Juno could potentially intercept the interstellar comet 3I/ATLAS

In 2017, astronomers working at Hawaii’s Pan-STARRS Observatory made headlines by spotting ‘Oumuamua, the first interstellar object (ISO) ever confirmed by observations.

A second visitor arrived two years later: the interstellar comet 2I/Borisov. Then, on July 1st, 2025, the Asteroid Terrestrial-impact Last Alert System (ATLAS) facility in Rio Hurtado identified a third interstellar object inside our Solar System-now designated 3I/ATLAS (or C/2025 N1 ATLAS).

As with the earlier discoveries, this newcomer has triggered intense scientific curiosity, alongside renewed discussion of spacecraft missions that might one day rendezvous with future ISOs.

Mission concepts for ISOs-and why 3I/ATLAS changes the conversation

Proposals that have been discussed in this context include Project Lyra, the Interstellar Object Explorer (IOE), and the ESA’s Comet Interceptor. A newer paper led by Prof. Abraham Loeb of Harvard University, however, considers an alternative: meeting 3I/ATLAS using a spacecraft that is already operating in deep space.

Their assessment suggests that NASA’s Juno mission could be redirected to intercept the object when it nears Jupiter on March 16th, 2026-offering the chance to observe at close range one of today’s most mysterious categories of cosmic bodies.

The team behind the proposal

Abraham Loeb holds the Frank B. Baird Jr. Professorship of Science at Harvard University and serves as Director of both the Institute for Theory and Computation (ITC) and the Galileo Project at the Harvard & Smithsonian Center for Astrophysics (CfA).

He collaborated with Adam Hibberd and Adam Crowl, both recognised scientists with the UK-based non-profit Initiative for Interstellar Studies (i4is). Their paper outlining the concept has recently been posted online and is under review for publication in The Astrophysical Journal Letters.

Prior work on ‘Oumuamua, propulsion concepts, and interstellar missions

Loeb, Hibberd, and Crowl have each previously focused on ISOs and on how spacecraft might be sent to meet them for detailed study. In 2018, Prof. Loeb attracted widespread attention after publishing the paper, "Could Solar Radiation Pressure Explain 'Oumuamua's Peculiar Acceleration?"

In that work-and in his later book Extraterrestrial-he proposed that ‘Oumuamua could have been an extraterrestrial spacecraft, which would account for its unusual behaviour and why it resisted straightforward classification.

Hibberd is also well known for his research with Marshall Eubanks on Project Lyra, a mission concept intended to “catch up” to ‘Oumuamua or later ISOs using lightsail technology and Directed-Energy Propulsion (DEP).

He and Eubanks additionally put forward the Swarming Proxima Centauri proposal, an allied idea that would use DEP to dispatch a swarm of spacecraft to examine the nearest rocky world beyond our Solar System, Proxima b.

Crowl, an independent researcher and propulsion engineer, previously contributed to Project Icarus-a design study for an interstellar probe developed in the tradition of Project Daedalus.

Why intercepting an interstellar object matters

Ever since ‘Oumuamua passed close to Earth, researchers have hoped for a dedicated mission capable of intercepting and analysing an interstellar traveller. Over the past decade, several spacecraft have already completed sample-return missions to Near-Earth Asteroids (NEAs), including JAXA’s Hayabusa and Hayabusa2 probes and NASA’s OSIRIS-REx probe.

Because asteroids and comets are largely remnants from the Solar System’s formation, studying their samples helps reconstruct the conditions that existed c. 4.5 billion years ago.

Likewise, examining ISOs while they traverse our System could reveal conditions in other planetary systems-without waiting for a purpose-built interstellar spacecraft to reach those distant stars.

The 3I/ATLAS hypothesis and the case for a rendezvous mission

As Prof. Loeb discussed in his 2018 paper, the chance-however small-that an ISO might be artificial (for example, an abandoned spacecraft) would multiply the scientific stakes dramatically. Hibberd, Crowl, and Loeb addressed comparable questions about 3I/ATLAS in a recent paper, "Is the Interstellar Object 3I/ATLAS Alien Technology?."

In an email to Universe Today, Prof. Loeb explained why this makes a close encounter particularly attractive:

[W]e show that applying a thrust of 2.675 kilometers per second on September 14th, 2025, can bring the Juno spacecraft from its orbit around Jupiter to intercept the path of 3I/ATLAS. The close encounter of 3I/ATLAS with Jupiter provides a rare opportunity to shift Juno from its current orbit around Jupiter to intercept the path of 3I/ATLAS at its closest approach to Jupiter.
The brightness of 3I/ATLAS implies a diameter of 20 kilometers for an asteroid with a typical reflectance (albedo) of 5%. As I showed in a published paper shortly after 3I/ATLAS was discovered, the detection of this object over 5 years of the ATLAS telescope's survey of the sky requires an untenable mass supply of rocky material from the Milky Way galaxy. If 3I/ATLAS is 20 kilometers in diameter, it might have targeted the inner Solar System as expected from alien technology.

