Screenshot of the Progress 51 launch. Via NASA TV.
The Russian Progress 51 cargo craft launched from the Baikonur Cosmodrome in Kazakhstan April 24, at 12:12 UTC (6:12 am EDT) and is on its way to the International Space Station. Unlike its three predecessors, Progress 51 will take the typical two-day rendezvous instead of the new 6-hour fast-track to reach the ISS. This is because of the phasing and orbital mechanics associated with this launch date. The unpiloted Progress is scheduled to dock to the aft port of the station’s Zvezda Service Module on April 26; however a problem arose when a rendezvous antenna did not deploy, which may affect the docking.
The Progress made it safely to orbit and deployed its solar arrays as planned. But one of the five sets of KURS automated rendezvous antennas used as navigational aids did not deploy. Russian ground controllers are assessing the antenna, which is used to measure orientation of the Progress vehicle, and how to troubleshoot the problem. We’ll keep you posted if the docking time changes.
On board are more than three tons of food, fuel, supplies and experiment hardware for the ISS Expedition 35 crew.
Composite Spitzer, Hubble, and Chandra image of supernova remnant Cassiopeia A. A new study shows that a supernova as far away as 50 light years could have devastating effects on life on Earth. (NASA/JPL-Caltech/STScI/CXC/SAO)
“The total number of stars in the Universe is larger than all the grains of sand on all the beaches of the planet Earth,” Carl Sagan famously said in his iconic TV series Cosmos. But when two of those grains are made of a silicon-and-oxygen compound called silica, and they were found hiding deep inside ancient meteorites recovered from Antarctica, they very well may be from a star… possibly even the one whose explosive collapse sparked the formation of the Solar System itself.
Researchers from Washington University in St. Louis with support from the McDonnell Center for the Space Sciences have announced the discovery of two microscopic grains of silica in primitive meteorites originating from two different sources. This discovery is surprising because silica — one of the main components of sand on Earth today — is not one of the minerals thought to have formed within the Sun’s early circumstellar disk of material.
Instead, it’s thought that the two silica grains were created by a single supernova that seeded the early solar system with its cast-off material and helped set into motion the eventual formation of the planets.
According to a news release by Washington University, “it’s a bit like learning the secrets of the family that lived in your house in the 1800s by examining dust particles they left behind in cracks in the floorboards.”
A 3.5-cm chondrite meteorite found in Antarctica in Nov. 1998. Dark meteorites show up well against the icy terrain of Antarctica. (Carnegie Mellon University)
Until the 1960s most scientists believed the early Solar System got so hot that presolar material could not have survived. But in 1987 scientists at the University of Chicago discovered miniscule diamonds in a primitive meteorite (ones that had not been heated and reworked). Since then they’ve found grains of more than ten other minerals in primitive meteorites.
The scientists can tell these grains came from ancient stars because they have highly unusual isotopic signatures, and different stars produce different proportions of isotopes.
But the material from which our Solar System was fashioned was mixed and homogenized before the planets formed. So all of the planets and the Sun have the pretty much the same “solar” isotopic composition.
Meteorites, most of which are pieces of asteroids, have the solar composition as well, but trapped deep within the primitive ones are pure samples of stars, and the isotopic compositions of these presolar grains can provide clues to their complex nuclear and convective processes.
The layered structure of a star about to go supernova; different layers contain different elements (Wikimedia)
Some models of stellar evolution predict that silica could condense in the cooler outer atmospheres of stars, but others say silicon would be completely consumed by the formation of magnesium- or iron-rich silicates, leaving none to form silica.
“We didn’t know which model was right and which was not, because the models had so many parameters,” said Pierre Haenecour, a graduate student in Earth and Planetary Sciences at Washington University and the first author on a paper to be published in the May 1 issue of Astrophysical Journal Letters.
Under the guidance of physics professor Dr. Christine Floss, who found some of the first silica grains in a meteorite in 2009, Haenecour investigated slices of a primitive meteorite brought back from Antarctica and located a single grain of silica out of 138 presolar grains. The grain he found was rich in oxygen-18, signifying its source as from a core-collapse supernova.
