An astronaut aboard the International Space Station took this nighttime panorama while looking north across Pakistan’s Indus River valley. The port city of Karachi is the bright cluster of lights facing the Arabian Sea, which appears completely black. City lights and the dark color of dense agriculture closely track with the great curves of the Indus valley. For scale, the distance from Karachi to the foothills of the Himalaya Mountains is 1,160 kilometers (720 miles). 
 
 This photograph shows one of the few places on Earth where an international boundary can be seen at night. The winding border between Pakistan and India is lit by security lights that have a distinct orange tone. Another night image (click here) shows the border zone looking southeast from the Himalaya. A daylight view shows the vegetated bends of the Indus Valley winding through the otherwise desert country. 
 
 More than two millennia ago, Alexander the Great entered the Indus plains in 327 BCE from the northwest. He then spent many months leading his army and navy down the length of the Indus valley shown in this view. From near Karachi, he then began the desert march back to Mesopotamia (modern Iraq). By contrast, it takes the space station just three minutes to travel this distance. 
 
 Astronaut photograph ISS045-E-27869 was acquired on September 23, 2015, with a Nikon D4 digital camera using a 28 millimeter lens, and is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. The image was taken by a member of the Expedition 45 crew. The image has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab 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. Caption by M. Justin Wilkinson, Texas State University, Jacobs Contract at NASA-JSC. ||||| The India-Pakistan boundary appears as an orange line in this picture taken from the International Space Station, Sept. 23, 2015. NASA 
 
 A nighttime picture taken from the International Space Station of the boundary between India and Pakistan—visible because of a string of Indian floodlights erected in an effort to prevent militants from crossing into its territory–went viral this week. 
 
 The U.S. National Aeronautics and Space Administration said the frontier was one of only a few visible from space at night. Among the others are the dividing lines between South Africa and Zimbabwe and North and South Korea – both marked by sharp differences in the prevalence of electric lighting on the two sides. 
 
 In the case of estranged neighbors India and Pakistan, the Indian floodlights, which extend for more than 1,900 kilometers, limn a boundary that is disputed in places. Fences extend along much of the 3,323-kilometerfrontier as well. 
 
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 An undated photo of the India-Pakistan boundary showing floodlighting along the Indian side of the divide. Indian Ministry of Home Affairs 
 
 Indian security forces say they are trying to prevent militants from smuggling weapons, ammunition and fighters across the line from Pakistan into the Indian-controlled part of Kashmir, a region also claimed by Pakistan. India blames Pakistan for harboring terrorists who strike in India, something Pakistan denies. 
 
 In July, Indian security forces shot dead four suspected militants who had mounted a deadly assault on a police station in northern India after attacking a bus. India said the gunmen came from Pakistan. 
 
 India’s Home Ministry wants to replace the current floodlights with LED bulbs, which are more energy efficient and has launched a pilot project along the boundary in the state of Punjab using this type of lighting. 
 
 A nighttime panorama taken looking north across the Indus River valley from the International Space Station on Sept. 23, 2015. NASA 
 
 The distinct, bright light above the horizon is known as airglow, a phenomenon caused by the excitation of atoms and molecules high in the atmosphere by ultraviolet radiation from the sun, NASA said. 
 
 This wasn’t the first time a photograph of the boundary from space has been widely shared. This picture was taken from the International Space Station in 2011. 
 
 Clusters of yellow lights on the Indo-Gangetic Plain indicate numerous cities in this photographof northern India and northern Pakistan taken by an astronauton Aug. 21, 2011, from the International Space Station. NASA 
 
 An undated NASA image of the Earth at night. NASA
– An astronaut above the International Space Station has captured a stunning image of what NASA refers to as "one of the few places on Earth where an international boundary can be seen at night." The Sept. 23 image shows the glowing, snaking border between India and Pakistan, visible because of the Indian floodlights that dot much of the way. The Wall Street Journal notes two other such borders: the boundary lines between the Koreas and between South Africa and Zimbabwe. In those cases, a dearth of electric lights on one side makes clear the difference. The India and Pakistan boundary is visible because of lights that wind their way through nearly 1,200 miles (the full border is about 2,000 miles long). Their intended purpose is to discourage militants from crossing from Pakistan into the part of Kashmir controlled by India, per the Journal. The Economic Times in March reported that due to the huge cost of lighting the border—owing to the electricity used and the diesel generators stationed there as back-up—the country intends to replace the floodlights with LED bulbs in the coming years. A pilot program along the border in Punjab will test out the LED bulbs, which can have 50 times the lifespan of regular ones. A neat note from NASA regarding distances, time, and progress: It points out that in 327 BC Alexander the Great traveled through the Indus River valley, which is identified in the image. He entered from the northwest and exited near Karachi, at which point he headed back to what is now Iraq. The journey took him "many months"; the ISS covers the same distance in three minutes. (See an incredible picture of the moon that NASA recently captured.)