Timing at Jupiter and why launching from Earth is not practical

As described in their analysis, 3I/ATLAS will pass to within roughly 53.6 million km (33.25 million mi), or 0.358 AU, of Jupiter on March 16th, 2026. Adjusting Juno’s trajectory could therefore place the probe on a path to intersect 3I/ATLAS during the comet’s closest approach to the gas giant.

This route sidesteps the difficulty of designing, building, and launching a new spacecraft quickly enough to reach 3I/ATLAS before its closest pass to the Sun on October 29th, 2025-after which it will leave our Solar System. Hibberd set out the issue as follows:

It is quite clear that a mission launched from Earth to 3I is completely infeasible, given how little warning we had of its arrival in the Solar System. Furthermore, it would not be within the performance envelope of the proposed ESA Comet Interceptor mission, so in other words, even if a spacecraft had been waiting at the Sun/Earth L2 point. Now, 3I coincidently comes quite close to Mars, Jupiter, and Venus, which is in itself a strange happenstance and will be unlikely to recur with any future ISO.
It seems reasonable, therefore, given the above serendipities and the impossibility of a dedicated probe being launched to encounter it in time, to enquire whether any existing spacecraft orbiting around Mars or Jupiter could be exploited for an intercept or a close approach. It is thus in this context that the work is worthwhile, and such analysis will only apply to ISOs that happen to have close encounters with the planets, which, as I have articulated, will be very rare indeed.

Trajectory modelling: OITS, SPICE, NOMAD, and the Lambert Problem

To find a workable flight path for a rendezvous, the researchers used Optimum Interplanetary Trajectory Software (OITS), a tool Hibberd originally created in 2017 in support of Project Lyra.

Hibberd noted that OITS enabled them to tackle the task of resolving the orbits and velocities of Juno and 3I/ATLAS (aka. the Lambert Problem), though only across a single orbital cycle. To estimate how near Juno could come to the interstellar comet while expending as little propellant as possible, Hibberd then turned to additional bespoke software.

That work relied on C code he wrote using three libraries-including NASA JPL, Navigation and Ancillary Information Facility (NAIF), and Spacecraft, Planet, Instrument, C-matrix, Events (SPICE) software-so the team could generate precise predictions for the orbits of Jupiter and Juno.

They also employed NOMAD software to integrate the motion of all three bodies and calculate the minimum velocity Juno would need to achieve in order to intercept 3I/ATLAS. As Loeb described, their findings point to the feasibility of an intercept enabled by a Jupiter Oberth Maneuver:

If doable, this exciting new goal will rejuvenate Juno's mission and extend its scientific lifespan beyond March 14th, 2026. So far, we have examined a zero-distance intercept of Juno with 3I/ATLAS. The optimal option involves a Jupiter Oberth Maneuver, which requires an application of ∆V on September 9th, 2025, only 8 days prior to the originally intended termination date for Juno's plunge into the atmosphere of Jupiter. Having delivered this thrust to diminish Juno's altitude, a further ∆V is subsequently delivered, constituting a Jupiter Oberth Maneuver and resulting in an eventual intercept of the target 3I/ATLAS on March 14th, 2026.

What Juno could measure during a close approach to 3I/ATLAS

The paper further notes that Juno’s full instrument complement could be brought to bear on 3I/ATLAS at close range. The payload includes a near-infrared spectrometer, magnetometer, microwave radiometer, gravity science instrument, energetic particle detector, radio and plasma wave sensor, UV spectrograph, and a visible-light camera.

The resulting observations-spanning spectra, imagery, and energetic emissions-could help determine the object’s composition, offering substantial insight into its system of origin and the physical conditions present at its formation.

"Our paper is contingent on a remarkable but testable hypothesis that 3I/ATLAS is a functioning technological artifact, to which I and my two co-authors do not necessarily ascribe," Loeb adds.

"Yet, this hypothesis is worthy of a scientific analysis for two reasons: The consequences, should the hypothesis turn out to be correct, could potentially be dire for humanity, and would possibly require defensive measures to be undertaken (though these might prove futile). [Second,] the hypothesis is an interesting exercise in its own right and is fun to explore, irrespective of its likely validity."

Hubble observations and what will ultimately settle the question

Even so, newly obtained images from the Hubble Space Telescope indicate that this scenario may already be unlikely. From the brightness profile of the surrounding coma, scientists have inferred that the nucleus has an effective radius of under 2.8 kilometres (~1.75 mi).

Ultimately, uncertainties about what 3I/ATLAS really is should be clarified once either (a) 3I/ATLAS moves closer to the Sun and starts venting gases through sublimation, or (b) the Juno probe is able to inspect it from close quarters.

Whatever the final result, the findings are expected to be compelling and should broaden our understanding of what exists beyond the Solar System.

This article was originally published by Universe Today. Read the original article.