The way Harvard astrophysicist Avi Loeb figures, rather than letting the Juno spacecraft end its mission with a suicidal plunge into Jupiter’s crushing atmosphere, why not use whatever propellant remains to get the orbiter into the most advantageous position to view a visiting object from another solar system.
The apple of Loeb’s eye is a 6-12 mi.-wide rocky body discovered on July 1 with a telescope in Rio Hurtado, Chile, that is part of NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS) survey team.
The comet-like object—although it currently lacks a distinctive comet-like tail—is just the third interstellar (3I) body discovered so far. Scientists suspect there could be many more similar objects zipping through the Solar System unobserved.
At the time of its discovery, 3I/ATLAS was about 4.5 astronomical units (AU) from the Sun—inside the orbit of Jupiter—and traveling at a speed of 137,000 mph. One AU is the distance between Earth and the Sun, roughly 93 million mi.
Hailing from the direction of the constellation Sagittarius and the central region of the Milky Way galaxy, 3I/ATLAS is in what is known as a hyperbolic orbit—too fast to be constrained by the gravity of the Sun and too elliptical for objects to loop back around it.
3I/ATLAS will fly within 18 million mi. of Mars on Oct. 2, and reach perihelion—its closest point to the Sun—on Oct. 29-30 at a distance of about 1.4 AU, which is just inside the orbit of Mars.
Before resuming its cold, dark journey in interstellar space, 3I/ATLAS will pass about 33 million mi. from Jupiter’s orbit on March 16. It is there and then Loeb and colleagues would like to put NASA’s Juno probe to work.
“A close encounter between Juno and 3I/ATLAS will provide a once-in-a-lifetime opportunity to bring a human-made spacecraft within a short distance from a large interstellar object,” Loeb and colleagues wrote in a series of papers posted on arxiv.org.
Built by Lockheed Martin and operated by NASA’s Jet Propulsion Laboratory, Juno launched from Cape Canaveral on Aug. 5, 2011, as part of the agency’s New Frontiers program. The spacecraft reached Jupiter and put itself into a polar orbit on July 5, 2016, to begin an extensive and extended scientific investigation of the planet.
Juno is currently slated to cease operations in September when it will be tugged from orbit and then crushed by Jupiter’s immense gravity. The disposal protocol was designed in part to reduce chances any super-resilient, hitchhiking Earth microbes that somehow managed to stay alive on the spacecraft since launch could inadvertently seed life on a potentially habitable (and possibly inhabited) moon of Jupiter.
Loeb and colleagues lay out an alternative plan for Juno. They propose using whatever propellant remains to reposition the spacecraft for 3I/ATLAS’s flyby on March 16.
“We do not presume any specific amount of fuel. Instead, we show how close Juno can get to the path of 3I/ATLAS as a function of the amount of fuel left,” Loeb wrote in an email to Aviation Week.
The instruments on Juno—a near-infrared spectrometer, magnetometer, microwave radiometer, gravity science instrument, energetic particle detector, radio and plasma wave sensor, UV spectrograph and visible light camera/telescope—“can all be used to probe the nature of 3I/ATLAS from a close distance far better than any observatories on Earth,” Loeb noted in an Aug. 2 post on Medium.
NASA did not specifically say it had studied a mission extension for Juno. The agency is in the process of trimming 20% of its staff and preparing for a possible near-50% cut to science programs, including Juno.
Instead, NASA spokeswoman Karen Fox told Aviation Week, “From what I understand, the Juno spacecraft has significantly less propellant than assumed. The team says any remaining propellant is typically used for small spacecraft maneuvers, done via thrusters.”
Rather than Juno, NASA says it is adding 3I/ATLAS observations to the Hubble and James Webb space telescopes’ schedules. “We are exploring whether some of our other telescopes can do so as well,” Fox said.
For now, astronomers worldwide have the interstellar visitor in sight. 3I/ATLAS is visible to ground-based telescopes through September. It will then pass behind by the Sun, relative to Earth’s line of sight, and—if it survives close approach—reappear in early December.
Interstellar objects are believed to be cosmic debris left over from the formation of planetary systems around their host stars. “As these remnants orbit their star, the gravity of nearby larger planets and passing nearby stars can launch them out of their home systems and into interstellar space, where they can cross paths with other solar systems,” the U.S. National Science Foundation (NSF) wrote in a press release about 31/ATLAS observations with its Gemini North telescope in Hawaii.
“These visitors from faraway regions of the cosmos are valuable objects to study since they offer a tangible connection to other star systems. They carry information about the chemical elements that were present when and where they formed, which gives scientists insight into how planetary systems form at distant stars throughout our galaxy’s history—including stars that have since died out,” the NSF added.
3I/ATLAS is traveling much faster than two previous interstellar objects, 1I/‘Oumuamua and 2I/Borisov, discovered in 2017 and 2019, respectively. The newcomer may be older as well, likely predating the Solar System, which is 4.6 billion yrs old.
Later this year, the Vera C. Rubin Observatory, jointly funded by the NSF and the U.S. Department of Energy’s Office of Science, is expected to join the hunt for more interstellar objects as part of its Legacy Survey of Space and Time program, a planned decade-long, all-sky survey of the southern hemisphere.