by Cindy Evans & Will Stefanov • design by Robert Simmon • April 22, 2008

To an observer in space,
humanity’s footprints on the surface of the Earth are large and varied.
They include the regular patterns of irrigated cropland, straight lines
of roads and railways running across continents, reservoirs on river
systems, and the cement rectangles of ports and seawalls along
coastlines. But what about humanity’s signature footprint—cities? By
day, cities viewed from space can blend into the countryside, or appear
as gray smudges, depending on the style of development and size of the
urban area.

Looking east
from a location southwest of Ireland, an astronaut took this nighttime
panorama of population centers in Ireland and the United Kingdom. Image
ISS016-E-27034 was taken on February 1, 2008, using a 28 mm lens.

At night however, city
lights present the space observer spectacular evidence of our
existence, our distribution, and our ability to change our environment.

A few years ago, NASA and NOAA joined forces to present the first world map of the nighttime Earth using 9 months of data
collected by the DMSP (Defense Meteorological Satellite Program)
satellite from an altitude of 830 kilometers (1 kilometer is 0.62
miles) above Earth. That “Night Lights” map, widely distributed on the
Internet, helped many people visualize the world’s distribution of
people and cities.

Astronauts circling the Earth have the wonderful vantage point of
observing the nighttime Earth from 350-400 kilometers above the
surface, taking in whole regions at once. Onboard cameras and a bit of
experimentation allow astronauts to take highly detailed images of our
cities at night and share them with the rest of us.

Chicago,
Illinois, is home to roughly three million people, but the wider
metropolitan area includes nearly 10 million. By day (top), the
cement-colored urban center of the city blends almost imperceptibly
into the gray-green colors of suburbs and then farmland. By night
(lower), the region’s ten million people cannot be missed. ISS007-E-16747 (top) was taken on October 8, 2003, with a 50 mm lens. ISS007-E-16525 (bottom) was taken on October 7, 2003, with a 50 mm lens.

But taking pictures in
the dark is difficult at best, made even more difficult by the fact
that the International Space Station moves more than 7 kilometers per
second (15,659 miles per hour) relative to Earth’s surface. With
daylight illumination, an onboard camera can be set to take an image
with an exposure time of just 1/500 of a second. With the Earth’s
surface in darkness, night images of cities require much longer
exposure times. As the Space Station (or Space Shuttle) flies over
Earth, however, the city the astronaut is trying to photograph will
move across the camera’s field of view while the shutter is open—a
recipe for blurry images. The longer the exposure, the more motion
blurring there will be.

Orange sodium
vapor lights illuminate the port facilities of Long Beach, California,
supporting the round-the-clock operations of one of the world’s busiest
commercial cargo ports. Image ISS016-E-27162 was taken on February 4, 2008, using the 400 mm lens, providing superior resolution.

In late 2002 and early 2003, astronaut Don Pettit, part of International Space Station Expedition 6, spent some time accumulating spare parts from around the space station, and constructed a device called a barn-door tracker.
A barn-door tracker is a camera mount commonly used by astronomers and
photographers on the ground to capture images of stars and planets in
the night sky. The camera is mounted on a hinged platform that can be
moved very slowly and precisely (by turning a knob). On the ground, the
device allows photographers to compensate for the rotation of the Earth
relative to the stars. In space, it allows astronauts to compensate for
the movement of the Space Station relative to the Earth below. The
careful coordination keeps the targeted city in the same position in
the camera’s field of view during the long exposure, even though both
the station and Earth’s surface are moving.

Pettit’s tracker and nighttime photography techniques produced
hundreds of images of cities from around the world that had estimated
resolutions (level of detail) of about 60 meters. Since then, a few
other crew members have been able to successfully master night
photography techniques. In late 2007 through early 2008, Expedition 16
Flight Engineer Dan Tani acquired a number of striking images of cities
at night, including some exciting images taken with the longer, 400 mm
lens, producing images with an estimated ground resolution of less than
10 meters.

Don Pettit built and
installed a “barn-door tracker” to enable him to take nighttime
photographs from the International Space Station. Astronauts use the
tracker to keep a camera steady during long exposures while the station
moves above the Earth’s surface. Image ISS006-E-44299 was taken on
April 5, 2003.