Finding that along with another oxygen-18-enriched silica grain identified within another meteorite by graduate student Xuchao Zhao, Haenecour and his team set about figuring out how such silica grains could form within the collapsing layers of a dying star. They found they could reproduce the oxygen-18 enrichment of the two grains through the mixing of small amounts of material from a star’s oxygen-rich inner zones and the oxygen-18-rich helium/carbon zone with large amounts of material from the outer hydrogen envelope of the supernova.
In fact, Haenecour said, the mixing that produced the composition of the two grains was so similar, the grains might well have come from the same supernova — possibly the very same one that sparked the collapse of the molecular cloud that formed our Solar System.
“It’s a bit like learning the secrets of the family that lived in your house in the 1800s by examining dust particles they left behind in cracks in the floorboards.”
Ancient meteorites, a few microscopic grains of stellar sand, and a lot of lab work… it’s an example of cosmic forensics at its best!
Antares maiden blastoff on April 21, 2013 from NASA Wallops Flight Facility. Credit: Mark Usciak/AmericaSpace
The privately developed Antares rocket built by Orbital Sciences Corp. successfully blasted off on its maiden test flight from the shores of Virginia on April 21 at 5 p.m. EDT from Mid-Atlantic Regional Spaceport (MARS) Pad-0A at NASA Wallops – thereby inaugurating the new commercial space race and delivered a pioneering trio of low cost NASA Smartphone nanosatellites dubbed PhoneSat to orbit.
The 13 story Antares rocket pierced the chilly but cloudless clear blue Virginia skies as “the biggest, loudest and brightest rocket ever to launch from NASA’s Wallops Flight Facility,” said former station astronaut and now Orbital Sciences manager Frank Culbertson.
Antares picture perfect liftoff marked the first step in a public/private collaboration between NASA and Orbital Sciences to restart cargo delivery services to the International Space Station (ISS) that were lost following the forced retirement of NASA’s space shuttle orbiters in 2011.
“Today’s successful test marks another significant milestone in NASA’s plan to rely on American companies to launch supplies and astronauts to the International Space Station, bringing this important work back to the United States where it belongs,” said NASA Administrator Charles Bolden.
Antares accelerates to orbit on April 21, 2013 from NASA Wallops Flight Facility. Credit: Mark Usciak/AmericaSpace
The test flight was dubbed the A-One Test Launch Mission and also signified the first launch from Americas newest space port at Pad-0A.
The primary goal of this test flight – dubbed the A-One mission – was to test the fully integrated Antares rocket and boost a simulated version of the Cygnus cargo carrier – known as a mass simulator – into a target orbit of 250 x 300 kilometers and inclined 51.6 degrees.
Antares also lofted the trio of off-the-shelf-smartphone “PhoneSats” to orbit. The three picture taking satellites are named Alexander, Graham and Bell and could be the lowest-cost satellites ever flown in space.
“The Phonesats cost about $3500 each,” said Andrew Petro, NASA Small Satellite Program executive, to Universe Today. “They are deployed after separation.”
Andrew Petro, NASA Small Satellite Program executive, holds NASA Smartphone Phonesat replica launched on Antares test flight on April 21, 2013. Credit: Ken Kremer (kenkremer.com)
The goal of NASA’s PhoneSat mission is to determine whether a consumer-grade smartphone can be used as the main flight avionics of a capable satellite but at a fraction of the cost.
NASA reports that all three lithium battery powered nanosats are functioning and transmitting data to multiple ground stations.
Two of the cubesats are PhoneSat version 1.0 while the other is the more advanced PhoneSat version 2.0. They were developed by engineers at NASA’s Ames Research Center in Calif.
Each square shaped smartphone measures about 4 inches (10 cm) per side, weighs about 4 pounds and is the size of a coffee mug. The smartphone serves as the cubesats onboard computer – see my photos.
NASA Smartphone Phonesat replica. Credit: Ken Kremer (kenkremer.com)
The cameras will be used for Earth photography. Imaging data will be transmitted in chunks and then stitched together later.
The third time was the charm for Antares following a pair of launch scrubs due to a technical glitch in the final minutes of the initial countdown attempt on Wednesday, April 17 and unacceptable winds on Saturday, April 20.
The rocket flew on a southeasterly trajectory and was visible for about 4 minutes.
This test flight was inserted into the manifest to reduce risk and build confidence for the follow on missions which will fly the fully outfitted Cygnus resupply spacecraft that will dock at the ISS, starting as early as this summer.
