Deep into the Kuiper Belt, New Horizons is still doing science

Enlarge / Artist’s impression of the New Horizons spacecraft at Arrokoth. This astronomical body is the most distant object visited by human spacecraft, with the flyby of NASA’s New Horizons spacecraft taking place on January 1, 2019.

New Horizons is now nearly twice as far from the Sun as Pluto, the outer planets are receding fast, and interstellar space is illuminated by the vast swath of the Milky Way ahead. But the spacecraft’s research is far from over. Its instruments are all functioning and responsive, and the New Horizons team has been working hard, pushing the spacecraft’s capabilities to carry out new tasks.

Since its launch in January 2006, the New Horizons spacecraft has traveled over 5 billion miles, passed by the moons of Jupiter, and surveyed the scaley frozen methane ice of its target planet Pluto. In January 2019, it buzzed by Arrokoth, another billion miles beyond Pluto—the most distant object to have ever been visited by a spacecraft. The data it returned from this intact remnant of our Solar System’s formation has given us important new insights into how that process happened.

But New Horizons’ mission is far from over. While it may never have another close encounter with an orbiting object, the team that operates the spacecraft is working out ways to put its instruments to new uses.

Budgets and power budgets

As New Horizons has gotten further from the Sun, piloting the spacecraft requires not only patience but a revised focus. Led by Alice Bowman—the mission’s version of Star Trek’s Scotty—engineers start building a command load three months in advance, then run them on a simulator at the Applied Physics Laboratory to check that they’re sound. Transmitting the commands currently takes eight hours to reach the craft from Earth and requires booking a slot on NASA’s Deep Space Network—three huge radio dishes located in California, Australia, and Spain, which handle communications with multiple space missions. So, like getting a table at a popular restaurant, bookings are required months in advance unless there’s an emergency.

New Horizons spins as it races through space, and while some instruments (like its particle detectors) operate best in spinning mode, to use its imagers, the craft has to be de-spun and pointed, using precious fuel. Power comes from an RTG (radioisotope thermoelectric generator), essentially a nuclear battery made from plutonium-238, which has a half-life of 87.7 years. It’s not currently known how long that power will last.

The two Voyager spacecraft, which already left the Solar System ahead of New Horizons, are still operational but have had to switch off some instruments, including the onboard cameras, which were “power hogs,” so now they run just a few instruments with a low power demand, then send back the data. As with the Voyagers, the more power-hungry instruments on New Horizons (e.g., the imagers) that need heaters to keep them at operating temperatures will likely be switched off first. It’s hard to predict when that will be, though, because the RTG’s lifespan is continually being extended by the engineering team, which keeps inventing ever more ingenious tweaks to eke out the power.

The mission also needs to continue paying those engineers. Happily, NASA recently announced that funding will continue for New Horizons through at least 2028 or 2029.

A new view of KBOs

One of the spacecraft’s missions is to continue to explore the Kuiper Belt, which extends from Neptune’s orbit at 30 AU to beyond 50 AU from the Sun. It consists of chunks of rock, ice, comets, and dust. Since leaving the largest Kuiper Belt Object (KBO) Pluto behind, the geology team has been using the spacecraft’s designed capabilities to study other KBOs, so far finding more than 100 new ones and passing almost 20 KBOs close enough to reveal surface properties, shapes, rotational periods, and close-in orbiting moons.

The Kuiper Belt holds the key to a big puzzle. Why did all the planets accrete from clouds of interstellar dust and gas rather than just smashing into each other in mutual annihilation? Asteroids are too battered and reshaped by multiple collisions to retain traces of their formation. So when the geology team learned the spacecraft would fly by a large KBO, they got very excited.

Sweeping past the contact binary Arrokoth at a distance of just 3,500 kilometers (2,198 miles) in 2019, the images that New Horizons returned appeared to an untrained eye to resemble an unspectacular lumpy potato. But its lonely location in the outer Kuiper Belt has kept Arrokoth intact, essentially a fossil from the early days of Solar System formation. Modeling the detailed data New Horizons obtained of this 36-kilometer-long (22-mile) by up to 20-kilometer-wide (13-mile) object, shows that the larger side was assembled from 8 to 10 smaller components, which all had to be moving quite slowly to successfully “dock.” “If they’d come together faster, their outlines would have been smooshed by the impact,” said Will Grundy, head of the mission’s Planetary Geology team at Lowell Observatory, where Pluto was discovered in 1930.

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