Great conjunctions between Saturn and Jupiter Moon are rare

Great conjunctions between Saturn and Jupiter Moon are rare, the next easily observable conjunction of the two planets is not expected for nearly 40 years.

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NASA probe snaps ‘great conjunction’ photo of Jupiter and Saturn from the moon

Behold, the view from the Lunar Reconnaissance Orbiter!

The Lunar Reconnaissance Orbiter captured an image of 2020's great conjunction of Jupiter and Saturn.
(Image credit: NASA/GSFC/Arizona State University)

A moon-orbiting probe got a stunning up-close view of the “great conjunction” of Jupiter and Saturn from Earth’s rocky satellite. 

Jupiter and Saturn appeared closer in the night sky than they had in about 800 years during what’s known as a “great conjunction.” People all around the globe watched and photographed the planets, which looked almost like a single, bright “star” in the sky. However, us Earthlings weren’t the only ones who got a celestial show. 

NASA’s Lunar Reconnaissance Orbiter (LRO), which launched in 2009 and has enough fuel to keep orbiting the moon for another six years, spotted the cosmic event all the way from the moon. 

The Lunar Reconnaissance Orbiter Camera’s (LROC) Narrow Angle Camera (NAC) captured an unbelievable image of the two planets just a few hours after the pair’s point of closest separation (0.1 degrees). Now, while Jupiter and Saturn may have looked like one glowing orb to the naked eye, with the detailed view of the NAC, you can clearly resolve the individual planets. In fact, the image provides so much detail that you can even faintly see Saturn’s rings. 

Here on Earth, skywatchers were able to see Jupiter’s moons with DSLR cameras and even basic telescopes, though Saturn’s rings were usually only visible with higher-powered telescopes. 

On Dec. 21, 2020, Jupiter and Saturn will appear just one-tenth of a degree apart, or about the thickness of a dime held at arm's length, according to NASA. During the event, known as a "great conjunction," the two planets (and their moons) will be visible in the same field of view through binoculars or a telescope.
On Dec. 21, 2020, Jupiter and Saturn appeared just one-tenth of a degree apart, or about the thickness of a dime held at arm’s length, according to NASA. During the event, known as a “great conjunction,” the two planets (and their moons) were visible in the same field of view through binoculars or a telescope.  (Image credit: NASA/JPL-Caltech)

When the NAC captured this image of the two planets, Jupiter was about four times brighter than Saturn, so the brightness of the original image was adjusted to make both equally visible. 

While Jupiter and Saturn have a close conjunction once every 20 years, the planets haven’t appeared this close since 1623. Additionally, the planetary alignment came just a few days before Christmas, with many dubbing the bright event a “Christmas Star,” adding even more to the astronomical excitement. 

Interstellar comets like Borisov may not be all that rare

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Astronomers calculate that the Oort Cloud may be home to more visiting objects than objects that belong to our solar system.


In 2019, astronomers spotted something incredible in our backyard: a rogue comet from another star system. Named Borisov, the icy snowball traveled 110,000 miles per hour and marked the first and only interstellar comet ever detected by humans.

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But what if these interstellar visitors — comets, meteors, asteroids and other debris from beyond our solar system — are more common than we think?

In a new study published Monday in the Monthly Notices of the Royal Astronomical Society, astronomers Amir Siraj and Avi Loeb at the Center for Astrophysics | Harvard & Smithsonian (CfA) present new calculations showing that in the Oort Cloud — a shell of debris in the farthest reaches of our solar system — interstellar objects outnumber objects belonging to our solar system.

“Before the detection of the first interstellar comet, we had no idea how many interstellar objects there were in our solar system, but theory on the formation of planetary systems suggests that there should be fewer visitors than permanent residents,” says Siraj, a concurrent undergraduate and graduate student in Harvard’s Department of Astronomy and lead author of the study. “Now we’re finding that there could be substantially more visitors.”

The calculations, made using conclusions drawn from Borisov, include significant uncertainties, Siraj points out. But even after taking these into consideration, interstellar visitors prevail over objects that are native to the solar system.

“Let’s say I watch a mile-long stretch of railroad for a day and observe one car cross it. I can say that, on that day, the observed rate of cars crossing the section of railroad was one per day per mile,” Siraj explains. “But if I have a reason to believe that the observation was not a one-off event — say, by noticing a pair of crossing gates built for cars — then I can take it a step further and begin to make statistical conclusions about the overall rate of cars crossing that stretch of railroad.”

But if there are so many interstellar visitors, why have we only ever seen one?

We just don’t have the technology to see them yet, Siraj says.

Consider, he says, that the Oort Cloud spans a region some 200 billion to 10 trillion miles away from our Sun — and unlike stars, objects in the Oort Cloud don’t produce their own light. Those two factors make debris in the outer solar system incredibly hard to see.

Senior astrophysicist Matthew Holman, who was not involved in the research, says the study results are exciting because they have implications for objects even closer than the Oort Cloud.

“These results suggest that the abundances of interstellar and Oort cloud objects are comparable closer to the Sun than Saturn. This can be tested with current and future solar system surveys,” says Holman, who is the former director of the CfA’s Minor Planet Center, which tracks comets, asteroids and other debris in the solar system.

“When looking at the asteroid data in that region, the question is: are there asteroids that really are interstellar that we just didn’t recognize before?” he asks.

Holman explains that there are some asteroids that get detected but aren’t observed or followed up on year after year. “We think they are asteroids, then we lose them without doing a detailed look.”

Loeb, study co-author and Harvard astronomy professor, adds that “interstellar objects in the planetary region of the solar system would be rare, but our results clearly show they are more common than solar system material in the dark reaches of the Oort cloud.”

Observations with next-generation technology may help confirm the team’s results.

The launch of the Vera C. Rubin Observatory, slated for 2022, will “blow previous searches for interstellar objects out of the water,” Siraj says, and hopefully help detect many more visitors like Borisov.

The Transneptunian Automated Occultation Survey (TAOS II), which is specifically designed to detect comets in the far reaches of our solar system, may also be able to detect one of these passersby. TAOS II may come online as early as this year.

The abundance of interstellar objects in the Oort Cloud suggests that much more debris is left over from the formation of planetary systems than previously thought, Siraj says.

“Our findings show that interstellar objects can place interesting constraints on planetary system formation processes, since their implied abundance requires a significant mass of material to be ejected in the form of planetesimals,” Siraj says. “Together with observational studies of protoplanetary disks and computational approaches to planet formation, the study of interstellar objects could help us unlock the secrets of how our planetary system — and others — formed.”