Could a decades-old Jupiter probe really be humanity’s first envoy to a possible alien craft? That is the audacious suggestion from Harvard astrophysicist Avi Loeb, whose latest proposal aims to turn NASA’s Juno spacecraft toward 3I/ATLAS, a newly discovered interstellar object with a trajectory and glow that have stirred both scientific intrigue and skepticism.
First sighted on July 1, 2025, by NASA’s Asteroid Terrestrial-impact Last Alert System in Chile, 3I/ATLAS is only the third known interstellar visitor to our planet after ‘Oumuamua and 2I/Borisov. It is moving at about 219,000 km/h on a retrograde orbit inclined by only five degrees from Earth’s orbital plane an arrangement Loeb estimates has only a 2 percent chance of happening by accident. The object will come unusually close to Venus, Mars, and Jupiter before it slingshots into interstellar space, a progression Loeb describes as “a rare opportunity” for in situ investigation.
What distinguishes 3I/ATLAS from average comets is its brightness and spectral profile. Though its approximate diameter of 20 kilometers would make it unusually bright if it were an asteroid, no gas emissions characteristic of comets were detected by spectroscopic observations using ESO’s Very Large Telescope, although it is heavily shrouded in dust and reddish in appearance like some Trans-Neptunian Objects. A July 21 image by the Hubble Space Telescope showed a dense glow on its front edge. Loeb compared it to “a vehicle turning on its headlights” and suggested it “could be a spacecraft powered by nuclear energy,” with dust potentially stripped from gathered interstellar trash.
Harvard’s plan relies on orbital mechanics. With Optimum Interplanetary Trajectory Software and the SPICE toolkit developed by NASA, Loeb and coauthors Adam Hibberd and Adam Crowl determined a 2.675 km/s delta-v deployed on September 14, 2025, on Juno could defocus it from Jupiter orbit to intercept 3I/ATLAS on March 14, 2026. The flyby would utilize a Jupiter Oberth effect burning Juno’s engine in the bottom of the planet’s gravity well to gain the most velocity change only days in advance of the scheduled deorbit into Jupiter’s atmosphere. This strategy avoids the impossibility of sending a new probe on this short notice.
If implemented, Juno’s instrument suite spanning a near-infrared spectrometer and microwave radiometer to a magnetometer and UV spectrograph can sample directly the object’s spectra, magnetic surroundings, and thermal radiation. Such information would make it clear whether 3I/ATLAS is a primordial remnant from the Milky Way’s thick disk and hence more than seven billion years old or much less natural.
Other observation plans are also on the cards. On October 2-3, 2025, 3I/ATLAS will fly just 2.7 million kilometers from Mars. The HiRISE camera on NASA’s Mars Reconnaissance Orbiter may take close-up, high-resolution photos of its nucleus and carbon-dioxide-filled coma, unhampered by the Earth-based telescopes’ viewing limitations when the comet is at perihelion on the far side of the Sun. Other missions, like ESA’s JUICE and NASA’s Psyche, will also be in good position for remote sensing, albeit without the direct intercept capability.
The engineering hurdles are daunting. Juno is old, fuel-starved, and only recently recovered from radiation-caused bugs. Could it be re-tasked? In theory, if it can be done, and the instruments work, then there is novel data there, Mark Burchell of the University of Kent said, with a note of warning regarding the probe’s compromised systems. Jason Wright of Pennsylvania State University has questioned its propulsion margins. But Loeb insists that the scientific reward either verifying an ancient cometary origin or finding signs of alien engineering makes the effort worthwhile.
To planetary scientists, 3I/ATLAS provides a brief opportunity to examine material created at the galaxy’s “cosmic noon,” a period of intense star formation billions of years before the Sun existed. For those open to more speculative interpretations, it is a test case for how quickly humanity can mobilize to investigate an object that might not be entirely natural. As Loeb put it, You never know what type of intent it has. We should observe it.