The two stage Antares is a medium class rocket similar to the Delta II and SpaceX Falcon 9.
The dummy Cygnus payload was outfitted with instrumentation to collect aerodynamic data until separation from the 2nd stage. That marked the successful conclusion of the A-One mission and the end of all data transmissions.
It will fly in earth orbit for about two weeks or so until atmospheric friction causes the orbit to decay and a fiery reentry.
Frank Culbertson post launch media interview. Credit: Brent Houston
The Antares first stage is powered by dual liquid fueled AJ26 first stage rocket engines that generate a combined total thrust of some 750,000 lbs – original built in the Soviet Union as NK-33 model engines.
The upper stage features an ATK Castor 30 solid rocket motor with thrust vectoring. Antares can loft payloads weighing over 5000 kg to LEO. The 2nd stage will be upgraded starting with the 4th flight.
Antares rocket erect at the Eastern shore of Virginia slated for maiden liftoff on April 17. Only a few hundred feet of beach sand and a miniscule sea wall separate the Wallops Island pad from the Atlantic Ocean waves and Mother Nature. Credit: Ken Kremer (kenkremer.com)
The Antares/Cygnus system was developed by Orbital Sciences Corp under NASA’s Commercial Orbital Transportation Services (COTS) program to replace the ISS cargo resupply capability previously tasked to NASA’s now retired Space Shuttle fleet.
Orbital’s Antares/Cygnus system is similar in scope to the SpaceX Falcon 9/Dragon system. Both firms won lucrative NASA contracts to deliver approximately 20,000 kilograms each of supplies and science equipment to the ISS.
The goal of NASA’s COTS initiative is to achieve safe, reliable and cost-effective transportation to and from the ISS and low-Earth orbit (LEO).
Orbital will launch at least eight Antares/Cygnus resupply missions to the ISS at a cost of $1.9 Billion
Up Close with Antares beautifully decaled nose NASA Wallops Pad 0-A. Credit: Ken Kremer (kenkremer.com)
Learn more about Antares, Orion, SpaceX, Curiosity and NASA robotic and human spaceflight missions at Ken’s upcoming lecture presentations:
April 28: “Curiosity and the Search for Life on Mars – (in 3-D)”. Plus the Space Shuttle, SpaceX, Antares, Orion and more. Washington Crossing State Park, Titusville, NJ, 130 PM
Antares accelerates to orbit on April 21, 2013 from NASA Wallops Flight Facility. Credit: Mark Usciak/AmericaSpaceAntares at MARS Launch Pad 0A at NASA Wallops Flight Facility, Virginia . Credit: Ken Kremer (kenkremer.com)
Screenshot of the meteorite that crashed through a house in Connecticut on April 19, 2013. Via NBC.
A rock that crashed through a house in Connecticut last weekend has been confirmed to be a meteorite.
Homeowner Larry Beck called police in Wolcott, CT at 10:30 a.m. on April 20, 2013 and said a baseball-sized rock crashed through his home the night before, causing damage to his roof and pipes in the attic before cracking the ceiling in his kitchen. Police reported that people from several towns in the area had called to report a loud boom that rattled windows last Friday evening.
Reports say that at first, police thought the rock was a broken piece of airport runway concrete that had dropped from a plane when landing gear was being lowered. The home is near two airports.
After examining the object on Tuesday, Stefan Nicolescu, the collections manager for the Mineralogy Division at the Yale Peabody Museum confirmed it was in fact a meteorite, likely a chondrite.
The Solar Impulse airplane flies over the Golden Gate Bridge on April 23, 2013. Credit: Solar Impulse.
The world’s first solar-powered plane is stretching its wings over the US. Today it took off from Moffett Field in Mountain View, California — the home of NASA’s Ames Research Center – and flew to San Fransisco, soaring over the Golden Gate Bridge.
Starting on May 1, Solar Impulse will fly across the US to New York, making several stops along the way as a kind of “get to know you” tour for the US while the founders of Solar Impulse, Swiss pilot Bertrand Piccard and and pilot Andre Borschberg, want to spread their message of sustainability and technology. You can read about the cross-country tour here on UT and also on the Solar Impulse website. You can follow Solar Impulse’s Twitter feed for the latest news of where they are.
