New Horizons is a long, long, long road from home.
Fifteen years after it was launched from Earth with a record speed, and six years since it became the first spacecraft ever to fly through Pluto, NASA’s New Horizons are about to reach a mile that only four others surpassed robot sins in history.
On Saturday (April 17) at 20:42 EDT (1242 GMT April 18) New Horizons will reach 50 AU (astronomical units) of the sun – or 50 times the distance the earth is from the sun. This is 7.5 billion kilometers. At 50 AU, it takes more than 6.5 hours for signals sent from New Horizons to reach Earth, and that is while traveling at the speed of light.
“I’m just thinking about the extent of it,” Alan Stern, lead researcher at New Horizons at the Southwest Research Institute in Boulder, Colorado, said in an interview with collectSPACE. “It has not been done for a generation, as the Voyagers have crossed these distances, and we are the only spacecraft out there in the outer heliosphere and the Kuiper belt.”
New Horizons is the fifth spacecraft from Earth.
Launched in 1972, Pioneer 10 was the first probe to move through the asteroid belt and fly through Jupiter, reaching 50 AU on September 22, 1990. Today it is approximately 129 AU from Earth.
His sister ship, Pioneer 11, reached AU50 a year later in 19950. It was launched in 1973 and, in addition to flying through Jupiter, was the first to make direct observations of Saturn. It is now about 105 AU from Earth.
NASA launched Voyager 1 on September 5, 1977, 16 days after its twin, Voyager 2. Voyager 1 studied Jupiter and Saturn, while Voyager 2 also encountered Uranus and Neptune. Voyager 1 is 152 AU from Earth today. Voyager 2 is at 127 AU. While Pioneer 10 and Pioneer were discontinued 11 years ago, both Voyagers remain active today.
The Pioneers and the Voyagers are so far away today that none of them are the closest investigation to New Horizons. NASA’s Juno spacecraft, in orbit around Jupiter, is approaching at this point.
“In the very distant future, we will be so far away from everyone that we will be closer to the Voyagers and the Pioneers, but we will never pass them by, because three of the four are going faster than us,” Stern said. “Right now we’re almost 100 AU from Voyager 1.”
To emphasize how far Voyager 1 traveled, NASA aimed the probe’s camera at the inner solar system in 1990 when it was approximately 40.11 AU from Earth. The resulting mosaic image, now known as the “Family Portrait”, captured six planets – Venus, Earth, Jupiter, Saturn, Neptune and Uranus – as just a few pixels each.
At 50 AU from the sun, New Horizons could not do the same.
“Mathematics tells us that our camera would burn out because we were facing the sun,” Stern said, noting that even at such a great distance the sun remains too bright for its long-range reconnaissance image, which is calibrated for the dimly lit encounter with Pluto. “So we do not want to do that until we are past the years of the Kuiper Belt.”
Instead, Stern and his team pointed New Horizons in the direction of Voyager 1, noting for the first time that a spacecraft in the Kuiper belt photographed the location of an even more distant spacecraft now traveling through interstellar space.
“Of course we did not see Voyager 1 because it is too faint, but we pictured the star field,” Stern told collectSPACE. “We looked with the camera at where the farthest spacecraft is and took a photo of the star field from our position in the Kuiper Belt. It’s just ghostly beautiful to me, even if it’s just a photo of stars.”
“It’s a tribute to Voyager’s groundbreaking mission, apart from noticing what we’re doing,” he said.
More than just a round milestone, reaching 50 AU means that everything New Horizons is doing now exceeds the planned lifespan of the design.
“One of the first things you do when designing a spacecraft is the requirements and one we had to set was the maximum distance we had to design the spacecraft to ride on,” Stern said. “Now you always build in margins so you can do better, but we had to have a number, so if we crossed the finish line, we could declare victory – that the spacecraft has achieved its design goals.”
“The finish line was 50 AU,” Stern said.
New Horizons flew through Pluto and returned the first glimpse of the world and its moons in July 2015, when the spacecraft was 39.2 AU from the sun. Four years later on New Year’s Day 2019, New Horizons made the farthest flies in history (so far) and made the first observations of a Kuiper Belt object (‘Arrokoth’) at a distance of 43.4 AU from the sun taken.
“We’re still getting data back from the fly,” Stern said. “As we fly over the Kuiper Belt, we do three other things: we study the helioferic environment, the plasma, the dust and the gas; we study other objects of the Kuiper Belt, we know that they are more than 30 what we in ways you can not see from Earth or from any other spacecraft, and we use the Subaru Telescope in Hawaii, which is one of the largest telescopes in the world, to find new objects from the Kuiper Belt to to study and hope we find a flight target, because we still have fuel in the tank and are able to do another fly. ‘
The hope is to find another target before New Horizons loses power. Although it draws electricity from a nuclear battery (a radioisotope thermoelectric generator, or RTG), the plutonium power supply produces 33 watts less every decade. By the end of the 2030s, when New Horizons will be at or near 100 AU of the sun, it may be too little to work.
Even though New Horizons did not reach 100 AU, Stern was impressed with how far the mission went and even more with how much his team was able to succeed.
“When the Voyagers flew, they were 450 people. New Horizons does this at about 50 belly buttons, so about ten times smaller,” he said.
‘If I think about what our team has accomplished during these 15 years with one spacecraft and no backup, to go study Pluto for the first time, then the Kuiper belt is for the first time and now passes the 50 AU mark it’s designed to be its maximum distance, it sounds to me just like science fiction, ‘Stern said. “I have to convince myself that this group of people could actually do this thing, it’s so much bigger than life.”