Illustration: NASA

Why space probe Juno took photos of Earth and the Moon

Tuesday 07 Apr 15


John Leif Jørgensen
Professor and Head of Measurement and Instrumentation
DTU Space
+45 45 25 34 48

The Juno Mission

  • Space probe Juno was launched by NASA in 2011.
  • Its final destination is Jupiter, the largest planet in our solar system.
  • Juno is expected to enter orbit around Jupiter in July 2016.
    For the probe to travel all the way to Jupiter as ‘cheaply’ as possible, researchers planned what is known as a fly-by. This meant that Juno was first to travel to Mars, circumnavigate the planet and then fly back to Earth to utilize Earth’s gravitational field to boost its acceleration.
  • Juno completed its fly-by on 9 October 2013.
  • The objectives of the Juno mission include increasing our knowledge of Jupiter’s magnetic and gravitational fields, its weather systems and the water content of its atmosphere—all knowledge that should expand our understanding of the creation and early development of our solar system.
  • To watch a video of Juno’s fly-by, go to:

Star cameras developed and hand-built by DTU Space supplied NASA with unique footage of Earth and the Moon seen from space. However, the Danish scientists had an ulterior motive with the photo shoot.

“This is the most gorgeous clip I have ever seen. God Speed, Juno! Hanx.”

That was the Tweet American actor Tom Hanks sent on 12 December 2013 after watching NASA’s short film of the Moon orbiting planet Earth.

The footage had been shot by two of the four hand-built DTU star cameras fitted to NASA’s space probe Juno. Juno’s final destination is Jupiter, but to penetrate that deeply into space, Juno was initially sent on a return trip to Mars, passing Earth once more in October 2013 to exploit the planet’s gravitational field for acceleration. The unique images were captured as the probe was travelling back towards Earth, providing the first footage of Earth and the Moon in motion as viewed from space.

NASA honours DTU researchers
The cameras on Juno started taking pictures every five minutes when the probe was four million kilometres away from Earth, and continued shooting for four days. Significant editing was required to merge all the images into a coherent film, a process handled by David Pedersen and Andreas Jørgensen, two PhD students at DTU Space

When the clip was posted on NASA’s website and set to music by Vangelis, it quickly went viral and has already become one of the most viewed clips in NASA’s history. It also garnered high praise for Professor John Leif Jørgensen and his colleagues in the Measurement and Instrumentation division at DTU Space, which supplied the star cameras for Juno. In September 2014, Professor John Leif Jørgensen travelled to the United States to accept the NASA Group Achievement Award on behalf of the team, in recognition of their outstanding achievement, which made a significant contribution to NASA’s mission. In addition, NASA has presented Professor John Leif Jørgensen with its prestigious ‘Exceptional Public Achievement Medal’.

New navigation method to be tested
Although delighted with the honours from NASA and the spectacular footage of the Earth and Moon, the professor even more excited by another aspect of the mission. Having the star cameras take photos in space gave him and his research team the opportunity to test a new form of navigation that may revolutionize space travel in future.

Simply put, the new method involves using triangulation to pinpoint the exact position of a vessel in space. Triangulation is already a popular method on Earth, where it is used for a variety of purposes including surveillance, navigation, and metrology. This approach makes it possible to locate an object by measuring angles to previously established points. In space, these ‘established points’ will be planets, the Earth and Moon, whose exact positions are already known—making them ideally suited to calculating the position of a spacecraft.

“The Juno mission presented us with a unique opportunity to test this navigation method using the star cameras on the spaceship. The whole idea behind recording the footage was to capture Earth and the Moon in the same shot in order to test the method,” explains Professor John Leif Jørgensen.

Montage: NASA, JPL, Caltach, MSSS
The pictures taken by the space probe Juno at a distance of 12,000 km allow us to see both Earth and the Moon from the perspective of space for the first time. 

NASA took some convincing
When DTU researchers originally aired the idea of testing this navigation method to NASA, they met a wall of resistance and were shown the door with a long list of reasons why it should not be attempted:

For the star cameras to capture both Earth and the Moon in the same shot, the space probe boosters would have to rotate the probe slightly in relation to its direction of travel. This would involve exposing a part of the side of the probe that was sheltered from the sun—and which was vulnerable to the immense heat radiation. The probe is designed to provide optimal performance at -180°C—the temperature in space close to Jupiter—so only the ‘Sun side’ is coated in a material to protect it from the effects of solar radiation.

In addition, Juno’s flight around the Earth—known as a ‘fly-by’—was originally set to last just a few hours, with the associated staffing of the control consoles. Starting a photographic assignment from a distance of 4 million km out in space would require staffing the consoles 24/7 for a week, along with back-up to guard against absence due to illness.

A scientific dream
Back home in Denmark, John Leif Jørgensen and his colleagues put on their thinking caps: How could they get NASA on board with the idea? The solution immediately presented itself when the scientists began to talk about how amazing it would look to approach Earth and the Moon from space. They created an animated film of the fly-by to visualize the film potential—and this time NASA needed no further convincing.

“It all started with a scientific dream of having the chance to test our idea for a new navigation method and the associated equipment—namely the star cameras that were going to be on board anyway. And we succeeded. In addition, we managed to capture the first ever footage from a spacecraft approaching Earth and the Moon,” says Professor John Leif Jørgensen.

“We subsequently found out that the bill for the photo shoot ran to more than a million dollars,” he adds. 

Edited article from DYNAMO no. 40, DTU's quarterly magazine in Danish.

The star camera

Stellar compasses are used to assist with the orientation of satellites and space probes, so scientists always know precisely which way they are facing in space.

A stellar compass consists of two main parts: a digital camera that photographs the night sky and a computer that matches the digital images against a stellar map stored in the computer. By comparing images of the night sky with the stellar map, the stellar compass is able to determine which direction it—and thus the satellite or probe—is facing.

The stellar compass was originally developed at DTU for the Danish Ørsted satellite, which was launched in 1999. However, the star cameras had been tested previously—in 1996—when they were installed on the NASA probe Thunderstorm that was used to test new technology in space. DTU Space has since continued to develop the star compass, making it increasingly compact and sophisticated. Star cameras have been used on more than 50 international missions to date.