Scientists criticise Nasa for scaling back mission to explore beyond Pluto

It may have reached the edge of the solar system and travelled more than 5 billion miles through space, but the New Horizons spacecraft is causing major ripples on Earth. A dispute has erupted between scientists and US space officials in the wake of Nasa’s decision to stop funding next year for the vessel’s main mission.

The move was described as “misguided and unfortunate” by Alan Stern, New Horizons’s principal investigator.

“Scientifically, this is a mistake,” he told the Observer. Several other senior scientists have backed him.

Nasa has said it is not going to shut down the spacecraft completely but will provide some funding so that the craft can continue to study space weather and other phenomena. But its prime task – the study of planet composition – will be halted.

New Horizons was designed to study the Kuiper belt, a doughnut-shaped ring of icy objects left over from the formation of the sun’s planets billions of years ago. For decades, researchers dreamed of getting a close-up of some of these fossils of the solar system’s birth, but were thwarted by their remoteness. New Horizons was built to put that right.

Launched from Cape Canaveral in January 2006, the probe used a flyby of the giant planet Jupiter to increase its velocity to more than 30,000mph, reaching Pluto, the largest Kuiper belt object, on 14 July 2015, and beaming data back to Earth. These messages took more than four hours to reach mission control, even though they were being transmitted at the speed of light. They revealed spectacular images of towering water-ice mountains and vast plains of frozen nitrogen.

Pluto was also discovered to have a thin, blue atmosphere, while a reddish-brown cap of material on its largest moon, Charon, was found to be composed of organic molecules that could be important ingredients of life. For a world in such a remote orbit around the sun, the dwarf planet and its moons proved surprisingly energetic.

After sweeping past Pluto and Charon, New Horizons plunged on into the Kuiper belt, and on 1 January 2019 swept close to Arrokoth, the most distant and the most primitive object ever explored by a spacecraft. Images showed that Arrokoth – a Native American term that means “sky” in the Powhatan-Algonquin language – consisted of two lobes that probably formed separately before gently merging within a cloud of particles early in the history of our solar system.

This finding, in combination with observations of other Kuiper belt objects by New Horizons, was crucial.

“It taught us so much about fundamental properties of planetary formation. It was completely transformational,” said Michele Bannister, a planetary scientist at University of Canterbury, New Zealand, in the journal Nature last week.

This point was backed by Stern, who said: “The Kuiper belt is made up of planetesimals, the building blocks of planets. Thanks to the data that was sent back by New Horizons, we now understand the way that these building blocks combine and coalesce and start the process of planetary formation. This is fundamentally important to understanding our own solar system and planets around other stars.”

New Horizons had been scheduled to take another four or five years to complete its journey through the Kuiper belt – named after the Dutch-American astronomer Gerard Kuiper, who proposed its existence in a paper in 1951. During this journey, it was hoped that it could rendezvous with another planetesimal like Arrokoth.

Related: Beyond Pluto: the hunt for our solar system's new ninth planet

But finding such a target near its path through the belt has been tricky.

“It is extremely hard to find another suitable object to get near but we have been trying really hard to find one,” said Stern.

“I think there is some frustration at Nasa that we don’t have another flyby target as yet, and I understand that frustration. We are working as hard as we can to put that right but it is such a tough problem. However, cutting off our funding only guarantees that there will never be another flyby target.”

New Horizons cost more than $800m (£650m) to build and fly to Pluto and beyond. Mission control costs have come in at about $10m a year. By shifting the mission away from planetary science and scrutiny of the Kuiper belt and by concentrating instead on heliophysics, the physics of the sun and its connection with the solar system, several million dollars are likely to be trimmed from that annual budget.

“It’s a false economy,” said Stern. “New Horizons can still do great science for the rest of its time in the Kuiper belt. But stopping it next year is both premature scientifically and unwise from the standpoint of fiscal policy. I am very concerned about this, and it is fair to say that I am in good company.”