What NASA's images of Earth show and how they were taken

Contents

On Friday, NASA released the first photos of the entire Earth taken from the Orion capsule of the Artemis 2 mission. Since then, the content and technology of the images have been widely discussed on social networks. Not only conspiracy theorists, but also photography enthusiasts are amazed by the bright image that NASA has named “Hello World”.

It shows the Earth in backlight from the sun, and in contrast to usual views, it is practically upside down: the North Pole is at the bottom left, the South Pole at the top right. Because it was taken with very high camera sensitivity and a long exposure time – more on the technology shortly – the Earth appears brightly lit. And that, even though the image shows the night side. This makes the auroras clearly visible, as well as the artificial lighting of large cities and the zodiacal light, which the sun generates. NASA has published the image here in full resolution as a JPEG with EXIF data.

One image, two exposure settings

The only light source for the Earth in the shot is moonlight, which was quite strong at the time of the image, April 3rd, shortly after the full moon on April 2nd. Also on the southern hemisphere, which “Hello, World” predominantly shows: Bottom left is Africa, below it the Iberian Peninsula, and at the right edge of the photo, parts of western South America can still be recognized. To illustrate that it is an intentionally composed shot of the Earth's night side, NASA has also released the same perspective with different camera settings. Metropolitan areas on the coasts of Africa and South America appear as bright points of light there.

The stars, which are otherwise missing from many Earth photos from space – including the famous “Blue Marble” from Apollo 17 – are clearly visible in "Hello World", even certain constellations are recognizable. This is explained by the large aperture of f/4.0 chosen by the photographer. He is the commander of the Artemis 2 mission, Reid Wiseman. Like all NASA astronauts, he was trained in the proper use of the cameras on board, as he demonstrated, among other things, in a Facebook post with a video.

Nikon D5 due to enormous dynamic range

The camera used to take the pictures is a proven professional workhorse: the digital full-frame single-lens reflex Nikon D5. It was launched almost exactly ten years ago and has a low resolution of 20.8 megapixels by today's standards. But: Its sensitivity (ISO) can be boosted up to 3,280,000, and even at lower sensitivities, it is characterized by a very high dynamic range – exactly what is needed for astrophotography. Reid Wiseman also praised the camera's low-light capabilities in his video. “Hello World” was taken with ISO 51,200, a setting at which even the most modern cameras do not deliver such clear images. Whether the published image, which comes from Adobe's Lightroom Classic 15.2.1, has been heavily denoised cannot be determined with certainty. ISO 51,200 still showed acceptable images with visible, but not overly disturbing noise in our test of the Nikon D5.

Another characteristic can be determined by visual inspection, namely that the image was probably not composed of multiple shots (compositing) or corrected for distortion. The former is very unlikely because the noise looks natural, as from a sensor at high ISO values. The focal length of 22 millimeters, according to EXIF data, suggests that no distortion correction was applied. This falls within the mid-range of typical DSLR optics such as the Nikkor 16–35 mm 1:4G ED VR or the Nikkor 14–24 mm 1:2.8G ED. Unlike extreme wide-angle lenses, this does not cause particularly strong distortions. NASA has not yet published which lens was used to take “Hello, World”. Each camera is supposed to have two lenses on board.

Little motion blur possible

This leaves the question of motion blur, which according to some commentators must undoubtedly occur at an exposure time of a quarter of a second. However, this is not the case, because, from a perspective point of view, the Orion capsule is moving away from the Earth almost linearly – even if the path appears as an arc when depicted two-dimensionally. Furthermore, the Earth does not rotate so fast that its depiction as a whole would cause significant blurring at 1/4s. In the case of a partial view of the night sky, as seen from Earth, the Earth's rotation is combined with the movements of stars and planets, which causes more blurring.

On board Artemis 2, in addition to two Nikon D5s, there is also a unit of the current, mirrorless Nikon flagship Z9. As already reported, NASA has been working for some time to convert this camera into a “Handheld Universal Lunar Camera” (HULC). Among other things, with a heating blanket and a simplified menu. According to Reid Wiseman, the Z9 is also a “great device”, but it is now primarily on board for radiation testing. In addition to the three system cameras, the astronauts also have their personal iPhones for photography.

(nie)