Comet ISON was used in a search for time travelers. NASA’s Hubble Space Telescope provides a close-up look of Comet ISON (C/2012 S1), as photographed on April 10. Credit: NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team.
Here’s our first good look at Comet (C/2012 S1) ISON. The Hubble Space Telescope captured this shot on April 10, when the comet was slightly closer than Jupiter’s orbit at a distance of 634 million kilometers (394 million miles) from Earth. Later this year, this comet could become a brilliant object in the sky, perhaps 10 times brighter than Venus.
Astronomers say preliminary measurements from the Hubble images suggest that the nucleus of ISON is no larger than 4-6 km (3-4 miles) across.
The astronomers said this is remarkably small considering the high level of activity observed in the comet so far. Astronomers are using these images to measure the activity level of this comet and constrain the size of the nucleus, in order to predict the comet’s activity when it will come with 1.1 million km (700,000 miles) of the Sun on November 28, 2013.
Even though Comet ISON was 620 million km from the Sun when this image was taken, the comet is already active as sunlight warms the surface and causes frozen volatiles to sublimate. A detailed analysis of the dust coma surrounding the solid, icy nucleus reveals a strong, jet blasting dust particles off the sunward-facing side of the comet’s nucleus.
The comet’s dusty coma, or head of the comet, is approximately 5,000 km (3,100 miles) across, or 1.2 times the width of Australia. A dust tail extends more than 92,000 km (57,000 miles), far beyond Hubble’s field of view.
Comet ISON belongs to a special category of comets called sungrazers. As the comet performs a hairpin turn around the Sun in November, its ices will vaporize in the intense solar heat. Assuming it defies death by evaporation, some predict it could become as bright as the full Moon. If so, that would occur for a brief time around at perihelion (closest approach to the Sun) when the comet would only be visible in the daytime sky very close to the Sun. When safely viewed, ISON might look like a brilliant, fuzzy star in a blue sky.
More careful analysis is currently underway to improve these measurements and to predict the possible outcome of the sungrazing perihelion passage of this comet.
ISON stands for International Scientific Optical Network, a group of observatories in ten countries who have organized to detect, monitor, and track objects in space. ISON is managed by the Keldysh Institute of Applied Mathematics, part of the Russian Academy of Sciences.
Talk about a light show! A massive bolide was captured on video during a middle-of-the-night rock concert in Argentina on April 21, 2013. The band, Los Tekis performed at an outdoor concert venue and in perfect timing, right after the band concluded a song, the person who shot the video panned out so that the sky was visible — just as the bolide lit up the sky. Continue reading “Bright Meteor Rocks Argentina Rock Concert”
International Space Station pass over Stonehenge, Wiltshire UK. Credit and copyright: Tim Burgess. Used by permission.
In a gorgeous mix of archeology and space exploration, photographer Tim Burgess captured a stunning view of the International Space Station passing over the historic and iconic Stonehenge on April 20, 2013. Tim said this composite image is composed of 11 shots, 10 sec, f2.8, 400 ISO. As one person commented on Flickr, “An amazing feat of human engineering passing over an amazing feat of human engineering, captured by an amazing feat of human engineering.”
Thanks to Tim for allowing us to post this image on UT; keep track of Tim’s photography on Flickr and Twitter.
Want to get your astrophoto featured on Universe Today? Join our Flickr group or send us your images by email (this means you’re giving us permission to post them). Please explain what’s in the picture, when you took it, the equipment you used, etc.
The very early partial phase of the 2010 December solstice total lunar eclipse. This week's shallow eclipse will appear similar at its maximum. (Photo by author)
Eclipse season is upon us this week with the first eclipse of 2013, a brief partial lunar eclipse.
The lunar eclipse on April 25, 2013 is a shallow one, meaning only a paltry 1.47% of the lunar limb will be immersed in the dark umbra or inner shadow of the Earth. Observers can expect to see only a dark diffuse edge of the inner shadow nick the the Moon as is grazes the umbra.
A partial lunar eclipse this shallow hasn’t occurred since May 3rd, 1958 (0.9%) and won’t be topped until September 28th, 2034 (1.4%). This is the second slightest partial lunar eclipse for this century.
Another term for this sort of alignment is known as a syzygy, a great triple-letter word score in Scrabble!
