solar panels and connecting trusses up to the station, so changing the shuttle rotation would affect construction. What to do?
“I’ve got this tank that takes us a bit over two years to manufacture, and essentially it looks like your car here that was peppered by a hailstorm, and what are we going to do?” said Bill Gerstenmaier, NASA’s associate administrator of the human exploration and operations directorate, speaking today (Nov. 14) at the Canadian Space Society’s annual summit in Ottawa, Canada.
“Mike Griffin was the administrator at the time. He said, ‘Get rid of that tank and put another one out there,’ and we didn’t have another one.”
To respond to the problem — based mostly on the word of two technicians who felt repairs were possible, Gerstenmaier said — NASA set out to fix the problem. Communications flew between the launch site in Florida and the manufacturer in New Orleans. NASA had a program that kept track of tiles on the shuttle, and modified it to take care of the dings. The mission lifted off successfully, using the repaired tank, in June 2007 — three months after the incident.
Bill Gerstenmaier in Kennedy Space Center’s Firing Room 4 for the of space shuttle Discovery on the STS-128 mission in 2009. In the background are Chris Scolese, NASA associate administrator, and Charlie Bolden, NASA Administrator. Credit: NASA
Gerstenmaier said this demonstrates that it’s possible to be innovative on reduced budgets, and drew parallels to what NASA is facing right now as it fights through fiscal 2014 budget discussions.
“We have to turn them not into a ‘woe is me’ kind of discussion, but rise above that and pull out the innovation, and that’s what we’re doing in this budget,” he said.
Reduced budgets have helped NASA make use of reduced resources before, he added. It encouraged the agency to tender out to commercial companies (such as SpaceX) for cargo flights to the space station, even though development would occur on the fly. Gerstenmaier, however, did not address concerns that the new budget could cut back commercial crew budgets even further.
Another example of past innovation by both NASA and the Canadian Space Agency, Gerstenmaier said, occurred when the space station’s Canadarm2 robotic arm was adapted to capture these commercial cargo vehicles and berth them into station. If the Canadians had been told in the 1990s — when the space station was just beginning — that the arm would have been required to do this, they likely would have balked, Gerstenmaier said.
While only touching lightly on the ongoing budget discussions, Gerstenmaier did say NASA is keeping an eye on the efforts of Canadian astronaut Chris Hadfield and others as it continues to develop outreach. He joked that the movie “Gravity” really showed the divide between space fans and the general public.
“We see it one way and [say] that isn’t physically correct … it doesn’t actually look like that in space. This is wrong,” he said. “Then the general public says ‘this is really stressful, she lost her child,’ — they’re in this other mode. We’re sitting next to each other in the theater. My non-space colleague is crying, and I’m saying this violates the law of physics.”
Some of Gerstenmaier’s past work in NASA includes top managerial positions in the shuttle//Mir program, space shuttle program integration, the International Space Station and NASA’s space operations directorate (where he oversaw the final 21 space shuttle missions.)
MESSENGER wide-angle camera images of comets Encke and ISON
Two comets currently on their way toward the Sun have been captured on camera from the innermost planet. The MESSENGER spacecraft in orbit around Mercury has spotted the well-known short-period comet Encke as well as the much-anticipated comet ISON, imaging the progress of each over the course of three days. Both comets will reach perihelion later this month within a week of each other.
While Encke will most likely survive its close encounter to continue along its 3.3-year-long lap around the inner Solar System, the fate of ISON isn’t nearly as certain… but both are making for great photo opportunities!
The figure above shows, on the left, images of comet 2P/Encke on three successive days from Nov. 6 to Nov. 8; on the right, images of C/2012 S1 (ISON) are shown for three successive days from Nov. 9 to Nov. 11. Both appear to brighten a little bit more each day.
MESSENGER image of ISON from Nov. 10 (enlarged detail)
MESSENGER is viewing these comets from a vantage point that is very different from that of observers on Earth. Comet Encke was approximately 0.5 AU from the Sun and 0.2 AU from MESSENGER when these images were taken; the same distances were approximately 0.75 AU and 0.5 AU, respectively, for ISON. More images will be obtained starting on November 16 when the comets should be both brighter and closer to Mercury. (Source: MESSENGER featured image article.)
