Explanation: For an Earth-orbiting gamma-ray telescope, Earth is actually the brightest source of gamma-rays, the most energetic form of light. Gamma-rays from Earth are produced when high energy particles, cosmic rays from space, crash into the atmosphere. While that interaction blocks harmful radiation from reaching the surface, those gamma-rays dominate in this remarkable Earth and sky view from the orbiting Fermi Gamma-ray Space Telescope's Large Area Telescope. The image was constructed using only observations made when the center of our Milky Way galaxy was near the zenith, directly above the Fermi satellite. The zenith is mapped to the center of the field. The Earth and points near the nadir, directly below the satellite, are mapped to the edges of the field resulting in an Earth and all-sky projection from Fermi's orbital perspective. The color scheme shows low intensities of gamma-rays as blue and high intensities as yellowish hues on a logarithmic scale. Our fair planet's brighter gamma-ray glow floods the edges of field, the high intensity yellow ring tracing Earth's limb. Gamma-ray sources in the sky along the relatively faint Milky Way stretch diagonally across the middle. Launched June 11, 2008 to explore the high-energy Universe, this week Fermi celebrated its 2,000th day in low Earth orbit.
What an utterly fascinating and informative image and text! Many thanks.
M
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
why the diagonal tilt? i find the whole concept of this pic confusing, but couldn't it have been simplified by rotating the galactic plane to a straight horizontal?
I imagine a fish-eye lens, displaying a "little island" picture like those that APOD is so fond of, that show a view of the actual horizon as a circle around nearer objects in the centre of the circle. Except this view is looking straight outwards at the Milky Way. Around the 'horizon' I see the gamma rays from the Earth's atmosphere and in the middle of the view, the MW's rays.
It is certainly an interesting projection, with the diagonal galactic disc indicating the inclination of the galactic coordinate system to the more familiar celestial coordinate system. But in comparison with the more conventional all-sky map in this link: http://physicscentral.com/explore/pictures/fermi.cfm
... it looks like today's APOD is only slightly more than a half-sky map (and probably half an Earth map too). This would make sense because the Fermi Satellite is only ~360 miles above Earth, so it could only ever see a bit more than half the sky at any one time.
inertnet wrote:Does anyone have an explanation for the two dark patches on the left and right 'inside' earth? Are these the earth poles or the equator?
I'm not really sure about the two dark patches, but based on the fact that the data points appear more coarse in these areas (and also around the yellow-red circular image perimeter representing the gamma-rays produced in Earth's atmosphere, which are flipped "inside-out") I'd say it is merely a result of the projection. It might be easier to understand with some grid lines, at least for the sky section.
Edit: looks like there might be a transition from galactic to celestial coordinates, in the outer parts of the sky section.
JohnD wrote:It is confusing - is the view as I imagine it?
I imagine a fish-eye lens, displaying a "little island" picture like those that APOD is so fond of, that show a view of the actual horizon as a circle around nearer objects in the centre of the circle. Except this view is looking straight outwards at the Milky Way. Around the 'horizon' I see the gamma rays from the Earth's atmosphere and in the middle of the view, the MW's rays.
John
Yes, if I understand correctly, that is sort of the perspective. As Nitpicker says, if the view of the satelite from 360 miles above the earth is somewhat more than half the sky, then the diameter of the pictured rays, from one magenta dot to the other side, would be, say, 210 to 240 degrees maybe. So the field is compressed, and the bright yellow gamma ray rim is the earth's horizon to the satelite, all around.
robo2 wrote:
why the diagonal tilt? i find the whole concept of this pic confusing, but couldn't it have been simplified by rotating the galactic plane to a straight horizontal?
inertnet wrote:
Does anyone have an explanation for the two dark patches on the left and right 'inside' earth? Are these the earth poles or the equator?
Fermi is performing a crazy nodding dance of looking to the north then to the south then to the north again. This defines a celestial coordinate system that contains east/west gaps when confined to one hemisphere.
"NASA's Fermi Gamma-ray Space Telescope orbits our planet every 95 minutes. Its wide-eyed Large Area Telescope (LAT) sweeps across the entire sky every three hours. In order to capture the entire sky every two orbits, scientists deliberately nod the LAT in a repeating pattern from one orbit to the next. It first looks north on one orbit, south on the next, and then north again. The spacecraft rolls to keep the sun from shining on and warming up the LAT's radiators, which regulate its temperature by bleeding excess heat into space. Every few weeks, the LAT deviates from this pattern to concentrate on particularly interesting targets, such as eruptions on the sun, brief but brilliant gamma-ray bursts associated with the birth of stellar-mass black holes, and outbursts from supermassive black holes in distant galaxies."