Recently, Don Pettit assembled a
sequence of several of the most striking images of city lights at night
into an animated “world tour” [high-resolution (126 MB MPEG), web-resolution (39 MB QuickTime)] of cities at night (script).
This video, produced entirely by Pettit, takes you on a quick trip
comparing cities from different regions, all viewed from the
International Space Station.

From a geographic perspective, cities at night tell different
stories about a region. City lights provide sharp boundaries that
delineate the densest concentrations of people, a characteristic that
has been used to assess the effect of urbanization on Earth’s ecosystems.
The increased detail of city lights available from astronaut
photography can help refine urban boundaries defined from satellite
data. Transportation corridors and major commercial development, such
as ports, shopping centers, and cultural icons—like the Las Vegas
strip—jump out of the landscape.

Dan Tani,
recently aboard the International Space Station as a member of
Expedition 16, extended Don Pettit’s techniques for photographing city
lights. He has taken nighttime photographs with a resolution of better
than 10 meters (about the length of a bus) using a 400 mm lens. Image ISS016-E-026454 was taken on January 30, 2008.

In many cities, neighborhoods of different generations
can be distinguished by the lighting color and patterns along their
streets. In many North American cities, older neighborhoods have less
regular street patterns and light green mercury vapor lighting, while
newer cities, especially in the western United States, have street
patterns aligned to the compass directions and use orange sodium vapor
lighting. The major Denver street patterns are rectilinear, aligned
north-south and east-west.

The “Vegas Strip” of
casinos and hotels—reputed to be the brightest spot on Earth—stands out
in the center of this image due to both its brightness and its
diversity of light colors. Image ISS016-E-27168 was taken on February 4, 2008, using the 400 mm lens.

Cities from different
regions of the Earth are also identified by differences in their
nighttime lights. Japanese cities glow a cooler blue-green than other
regions of the world. Newer developments along the shore of Tokyo Bay
are characterized by orange sodium vapor lamps, while the majority of
the urban area has light green mercury vapor lamps.

The streets of Denver, Colorado, are aligned with the cardinal directions. Image ISS016-E-26150 was taken on January 31, 2008, with an 85 mm lens.

Border cities like Ciudad
Juaréz, Mexico, and El Paso, Texas, illustrate different city patterns
side-by-side, suggesting cultural influences on the development and
growth of cities and infrastructure. Ciudad Juaréz, supports at least
1,300,000 people. On the U.S. side of the Rio Grande, El Paso is marked
by the brightly-lit Interstate Highway I-10 that cuts across the city.
Although the area of El Paso, with an estimated population of slightly
more than 600,000 is roughly on the order of the area of built-up
Ciudad Juaréz, the density of settlement evidenced by the distribution
of lights, is much less.

Like many
Japanese cities, the night lights of Tokyo, Japan, have a blue-green
glow that comes from mercury vapor lighting. Image ISS016-E-27586 was taken on February 5, 2008.

The rapid growth in
Jiddah and Mecca in Saudi Arabia can be mapped from the lighting
patterns, and the road connecting them stands out as a bright string in
the surrounding dark desert.

More densely populated Ciudad Juaréz, Mexico, is separated from El Paso, Texas, by the Rio Grande. Image ISS006-E-44123 was taken on April 7, 2003, with an 85 mm lens.

What’s next? Earth is
becoming an urban planet. As more and more people move to cities, and
the surrounding rural and suburban areas are increasingly developed,
the pattern of lights in cities around the world will change.
Individual city footprints will coalesce into ever larger bright blobs.
More roads will connect those cities to form an illuminated, lace-like
web on the habitable parts of the continents. Nighttime photographs
from astronauts on upcoming missions will document these changes,
providing dramatic illustrations for the continuing story of humanity’s
footprints on the Earth.

Astronaut photographs are provided by the ISS Crew Earth
Observations experiment, and the Image Science & Analysis
Laboratory, Johnson Space Center. The images in this article have been
cropped and enhanced to improve contrast. Lens artifacts have been
removed. The International Space Station Program
supports the laboratory to help astronauts take pictures of Earth that
will be of the greatest value to scientists and the public, and to make
those images freely available on the Internet. Additional images taken
by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Text and captions by Cynthia A. Evans and William L. Stefanov, NASA-JSC.