Encke will reach its perihelion on Nov. 21; ISON on Nov. 28.
“We are thrilled to see that we’ve detected ISON,” said Ron Vervack, of the Johns Hopkins University Applied Physics Laboratory, who is leading MESSENGER’s role in the ISON observation campaign. “The comet hasn’t brightened as quickly as originally predicted, so we wondered how well we would do. Seeing it this early bodes well for our later observations.”
Comet 2P/Encke photographed on October 30 by Damian Peach.
Unlike ISON, Encke has been known for quite a while. It was discovered in 1786 and recognized as a periodic comet in 1819. Its orbital period is 3.3 years — the shortest period of any known comet — and November 21 will mark its 62nd recorded perihelion. (Source)
“Encke has been on our radar for a long time because we’ve realized that it would be crossing MESSENGER’s path in mid-November of this year,” Vervack explained. “And not only crossing it, but coming very close to Mercury.”
These early images of both comets are little more than a few pixels across, Vervack said, but he expects improved images next week when the comets make their closest approaches to MESSENGER and Mercury.
“By next week, we expect Encke to brighten by approximately a factor of 200 as seen from Mercury, and ISON by a factor of 15 or more,” Vervack said. “So we have high hopes for better images and data.”
– Ron Vervack, JHUAPL
Read more about the MESSENGER cometary observation campaign in the full news release here.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Southwest Research Institute
November 2013 offers a chance to catch a dependable meteor shower, albeit on an off year. The Leonid meteors are set to reach their annual peak this coming weekend on Sunday, November 17th. We say it’s an off-year, but not that it should discourage you from attempting to catch the Leonids this weekend in the early dawn.
Projections for 2013 suggest a twin-peaked maximum, with the first peak arriving on November 17th at 10:00 UT/5:00 AM EST favoring North America, and the second one reaching Earth on the same date six hours later at 16:00 UT/11:00, favoring the central Pacific.
Unfortunately, the Full Moon also occurs the on very date that the Leonids peak at 10:16 AM EST/ 15:16UT, right between the two peaks! This will definitely cut down on the number of meteors you’ll see in the early AM hours.
That’s strike one against the 2013 Leonids. The next is the curious sporadic nature of this shower. Normally a minor shower with a zenithal hourly rate (ZHR) in the range of 10-20 per hour, the Leonids are prone to great storms topping a ZHR of 1,000+ every 33 years. We last experienced such an event in 1998 and 1999, and we’re now approaching the mid-point lull between storms in the 2014-2016 time frame.
An early Leonid meteor captured last week from the United Kingdom Meteor Observing Network’s Church Crookham station. (Credit: UKMON/Peter-Campbell-Burns).
Still, this is one shower that’s always worth monitoring. The source of the Leonids is Comet 55p/Tempel-Tuttle, which is on a 33-year orbit and is due to reach perihelion again in 2031.
Note that the Leonids have also continued to show enhanced activity in past years even when the Moon was a factor:
2012- ZHR=47.
2011- ZHR=22, Moon=8% waning gibbous.
2010- ZHR=40, 86% waxing gibbous.
2009- ZHR=79.
2008-70 ZHR=72% waning gibbous
We even managed to observe the Leonid meteors from Vail, Arizona in 2002 and 2005, on years when the Moon was nearly Full.
Now, for the good news. The Leonids have a characteristic r value of 2.5, meaning that they produce a higher than normal ratio of fireballs. About 50-70% of Leonid meteors are estimated to leave persistent trains, a good reason to keep a pair of binoculars handy. And hey, at least the 2013 Leonids peak on the weekend, and there’s always comet’s ISON, X1 LINEAR, 2P/Encke and R1 Lovejoy to track down to boot!
A 2002 Leonid captured over Redstone Arsenal, Alabama. (Credit: NASA/MSFC/MEO/Bill Cooke).
