While it looks a lot like the photo taken by Apollo astronauts,
you may be surprised to know that this popular image of Earth
is not actually a snapshot. Robert Simmon, who worked on this for NASA's
Earth Observatory, was kind enough to answer a few quick questions about this
image from the Blue
KH: What's different from the picture that the astronauts did take?
RS: Instead of being a snapshot, this picture is a composite: one month of
images taken from a satellite were combined together into a single map.
KH: How did you create that image?
RS: I used a 3D program (Electric Image) to render out separate "layers" (surface,
clouds, fake atmosphere, and a fake specular highlight (the shiny spot)). I then
combined the separate layers in Photoshop, which gave me individual control over
opacity, brightness, contrast, color, and saturation. Source imagery and the
high-resolution final results are here: http://earthobservatory.nasa.gov/Newsroom/BlueMarble/BlueMarble_2002.html
KH: How much science goes on before you do your part?
RS: Lots! The raw data--collected as individual pixels from 440 miles above
the Earth--need to be processed to account for everything from the curvature
of the Earth to influence of the atmosphere and optical properties of the instrument.
These are then stored in a data archive as thousands of individual images,
called scenes. Literally hundreds of NASA workers are involved in this part
of the process, which is automated and goes on continuously, day and night
(about a terabyte a day of data are processed for a single instrument). A colleague
of mine, Reto Stckli, then processed one season's worth of these scenes to
remove clouds, snow, and other things blocking the satellite's view of the
surface, and built a single composite image of the surface. He also made an
image of the clouds from two days of data. We've subsequently developed a series
of even more detailed maps for each month of 2004: http://earthobservatory.nasa.gov/Newsroom/BlueMarble/BlueMarble_monthlies.html
KH: Why are clouds hard to get right?
RS: The hardest part (for me) of creating the image was getting the cloud
to look realistic. The color and brightness of a real cloud is due to a combination
of its thickness and the angle of the surface to the sun. Unfortunately our
cloud map is only a two-dimensional approximation of the cloud's opacity. It
took many hours of experimenting with brightness curves and layer transfer
modes in Photoshop to get clouds that looked right, and I'm still not happy
with the final result.
KH: Why didn't you get credit for it?
RS: I work (through a contracting company) for NASA--part of the federal government,
so the image is in the public domain.
KH: What does it mean that the image is in the public domain?
RS: Anyone can use it however they like, with the exception of using an image
form NASA to imply that NASA endorses your product or service.
KH: What other images have you worked on?
RS: The most widely-seen is probably an image of the Earth's
city lights at night.
I also create images on a daily basis for the NASA
For more information on the Blue Marble, visit the History of Blue Marble: BlueMarble_2002 and BlueMarble_history.