As the New Horizons spacecraft hurtles out towards interstellar space, it has now reached an historical milestone. On April 17, 2021, New Horizons passed 50 astronomical units, or 50 times Earth’s distance from the Sun. It is just the 5th spacecraft to reach that distance, joining the Voyagers 1 and 2 and the Pioneers 10 and 11.
“Although four other missions reached this distance back in the 20th Century, none was in perfect health, but New Horizons is,” said New Horizon’s principal investigator Alan Stern, on Twitter. “This is an amazing testament to the skill, care, and attention to detail of those who designed and built New Horizons and those who have been its flight crew now for over 15 years.”
This summer, it will be six years since New Horizons made its flyby of Pluto and its system of Moons in July of 2015.
To celebrate the achievement of reaching this new distance marker, scientists sent instructions a few months ago to New Horizons to attempt to image the location of another deep space traveler, Voyager 1, which is now in interstellar space. Although Voyager 1 is far too faint to be seen directly in the image, its location is known precisely due to NASA’s radio tracking.
“That’s a hauntingly beautiful image to me,” said Stern.
Converting the AU scale to one we are more familiar with, New Horizons is now almost 5 billion miles (7.5 billion kilometers) away. This means communicating with the spacecraft takes a lot of time
At the time of the Pluto flyby, two-way communication between New Horizons and Earth required a nine-hour round trip — 4.5 hours to the spacecraft and another 4.5 back. Since radio signals travel at light speed (186,000 miles per second, 300,000 km per second), this exemplifies Pluto’s great distance from Earth, nearly three billion miles (4 billion km). And at its current distance, signals take 7 hours to reach the far flung spacecraft, and another 7 hours before its control team on Earth finds out if the message was received.
“Working with a spacecraft so far away is a challenge,” said Alice Bowman told me in 2016, for my book “Incredible Stories From Space.” Bowman is New Horizons’ mission operations manager at the Johns Hopkins Applied Physics Laboratory, where New Horizons was built and is operated. “I always say you need to have a split personality when you work in ops (Mission Operations) because of all the variations in time. When you send a real-time command from Earth, you have to know where the spacecraft will be in the future.”
The New Horizons team provided another way to imagine just how far 50 AU is: Think of the solar system laid out on a neighborhood street; the Sun is one house to the left of “home” (or Earth), Mars would be the next house to the right, and Jupiter would be just four houses to the right. New Horizons would now be 50 houses down the street, 17 houses beyond Pluto.
New Horizons is by no means finished with its mission. After the Pluto flyby, the spacecraft took the first close-up look at a Kuiper Belt object (KBO) with its flight past Arrokoth on New Year’s day 2019. From its unique perch in the Kuiper Belt, New Horizons is making observations that can’t be made from anywhere else even the stars look different from the spacecraft’s point of view.
New Horizons has been collecting data on the solar wind and space environment in the Kuiper Belt, and scanning for other Kuiper Belt objects, with an eye towards visiting one that shows up along the way, “within fuel reach.” Stern said on Twitter. This summer, the mission team will transmit a software upgrade to boost New Horizons’ scientific capabilities. For future exploration, the spacecraft’s nuclear battery should provide enough power to keep New Horizons operating until the late-2030s.
Further reading and more images: JHUAPL