Image credit: Scott Ehardt, Wikimedia Commons; image source; larger image

How does GPS work? - Sep 1, 2011

The photo shows a Global Positioning System satellite. To find out how the system works, visit How does GPS work?, and be sure to see the video to understand how three or four different GPS satellites specify your position on Earth. For more, check out this Beyond Discovery page from the National Academy of Sciences.

image credit: Wolfgang Beyer (Wikimedia Commons); image source; larger image

Benoît Mandelbrot, Novel Mathematician, Dies at 85 - Aug 2, 2011

This famous fractal is the Mandelbrot set. Click to make the image larger, and look at the boundary between the black and blue--it is made up of the larger image, at smaller and smaller scales. Check out this Fractal Geometry page from IBM, especially the video (scroll down) that zooms in on the Mandelbrot set.

The image is named for Benoît B. Mandelbrot, who made it by graphing a set of complex numbers described by an equation. He coined the term "fractal" and popularized fractal research by showing its importance to other fields besides mathematics. Read more about his life and accomplishments at Benoît Mandelbrot, Novel Mathematician, Dies at 85.

Smaller image credit: NASA; smaller image source; larger image credit: STS-41B, NASA; larger image source

Footloose - Jul 1, 2011

In 1984, astronaut Bruce McCandless made this untethered spacewalk--the first ever, and one of only a few. He maneuvered with a "jet pack" strapped to his body as he orbited Earth at about 18,000 miles an hour. Click on the image to see McCandless at his maximum distance from the shuttle.

Whenever the jet pack's engines were off, McCandless was in free fall--the only force on him was Earth's gravity (neglecting air resistance and the gravitational attraction of the shuttle). To learn more, visit Footloose and this APOD page.

image credit: Paul Bourke, University of Western Australia; larger image

Chaos Theory: A Brief Introduction - Jun 8, 2011

This image shows the "Lorentz attractor," a graph that represents the behavior of a simple model of Earth's weather. Weather is just one example of a chaotic system, in which seemingly irregular behavior does follow certain patterns.

For an overview of chaos theory that concisely explains the characteristics of chaotic systems, see Chaos Theory: A Brief Introduction.

image credit: NRC file photo; image source; larger image

Mechanics of a Meltdown Explained - May 12, 2011

The long red tubes are zirconium-alloy-clad fuel rods being fastened together into large bundles that will form the core of a nuclear reactor. Inside the zirconium cylinders are stacked pellets of uranium oxide, the reactor fuel.

To find out what happens to the zirconium cladding and the fuel rods in a "nuclear meltdown", visit Mechanics of a Meltdown Explained. The article explains the problems faced by the Fukushima power plant after the March 2011 earthquake in Japan.

image credit: NASA; larger image

Not Just Another Old Flame - Apr 12, 2011

The photo shows two flames, one on Earth and one in space, with both flames on identical candles.

For a NASA website on flames in space, see Not Just Another Old Flame.

image credit : NASA/CXC/SAO/H.Marshall et al.; image source; larger image

Chandra Observes Cosmic Traffic Pile-Up In Energetic Quasar Jet - Feb 1, 2011

This X-ray image of 3C273 shows a jet of energy shooting out of the quasar's bright center, thought to be home to a supermassive black hole. If you look closely you can see a small thread connecting the center to the bright spots of the jet. Scientists have observed that matter from that small thread moves very fast, then appears to slow down in the luminous part of the jet, akin to a "cosmic traffic pile-up" of matter. For more details, see Chandra Observes Cosmic Traffic Pile-Up In Energetic Quasar Jet.

The Doppler Effect and Sonic Booms - Dec 1, 2010

This image of the shockwave made by a speeding bullet is a shadowgraph--simply a photograph of the shadow of the bullet and the shockwave. Variations in the density of air refract the light used to make the shadow and produce bright and dark regions.

- Note the little fragment that is moving just below the speed of sound (the disturbance is slightly ahead of it).
- To see animations of shockwaves made by objects moving at various speeds, visit The Doppler Effect and Sonic Booms.

image credit: Fatima Toor; image source; larger image

Quantum Cascade Lasers - Nov 1, 2010

The image above shows a quantum cascade laser captured by a camera that images infrared light. The laser light is the small dot in the middle of the round window. The laser itself--behind the window--is kept at a temperature of -193° C, just above the temperature of liquid nitrogen.

Quantum cascade lasers work like this:
- In a thin slice of a semiconductor, an electron can occupy discrete energy levels.
- When an electron moves from a higher level to a lower level, a photon is emitted.
- If many thin slices of semiconductor are stacked together, the electron can cascade from one to another, emitting a photon of the same frequency in each slice. These photons form the laser beam.
- To learn more, see Quantum Cascade Lasers.

image credit: A. Hildebrand, M. Pilkington, and M. Connors; image source; larger image

Do We Know What Killed the Dinosaurs? - Oct 16, 2010

In the image above you can see two concentric circles. These mark the location of the Chicxulub crater, and the white line shows the coast of the Gulf of Mexico. See this Canadian site (scroll down to the third image) for a fuller explanation of how the image was made and for related images.

- The Chicxulub crater is a candidate for the asteroid impact that killed the dinosaurs, but not all scientists are convinced. Some believe the iridium layer (pictured at left) could also have been produced by volcanic eruptions at the same time as the layer was created.  

- For more about the Cretaceous extinction and the debate over what caused it, see these articles from the University of California Museum of Paleontology and the Geological Society of America.

(This feature was updated on July 13, 2012.)