What it is: One of the last photos of Neptune taken by NASA’s Voyager 2 probe
Where it is: 2.8 billion miles (4.5 billion kilometers) of the sun
When was it recorded: 25 August 1989
When it was shared: 19 August 2024
Why it is so special: Only one space probe has so far visited the eighth planet, which is the most distant from the sun.
On 25 August 1989 Trip 2 The spacecraft took the first-ever close-up images of Neptune. This image—one of the last full-frame photos taken before the probe ended its “grand tour” of the planets—became one of the most famous, showing Neptune in a deep azure blue that shaped public perception of the planet for decades. (That is, until a reprocessing of the images from Voyager 2 earlier this year revealed Neptune’s true color to be a much lighter blue-green.)
The original images from Voyager 2 were taken in false color using filters, a standard technique used by planetary astronomers. In this case, blue and green filters were used, as well as a filter that allows light to pass at a wavelength that is absorbed by methane gas. According to scientistHydrogen and helium dominate Neptune’s atmosphere, but methane gives it its blue appearance by absorbing red light. The filters make methane appear dark blue in this image, but they also reveal a semi-transparent layer of haze above the planet. The bright red rim around Neptune is caused by haze scattering sunlight at higher altitudes above the bulk of the methane.
Related: New study suggests Uranus and Neptune are not what we thought
Voyager 2 took this photo almost exactly 12 years after its launch on a Titan-Centaur rocket from Cape Canaveral, Florida. After visiting Jupiter 1979, Saturn in 1981 and Uranus in 1985, Voyager came closest to Neptune on August 25, 1989. During the flyby, Voyager also visited two of Neptune’s moons, Triton and Nereid, and discovered six new moons and four rings.
Because Neptune is about 30 times farther from the sun than Earth, it receives only a fraction of Earth’s sunlight, meaning Voyager 2 had to take long-exposure images. So engineers fired the fast-flying spacecraft’s thrusters to spin them so the camera could stay focused.
Voyager 2’s images of Neptune were its last and were sent back as radio signals using 13-watt transmitters—about enough power to light a refrigerator bulb, according to Voyager 2. NASA – and took four hours to travel through the solar system to NASA’s Deep Space Network of radio antennas around the world.
Neptune was Voyager 2’s last stop before traveling to the edge of the solar system. The probe reached interstellar space on November 5, 2018. Voyager 2 remains NASA’s longest-running mission, even after encounter some communication problems last summer.
You can find more cool space photos in our Space Photo of the Week Archive. New stories are posted every Sunday.