Here’s a few tips and tricks that you can use to “beat the Moon” on your Leonid quest. One is to start observing now, on the moonless mornings leading up to the 17th. You’ll always see more Leonid meteors past local midnight as the radiant rises to the northeast. This is because you’re standing on the portion of the Earth turning forward into the meteor stream. Remember, the front windshield of your car (the Earth) always collects the most bugs (meteors). Observers who witnessed the 1966 Leonid storm reported a ZHR in excess of thousands per hour, producing a Star Trek-like effect of the Earth plowing through a “snowstorm” of meteors!
The radiant of the Leonids sits in the center of the backwards question mark asterism of the “Sickle” in the astronomical constellation Leo (hence name of the shower).
You can also improve your prospects for seeing meteors by blocking the Moon behind a building or hill. Though the Leonids will appear to radiate from Leo, they can appear anywhere in the sky. Several other minor showers, such as the Taurids and the Monocerotids, are also active in November.
Meteor shower photography is simple and can be done with nothing more than a DSLR camera on a tripod. This year, you’ll probably want to keep manual exposures short due to the Full Moon and in the 20 seconds or faster range. Simply set the camera to a low f-stop/high ISO setting and a wide field of view and shoot continuously. Catching a meteor involves luck and patience, and be sure to examine the frames after a session; every meteor I’ve caught on camera went unnoticed during observation! Don’t be afraid to experiment with different combinations to get the sky conditions just right. Also, be sure to carry and extra set of charged camera batteries, as long exposures combined with chilly November mornings can drain DSLR batteries in a hurry!
A Woodcut print depicting the 1933 Leonids as seem from Niagara Falls. (Wikimedia Commons image in the Public Domain).
The Leonids certainly have a storied history, dating back to before meteors where understood to be dust grains left by comets. The 1833 Leonids were and awesome and terrifying spectacle to those who witnessed them up and down the eastern seaboard of the U.S. In fact, the single 1833 outburst has been cited as contributing to the multiple religious fundamentalist movements that cropped up in the U.S. in the 1830s.
We witnessed the 1998 Leonids from the deserts of Kuwait while stationed at Al Jabber Air Base. It was easily one of the best meteor displays we ever saw, with a ZHR reaching in access of 500 per hour before dawn. It was intense enough that fireballs behind us would often light up the foreground like camera flashes!
Reporting rates and activity for meteor showers is always fun and easy to do — its real science that you can do using nothing more than a stopwatch and your eyes. The International Meteor Association is always looking for current meteor counts from observers. Data goes towards refining our understanding and modeling of meteor streams and future predictions. The IMO should also have a live ZHR graph for the 2013 Leonids running soon.
Have fun, stay warm, send those Leonid captures in to Universe Today, and don’t forget to tweet those meteors to #Meteorwatch!
If you’re into astronomy, or just a fan of any science fiction franchise worth its salt, then chances are you’ve heard the term parsec thrown around. But what is a parsec exactly? Basically, it’s a unit of length used to measure the astronomically large distances between objects beyond our Solar System.
As we’ve mentioned before, the Sun is a terrifying ball of plasma. It’s a good thing we’re keeping an eye on it. And that eye is the Solar and Heliospheric Observatory, or SOHO. Operating for more than 18 years now, SOHO has been making detailed observations of the Sun’s activity though an almost entire solar cycle. With so many years of operation, SOHO has some amazing stories to tell.
This scene shows the "Murray Ridge" portion of the western rim of Endeavour Crater on Mars. The ridge is the NASA's Mars Exploration Rover Opportunity's work area for the rover's sixth Martian winter. Image Credit: NASA/JPL-Caltech/Cornell/ASU
Just where has the Curiosity rover traveled so far and where is it going? This new video, narrated by John Grotzinger, the principal investigator for the Mars Science Laboratory mission, provides an aerial tour of the rover’s past, present and future traverses on the Red Planet.
Curosity landed in a flat, “hummocky” area in Gale Crater and is heading towards the Aeolis Mons, also known as Mount Sharp, a mountain 5 kilometers (3 miles) high. Right now the rover is among a cluster of small, steep-sided knobs, or buttes that are quite large — up to about the size of a football field and the height of a goal post. They sit in a gap in a band of dark sand dunes that lie at the foot of the mountain. Deep sand could present a hazard for driving, so this break in the dunes is the access path to the mountain.