A video simulation of the eclipse:
The eclipse will be visible in its entirety from eastern Europe & Africa across the Middle East eastward to southeast Asia and western Australia. The eclipse will be visible at moonrise from South America to Western Europe and occurring at moonset for eastern Australia and the Far East. The partially eclipsed Moon will be directly overhead just off the northeastern coast of Madagascar. The eclipse will not be visible from North America.
Two eclipse seasons occur each year when the nodal points of the Moon’s orbit intersect the ecliptic while aligned with the position of the Sun and the Earth’s shadow. The Moon’s orbit is inclined 5.15° degrees with respect to the ecliptic, which traces out our own planet’s path around the Sun. If this intersection occurs near New or Full Moon, a solar or lunar eclipse occurs.
The visibility region of Thursday’s partial lunar eclipse (Credit: NASA/GSFC/F. Espenak).
If the Moon’s orbit was not inclined to our own, we’d get two eclipses per lunation, one solar and one lunar.
2013 has 5 eclipses, 3 lunar and 2 annular. The minimum number of eclipses that can occur in a calendar year is 4, and the maximum is 7, as will next occur in 2038.
The 3 lunar eclipses in 2013 are this week’s partial eclipse on April 25th and two faint penumbral eclipses, one on May 25th and another on October 18th. There is no total lunar eclipse in 2013. The last one occurred on December 10th 2011, and the next one won’t occur until April 15th 2014, favoring the Pacific Rim region.
This eclipse will also set us up for the first solar eclipse of 2013, an annular eclipse crossing NE Australia (in fact crossing the path of last year’s total eclipse near Cairns) and the south Pacific on May 10th. The only solar totality that will touch the surface of the Earth in 2013 is the hybrid eclipse on November 3rd spanning Africa and the South Atlantic with a maximum totality of 1 minute & 40 seconds.
Contact times for the April 25 shallow eclipse:
P1-The Moon touches the penumbra-18:03:41 UT
U1-The Moon touches the umbra-19:54:04 UT
Mid-Eclipse-20:08:37.5 UT
U4 -The Moon quits the umbra-20:21:04 UT
P4-The Moon quits the penumbra- 22:11:23 UT
The length of the partial phase of the eclipse is exactly 27 minutes, and the length of the entire eclipse is 4 hours, 7 minutes and 42 seconds.
This particular eclipse is part of saros series 112 and is member 65 of 72.
This saros cycle began in 859 C.E. on May 20th and will end in 2139 on July 12th with a penumbral lunar eclipse. One famous member of this series was 52. This eclipse was one of many used by Captain James Cook to fix his longitude at sea on December 4th 1778. Christopher Columbus also attempted this feat while voyaging to the New World. It’s a fun project that anyone can try!
I also remember watching the last eclipse in this series from South Korea on April 15th 1995, a slightly better partial of 11.14%.
An occultation of the bright star Spica occurs just 20 hours prior as seen from South Africa across the southern Atlantic. This the 5th in a series of 13 occultations of the star by the Moon in 2013.
The visibility region for the April 25th occultation of Spica. (Created by the Author using Occult 4.1.0.2).
The +2.8th magnitude star Zubenelgenubi (Alpha Librae) is occulted by the waning gibbous Moon just 15 hours after the eclipse for Australia and the South Pacific.
Another occultation of a bright star with potential this week is +4.7th magnitude Chi Virginis across North America on the morning of Wednesday, April 24th centered on 4:24 UT.
Visibility region for the occultation of Chi Virginis on April 24th. (Created by the author using Occult 4.1.0.2 software).
Also keep an eye out for +0.1 magnitude Saturn near the Full Moon. Saturn reaches opposition this weekend for 2013 on April 28th
Full Moon occurs near mid-eclipse at 20:00 UT/16:00 EDT on April 25th. Colloquial names for the April Full Moon are the Pink, Fish, Sprouting Grass, Egg, Seed, & Waking Moon.
Sure, the penumbral phases of an eclipse are subtle and may not be noticeable to the naked eye… but it is possible to see the difference photographically. Simply take a photo of the Moon before it enters the Earth’s penumbra, then take one during the penumbral phase and then another one after. Be sure to keep the ISO/f-stop and shutter speed exactly the same throughout. Also, this project only works if the eclipsed Moon is high in the sky throughout the exposures, as the thick air low to the horizon will discolor the Moon as well. Compare the shots; do you see a difference?