NASA's Fermi Gamma-ray Space Telescope orbits our planet every 95 minutes, building up increasingly deeper views of the universe with every circuit. Its wide-eyed Large Area Telescope (LAT) sweeps across the entire sky every three hours, capturing the highest-energy form of light -- gamma rays -- from sources across the universe. These range from supermassive black holes billions of light-years away to intriguing objects in our own galaxy, such as X-ray binaries, supernova remnants and pulsars.
Now a Fermi scientist has transformed LAT data of a famous pulsar into a mesmerizing movie that visually encapsulates the spacecraft's complex motion.
Pulsars are neutron stars, the crushed cores of massive suns that destroyed themselves when they ran out of fuel, collapsed and exploded. The blast simultaneously shattered the star and compressed its core into a body as small as a city yet more massive than the sun. The result is an object of incredible density, where a spoonful of matter weighs as much as a mountain on Earth. Equally incredible is a pulsar's rapid spin, with typical rotation periods ranging from once every few seconds up to hundreds of times a second. Fermi sees gamma rays from more than a hundred pulsars scattered across the sky.
One pulsar shines especially bright for Fermi. Called Vela, it spins 11 times a second and is the brightest persistent source of gamma rays the LAT sees. Although gamma-ray bursts and flares from distant black holes occasionally outshine the pulsar, they don't have Vela's staying power. Because pulsars emit beams of energy, scientists often compare them to lighthouses, a connection that in a broader sense works especially well for Vela, which is both a brilliant beacon and a familiar landmark in the gamma-ray sky.
Most telescopes focus on a very small region of the sky, but the LAT is a wide-field instrument that can detect gamma rays across a large portion of the sky at once. The LAT is, however, much more sensitive to gamma rays near the center of its field of view than at the edges. Scientists can use observations of a bright source like Vela to track how this sensitivity varies across the instrument's field of view.
With this in mind, LAT team member Eric Charles, a physicist at the Kavli Institute for Particle Astrophysics and Cosmology and the SLAC National Accelerator Laboratory at Stanford University in California, used the famous pulsar to produce a novel movie. He tracked both Vela's position relative to the center of the LAT’s field of view and the instrument's exposure of the pulsar during the first 51 months of Fermi’s mission, from Aug. 4, 2008, to Nov. 15, 2012.
The movie renders Vela’s position in a fisheye perspective, where the middle of the pattern corresponds to the central and most sensitive portion of the LAT’s field of view. The edge of the pattern is 90 degrees away from the center and well beyond what scientists regard as the effective limit of the LAT's vision.
The pulsar traces out a loopy, hypnotic pattern reminiscent of Art produced by the colored pens and spinning gears of a Spirograph, a children's toy that produces geometric patterns.
The pattern created in the Vela movie reflects numerous motions of the spacecraft. The first is Fermi's 95-minute orbit around Earth, but there's another, subtler motion related to it. The orbit itself also rotates, a phenomenon called precession. Similar to the wobble of an unsteady top, Fermi's orbital plane makes a slow circuit around Earth every 54 days.
In order to capture the entire sky every two orbits, scientists deliberately nod the LAT in a repeating pattern from one orbit to the next. It first looks north on one orbit, south on the next, and then north again. Every few weeks, the LAT deviates from this pattern to concentrate on particularly interesting targets, such as eruptions on the sun, brief but brilliant gamma-ray bursts associated with the birth of stellar-mass black holes, and outbursts from supermassive black holes in distant galaxies.
The Vela movie captures one other Fermi motion. The spacecraft rolls to keep the sun from shining on and warming up the LAT's radiators, which regulate its temperature by bleeding excess heat into space.
The braided loops and convoluted curves drawn by Vela hint at the complexity of removing these effects from the torrent of data Fermi returns, but that’s a challenge LAT scientists long ago proved they could meet. Still going strong after more than four years on the job, Fermi continues its mission to map the high-energy sky, which is now something everyone can envision as a celestial Spriograph traced by a pulsar pen.
tomatoherd wrote:Yes, if I understand correctly, that is sort of the perspective. As Nitpicker says, if the view of the satelite from 360 miles above the earth is somewhat more than half the sky, then the diameter of the pictured rays, from one magenta dot to the other side, would be, say, 210 to 240 degrees maybe. So the field is compressed, and the bright yellow gamma ray rim is the earth's horizon to the satelite, all around.