These buttes have been named the Murray Buttes in honor of influential planetary scientist Bruce Murray (1931-2013).
“Bruce Murray contributed both scientific insight and leadership that laid the groundwork for interplanetary missions such as robotic missions to Mars, including the Mars rovers, part of America’s inspirational accomplishments,” said NASA Mars Exploration Program Manager Fuk Li from JPL. “It is fitting that the rover teams have chosen his name for significant landmarks on their expeditions.”
Meanwhile at Endeavour Crater, where the Opportunity is still exploring, nearly a decade on, and is now preparing for winter. A feature there has also been named for Bruce Murray, Murray Ridge, part of an uplifted crater rim.
“Murray Ridge is the highest hill we’ve ever tried to climb with Opportunity,” said the mission’s principal investigator, Steve Squyres of Cornell University, Ithaca, N.Y. The ridge has outcrops with clay minerals detected from orbit. It also provides a favorable slope for Martian winter sunshine to hit the rover’s solar panels, an advantage for keeping Opportunity mobile through the winter.
“Bruce Murray is best known for having been the director of JPL, and JPL is where our rovers were built,” Squyres said. “He led JPL during a time when the planetary exploration budget was under pressure and the future for planetary missions was not clear. His leadership brought us through that period with a strong exploration program. He was also a towering figure in Mars research. His papers are still cited abundantly today.”
Back to the video, interestingly, the “fly-through” data comes from a variety of missions representing some of the history of Mars exploration. Doug Ellison, who works with JPL’s Eyes on the Solar System – which uses spacecraft data to create realistic simulated views of spacecraft, planets and other features within our solar system – said on Twitter that the video uses data from Viking to narrow down the color, Mars Express High Resolution Stereo Camera (MEX-HRSC_ and the Mars Reconnassaince Oribiter’s HiRISE camera for topography, and the MRO Context camera (MRO-CTX) and HiRISE for imagery.
Black hole with disc and jets visualization courtesy of ESA
The concept of a black hole jet isn’t a new one, but we still have a lot to learn about the mixture of particles found in the vicinity of them. Through the use of ESA’s XMM-Newton Observatory, astronomers have been taking a look at a black hole in our galaxy and found some surprising results.
As we know, stellar mass black holes take on materials from nearby stars. Matter from these companion stars is pulled away from the parent body toward the black hole and radiates a temperture so intense that it emits X-rays. However, a black hole doesn’t always ingest everything that comes its way. Sometimes they reject small portions of this incoming mass, pushing it away in the form of a set of powerful jets. These jets also feed the surroundings, releasing both mass and energy… robbing the black hole of fuel.
Through the study of jet composition, researchers are able to better determine what gets taken into a black hole and what doesn’t. Through observations taken at the radio wavelength of the electromagnetic spectrum, we have seen electrons crusing along at nearly the speed of light. However, it hasn’t been clearly determined whether the negative charge of the electrons is complemented by their anti-particles, positrons, or rather by heavier positively-charged particles in the jets, like protons or atomic nuclei.” With XMM-Newton’s power behind them, astronomers have had the opportunity to examine a black hole binary system called 4U1630–47 – a candidate known to have unexpected outbursts of X-rays for segments of time which last between months and years.
“In our observations, we found signs of highly ionised nuclei of two heavy elements, iron and nickel,” says María Díaz Trigo of the European Southern Observatory in Munich, Germany, lead author of the paper published in the journal Nature. “The discovery came as a surprise – and a good one, since it shows beyond doubt that the composition of black hole jets is much richer than just electrons.”
During September 2012, a team of astronomers headed up by Dr. Díaz Trigo and collaborators, observed 4U1630–47 with XMM-Newton. They also backed up their observations with near-simultaneous radio observations taken from the Australia Telescope Compact Array. Even though the studies were done close to each other – within just a couple of weeks – the results couldn’t have been more different.
According to Trigo’s team, the initial set of observations picked up X-ray signatures from the accretion disc, but there was no activity in the radio band. This is an indicator that the jets weren’t active at that time. However, in the second set of observations, there was activity in both X-ray and radio… the jets had turned back on! While examining the X-ray data from the second set, they also found iron nuclei in motion. These particles were moving both toward and away from XMM-Newton – proof the ions were part of twin jets aimed in opposite directions. However, that’s not all. There was also evidence of nickel nuclei pointing toward the observatory.