A penumbral eclipse would offer a good proof of concept test for hunting for transiting exoplanets as well, although to our knowledge, no one has ever attempted this.
Finally, calling out to all Universe Today readers in Madagascar. YOU may just be able to catch a transit of the International Space Station in front of the Moon just as the ragged edge of the umbra becomes apparent on the limb of the Moon. Check CALSky a day or so prior to the eclipse for a refined path… it would be an unforgettable pic!
The position of the ISS on April 25th at 19:48UT, just minutes before the partial phase of the eclipse begins. (Credit: Orbitron).
And if any ambitious observer is planning to live stream the eclipse, let us know and we’ll add your embed to this post. We do not expect an avalanche of web broadcasts, but hey, we’d definitely honor the effort! Slooh is usually a pretty dependable site for live eclipse broadcasts, and as of this writing seems to have broadcast scheduled in the cue.
Shoemaker-Levy 9 impact site G. The comet collided with Jupiter in 1994. Credit: R. Evans, J. Trauger, H. Hammel and the HST Comet Science Team
A large comet that peppered Jupiter two decades ago brought water into the giant planet’s atmosphere, according to new research from the Herschel space observatory.
Shoemaker-Levy 9 astounded astronomers worldwide when its 21 fragments hit Jupiter in June 1994. The event was predicted and observatories were trained on Jupiter as the impact occurred. The dark splotches the comet left behind were even visible in small telescopes. But apparently, those weren’t the only effects of the collision.
Herschel’s infrared camera revealed there is two to three times more water in the southern hemisphere of the planet, where the comet slammed into the atmosphere, than in the northern hemisphere. Further, the water is concentrated in high altitudes, around the various sites where Shoemaker-Levy 9 left its mark.
It is possible, researchers acknowledged, that water could have come from interplanetary dust striking Jupiter, almost like a “steady rain.” If this were the case, however, scientists expect the water would be evenly distributed and also would have filtered to lower altitudes. Jupiter’s icy moons were also in the wrong locations, researchers said, to have sent water towards the massive planet.
Internal water rising up was ruled out because it cannot penetrate the “cold trap” between Jupiter’s stratosphere and cloud deck, the researchers added.
“According to our models, as much as 95 percent of the water in the stratosphere is due to the comet impact,” said Thibault Cavalié of the Astrophysical Laboratory of Bordeaux, in France, who led the research.
Eight impact sites from Comet Shoemaker-Levy 9 are visible in this 1994 image. Credit: Hubble Space Telescope
While researchers have suspected for years that Jupiter’s water came from the comet — ESA’s Infrared Space Observatory saw the water there years ago — these new observations provide more direct evidence of Shoemaker-Levy 9’s effect. The results were published in Astronomy and Astrophysics.
Herschel’s find provides more fodder for two missions that are scheduled for Jupiter observations in the coming few years. The first goal for NASA’s Juno spacecraft, which is en route and will arrive in 2016, is to figure out how much water is in Jupiter’s atmosphere.
Additionally, ESA’s Jupiter Icy moons Explorer (JUICE) mission is expected to launch in 2022. “It will map the distribution of Jupiter’s atmospheric ingredients in even greater detail,” ESA stated.
While ESA did not link the finding to how water came to be on Earth, some researchers believe that it was comets that delivered the liquid on to our planet early in Earth’s history. Others, however, say that it was outgassing from volcanic rocks that added water to the surface.
Conventional theory dictates ice was in our solar system from when it was formed, and today we know that many planets have water in some form. Last year, for example, water ice and organics were spotted at Mercury’s north pole.
Mars appeared to be full of water in the ancient past, as evidenced by a huge, underground trench recently discovered by scientists. There is frozen water at the Martian poles, and both the Curiosity and Spirit/Opportunity rover missions have found evidence of flowing water on the surface in the past.
The outer solar system also has its share of water, including in all four giant planets (Jupiter, Saturn, Uranus and Neptune) and (in ice form) on various moons. Even some exoplanets have water vapor in their atmospheres.
“All four giant planets in the outer solar system have water in their atmospheres, but there may be four different scenarios for how they got it,” added Cavalié. “For Jupiter, it is clear that Shoemaker-Levy 9 is by far the dominant source, even if other external sources may contribute also.”
