NASA's New Horizons spacecraft flew by Pluto seven years ago, but the probe's work is far from done.
New Horizons is still on duty in extended mission mode, diving ever deeper into the Kuiper Belt to examine ancient, icy mini-worlds in that vast region beyond the orbit of Neptune.
New Horizons launched in January 2006 and carried out a reconnaissance study of Pluto and its moons in the summer of 2015, culminating in a close flyby of the dwarf planet on July 14, 2015. That encounter revealed Pluto to be an incredibly diverse world, complete with towering water-ice mountains and huge plains of exotic nitrogen ice.
But the nuclear-powered probe kept its eyes open even after Pluto was in the rear-view mirror.
New Horizons next flew by Arrokoth, a small Kuiper Belt object (KBO), on Jan. 1, 2019. Arrokoth, which the New Horizons science team discovered in 2014 using the Hubble Space Telescope, is the most distant and most primitive object ever explored up close by a spacecraft.
And there could be another flyby in New Horizons' future as well.
At a meeting of NASA's Outer Planets Assessment Group (OPAG) in June, New Horizons principal investigator Alan Stern, of the Southwest Research Institute (SwRI) in Colorado, related that both the spacecraft and its scientific payload are entirely healthy. The probe's lifetime is presently limited only by its nuclear fuel supply, which is likely sufficient to keep New Horizons flying through 2040.
And NASA recently granted another mission extension for New Horizons, which will keep the spacecraft going through 2025.
"I am very excited about this second extended mission," Stern told Space.com. NASA and the New Horizons team are discussing budget numbers for fiscal year 2025, he added.
Main action items
Now on the New Horizons agenda are a trio of main action items, as approved by NASA. One involves looking for another flyby target "and also more KBOs that we can study, not up close, but in the distance," Stern said.
In addition, New Horizons is still transmitting the last bytes of data gathered during the Arrokoth flyby in 2019.
"We got delayed in that, mostly because the Deep Space Network had some upgrades. They took antennas down, and one was down for a year," Stern said. "We've got roughly 90% of the Arrokoth flyby [data] on the ground, but we want everything, and that takes time. So that's a significant activity."
Then there's the centerpiece of New Horizons' second extended mission — a diversity of observations across a variety of fields.
"While we are flying across the Kuiper Belt," added Stern, "we are going to be doing a very interdisciplinary mission in all the space sciences — astrophysics, planetary science and heliophysics. We're going to use this spacecraft to do things that really cannot be done except if you have a spacecraft out there. There's really never been anything like this … We're doing all three different space sciences by making New Horizons an observatory for all three purposes."
For example, in heliophysics, the spacecraft will study "pickup ions." These charged particles dominate the pressure of the outer heliosphere — the huge bubble of magnetic fields and particles that the sun blows around itself — and control where the boundary with the interstellar medium is situated.
In astrophysics, New Horizons will study the cosmic optical and ultraviolet background, getting a nice view beyond the obscuring dust and other scattered light sources of the solar system's inner regions. New Horizons has already produced the most sensitive measurements of these backgrounds to date, with "deep implications for cosmology," Stern noted.
In the planetary science column, the probe is slated to study Uranus and Neptune from unique "high phase angle" geometries, shedding light on the important energy balances of those planets.
"There's never been anything really deeply interdisciplinary like New Horizons is going to become for this next three years in extended mission," said Stern.
The New Horizons team also plans to obtain time on ground-based telescopes such as Keck and Subaru to find new KBOs to study as the probe zips by them, "or if you get lucky and pass one that's close enough we could get to, we'll have a close flyby," Stern said.
The New Horizons team is employing machine learning to hunt for new KBOs using such scopes. "It turns out it is faster, more accurate, more reliable," said Stern. Weighed against human sleuthing, machine learning "is better and finds more KBOs. So that's a breakthrough and saves us a lot of work and turns out a better product."
Ground-based observations have shown that there are different classes of KBOs that have different colors and compositions. "So we know there's a lot of heterogeneity among the KBOs," Stern said. "If we had a flyby of a second KBO, I would not expect the same thing at all. It would be a completely different place than Arrokoth."
KBOs teach scientists about planetesimals, the building blocks of planets thought to exist in protoplanetary disks and debris disks.
The Arrokoth KBO flyby yielded a breakthrough result, Stern said: That at least some planetesimals formed very gently, in a process called a local cloud-collapse phenomenon. The New Horizons team would love to study another KBO up close, to see if its formation and evolution match what was observed at Arrokoth.
"We're turning this into a machine that does good for astrophysics and heliophysics while it's doing good for planetary science," Stern said about New Horizons and its second extended mission. "They are equal partners in science, and that is a first for a planetary science mission."
Leonard David is author of the book "Moon Rush: The New Space Race," published by National Geographic in May 2019. A longtime writer for Space.com, David has been reporting on the space industry for more than five decades. Follow us on Twitter @Spacedotcom or on Facebook.