In this projection, the zenith is in the center of the image, and the nadir (the point on the Earth directly below the satellite) is the outermost ring. The nadir represents a kind of discontinuity or singularity. It's why the gamma ray density drops off as we move outwards in that outer ring- the physical angle being recorded is being mapped to a larger and larger area on the projection. The inside of the outer ring (seen as a red line) is the Earth's horizon, where a given angle of measurement covers the most physical surface of the Earth.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
workgazer wrote:
So plannets glow in gammer rays, could this be used to help find exo plannets?
Probably not.
The Earth's horizontal atmospheric ring is impressive here only because the telescope is in low Earth orbit.
From almost any distance in our own solar system the Earth's thin gamma ray ring would be unresolvable.
RJTONNIS wrote:
Why is the lower right brighter than the upper left?
The asymmetry in intensity between East and West is caused by the Earth's magnetic field.
<<A NASA-funded scientist has produced a new type of picture of the Earth from space, which complements the familiar image of our "blue marble". This new picture is the first detailed image of our planet radiating gamma rays, a type of light that is millions to billions of times more energetic than visible light. The image portrays how the Earth is constantly bombarded by particles from space. These particles, called cosmic rays, hit our atmosphere and produce the gamma-ray light high above the Earth. The atmosphere blocks harmful cosmic rays and other high-energy radiation from reaching us on the Earth's surface. "If our eyes could see high-energy gamma rays, this is what the Earth would look like from space," said Dr. Dirk Petry of NASA Goddard Space Flight Center in Greenbelt, Md. "Other planets -- most famously, Jupiter -- have a gamma-ray glow, but they are too far away from us to image in any detail."
Petry assembled this image from seven years of data from NASA's Compton Gamma-Ray Observatory, which was active from 1991 to 2000. The Compton Observatory orbited the Earth at an average altitude of about 420 km. From this distance, the Earth appears as a huge disk with an angular diameter of 140 degrees. The long exposure and close distance enabled Petry to produce a gamma-ray image of surprisingly high detail. "This is essentially a seven-year exposure," Petry said.
The gamma rays produced in the Earth's atmosphere were detected by Compton's EGRET instrument, short for Energetic Gamma-Ray Experiment Telescope. In fact, 60 percent of the gamma rays detected by EGRET were from Earth and not deep space. Although it makes a pretty image, local gamma-ray production interferes with observations of distant gamma-ray sources, such as black holes, pulsars, and supernova remnants.
Petry created this gamma-ray Earth image to better understand the impact of "local" cosmic-ray and gamma-ray interactions on an upcoming NASA mission called GLAST, the Gamma-ray Large Area Space Telescope. GLAST is planned for launch in 2007. Its main instrument, the Large Area Telescope, is essentially EGRET's successor. In 1972 and 1973 the NASA satellite SAS-II captured the first resolved image of the Earth in gamma rays, but the detectors had less exposure time (a few months) and worse energy resolution.
Petry, a member of the GLAST team at NASA Goddard, is an assistant research professor at the Joint Center for Astrophysics of the University of Maryland, Baltimore Country. A scientific paper describing his work is available at: http://xxx.lanl.gov/abs/astro-ph/0410487
Gamma rays are millions to trillions of times more energetic than visible light; and they span an energy range far wider than the familiar visible rainbow from red to violet. Visible light has an energy value of about 1.6 to 3.3 eV. Gamma rays are measured in MeV and GeV -- and sometimes TeV. The red image corresponds to gamma rays at 35-100 MeV; the green image corresponds to gamma rays at 100 MeV to 1 GeV; the blue image corresponds to gamma rays at 1-10 GeV; and the brownish image shows the full mix of energies.
Because the satellite was so close to the Earth, the image has an extreme wide-angle view as if one were taking a picture with a 35 mm photo camera equipped with an 8 mm fish-eye lens. The Earth's rim is much brighter than the center because cosmic rays hitting the rim at a shallow angle are more likely to create detectable gamma rays. The asymmetry in intensity between East and West is caused by the Earth's magnetic field.>>
Looking at the image, I have a question. The apparent thickness of the 'yellow' gamma level varies. It is thinnest at what would correspond to 10 o"clock on a clock face. Why? What does that observation reflect?
johnstbd wrote:Looking at the image, I have a question. The apparent thickness of the 'yellow' gamma level varies. It is thinnest at what would correspond to 10 o"clock on a clock face. Why? What does that observation reflect?
See the post just above yours by neufer.
Just call me "geck" because "zilla" is like a last name.
Gamma Ray Earth and Sky