“From these ‘fingerprints’ of iron and nickel, we could show that the speed of the jet is very high, about two-thirds of the speed of light,” says co-author James Miller-Jones from the Curtin University node of the International Centre for Radio Astronomy Research in Perth, Australia.
“Moreover, the presence of heavy atomic nuclei in black hole jets means that mass and energy are being carried away from the black hole in much larger amounts than we previously thought, which may have an impact on the mechanism and rate by which the black hole accretes matter,” adds co-author Simone Migliari from the University of Barcelona, Spain.
Astounding new findings? Well… yeah. For a typical stellar-mass black hole, this is the first time that heavy nuclei has been detected within the jets. As of the present, there is only “one other X-ray binary that shows similar signatures from atomic nuclei in its jets – a source known as SS 433. This black hole system, however, is characterised by an unusually high accretion rate, which makes it difficult to compare its properties to those of more ordinary black holes.” Through these new observations of 4U1630–47, astronomers will be able to fill in information gaps about what causes jets to occur in black hole accretion disks and what drives them.
“While we now know a great deal about black holes and what happens around them, the formation of jets is still a big puzzle, so this observation is a major step forward in understanding this fascinating phenomenon,” says Norbert Schartel, ESA’s XMM-Newton Project Scientist.
The thickness of the moon's crust as calculated by NASA's GRAIL mission. The near side is on the left-hand side of the picture, and the far side on the right. Credit: NASA/JPL-Caltech/S. Miljkovic
The familiar blotches that make up “the man in the moon”, from the vantage point of Earth, happened because the moon’s crust is thinner on the near side than the far side to our planet, new research reveals.
The twin GRAIL spacecraft provided the most accurate sizes yet of lunar impact craters on the moon, providing more insight into what happened when Earth’s closest large neighbor was hammered with meteorites over billions of years.
“Since time immemorial, humanity has looked up and wondered what made the man in the moon,” stated Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology in Cambridge.
Artist’s conception of the twin GRAIL spacecraft, called Ebb and Flow. Credit: NASA/JPL-Caltech
“We know the dark splotches are large, lava-filled, impact basins that were created by asteroid impacts about four billion years ago. GRAIL data indicate that both the near side and the far side of the moon were bombarded by similarly large impactors, but they reacted to them much differently.”
The moon’s near side is easily visible in a telescope, but it’s hard to measure the size of the impacts because lava is obscuring their dimensions. The GRAIL spacecraft, however, peered at the internal structure of the moon and also produced information showing how thick the crust is. This showed that there are more, bigger craters on the closer side of the moon to us than the further side.
Closeup of the Moon showing Endymion, Atlas and the distant Mare Humboldtianum. Credit and copyright: Danny Robb.
“Impact simulations indicate that impacts into a hot, thin crust representative of the early moon’s near-side hemisphere would have produced basins with as much as twice the diameter as similar impacts into cooler crust, which is indicative of early conditions on the moon’s far-side hemisphere,” stated lead author Katarina Miljkovic of the Paris Institute of Earth Physics (Institut de Physique du Globe de Paris).
As is common with research projects, learning more about the moon is revealing a new mystery that needs to be examined. It’s commonly cited that the moon was walloped during something called the late heavy bombardment, a period four billion years ago when it was believed that more meteorites impacted the moon.
“The late heavy bombardment is based largely on the ages of large near-side impact basins that are either within, or adjacent to the dark, lava-filled basins, or lunar maria, named Oceanus Procellarum and Mare Imbrium,” NASA stated.
“However, the special composition of the material on and below the surface of the near side implies that the temperatures beneath this region were not representative of the moon as a whole at the time of the late heavy bombardment. The difference in the temperature profiles would have caused scientists to overestimate the magnitude of the basin-forming impact bombardment.”
A research paper on the topic recently appeared in Science. GRAIL successfully concluded its mission last year after nine months of operations, flying into the side of a mountain as planned.
