Comet Makes a Pit Stop Near Jupiter’s Asteroids

After traveling several billion miles toward the Sun, a wayward young comet-like object orbiting among the giant planets has found a temporary parking place along the way. The object has settled near a family of captured ancient asteroids, called Trojans, that are orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan population.

A wayward object took a rest stop in a field of captured asteroids.

Astronomers found a roaming comet taking a rest stop before possibly continuing its journey. The wayward object made a temporary stop near giant Jupiter. The icy visitor has plenty of company. It has settled near the family of captured asteroids known as Trojans that are co-orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan asteroid population. Hubble Space Telescope observations reveal the vagabond is showing signs of transitioning from a frigid asteroid-like body to an active comet, sprouting a long tail, outgassing jets of material, and enshrouding itself in a coma of dust and gas.Credits: NASA, ESA, and B. Bolin (Caltech)

The unexpected visitor belongs to a class of icy bodies found in space between Jupiter and Neptune. Called “Centaurs,” they become active for the first time when heated as they approach the Sun, and dynamically transition into becoming more comet-like.

Visible-light snapshots by NASA’s Hubble Space Telescope reveal that the vagabond object shows signs of comet activity, such as a tail, outgassing in the form of jets, and an enshrouding coma of dust and gas. Earlier observations by NASA’s Spitzer Space Telescope gave clues to the composition of the comet-like object and the gasses driving its activity.

“Only Hubble could detect active comet-like features this far away at such high detail, and the images clearly show these features, such as a roughly 400,000-mile-long broad tail and high-resolution features near the nucleus due to a coma and jets,” said lead Hubble researcher Bryce Bolin of Caltech in Pasadena, California.

Describing the Centaur’s capture as a rare event, Bolin added, “The visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. We’re investigating how it was captured by Jupiter and landed among the Trojans. But we think it could be related to the fact that it had a somewhat close encounter with Jupiter.”

The team’s paper appears in the February 11, 2021 issue of The Astronomical Journal.

The research team’s computer simulations show that the icy object, called P/2019 LD2 (LD2), probably swung close to Jupiter about two years ago. The planet then gravitationally punted the wayward visitor to the Trojan asteroid group’s co-orbital location, leading Jupiter by about 437 million miles.

After traveling several billion miles toward the Sun, a wayward young comet-like object orbiting among the giant planets has found a temporary parking place along the way. The object has settled near a family of captured ancient asteroids, called Trojans, that are orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan population.Credits: NASA

Bucket Brigade

The nomadic object was discovered in early June 2019 by the University of Hawaii’s Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes located on the extinct volcanoes, one on Mauna Kea and one on Haleakala. Japanese amateur astronomer Seiichi Yoshida tipped off the Hubble team to possible comet activity. The astronomers then scanned archival data from the Zwicky Transient Facility, a wide-field survey conducted at Palomar Observatory in California, and realized that the object was clearly active in images from April 2019.

They followed up with observations from the Apache Point Observatory in New Mexico, which also hinted at the activity. The team observed the comet using Spitzer just days before the observatory’s retirement in January 2020, and identified gas and dust around the comet nucleus. These observations convinced the team to use Hubble to take a closer look. Aided by Hubble’s sharp vision, the researchers identified the tail, coma structure and the size of the dust particles and their ejection velocity. These images helped them confirm that the features are due to relatively new comet-like activity.

Although LD2’s location is surprising, Bolin wonders whether this pit stop could be a common pull-off for some sunward-bound comets. “This could be part of the pathway from our solar system through the Jupiter Trojans to the inner solar system,” he said.

The unexpected guest probably will not stay among the asteroids for very long. Computer simulations show that it will have another close encounter with Jupiter in about another two years. The hefty planet will boot the comet from the system, and it will continue its journey to the inner solar system.

“The cool thing is that you’re actually catching Jupiter flinging this object around and changing its orbital behavior and bringing it into the inner system,” said team member Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. “Jupiter controls what’s going on with comets once they get into the inner system by altering their orbits.”

The icy interloper is most likely one of the latest members of the so-called “bucket brigade” of comets to get kicked out of its frigid home in the Kuiper belt and into the giant planet region through interactions with another Kuiper belt object. Located beyond Neptune’s orbit, the Kuiper belt is a haven of icy, leftover debris from our planets’ construction 4.6 billion years ago, containing millions of objects, and occasionally these objects have near misses or collisions that drastically alter their orbits from the Kuiper belt inward into the giant planet region.

The bucket brigade of icy relics endure a bumpy ride during their journey sunward. They bounce gravitationally from one outer planet to the next in a game of celestial pinball before reaching the inner solar system, warming up as they come closer to the Sun. The researchers say the objects spend as much or even more time around the giant planets, gravitationally pulling on them—about 5 million years—than they do crossing into the inner system where we live.

“Inner system, ‘short-period’ comets break up about once a century,” Lisse explained. “So, in order to maintain the number of local comets we see today, we think the bucket brigade has to deliver a new short-period comet about once every 100 years.”

An Early Bloomer

Seeing outgassing activity on a comet 465 million miles away from the Sun (where the intensity of sunlight is 1/25th as strong as on Earth) surprised the researchers. “We were intrigued to see that the comet had just started to become active for the first time so far away from the Sun at distances where water ice is barely starting to sublimate,” said Bolin.

Water remains frozen on a comet until it reaches about 200 million miles from the Sun, where heat from sunlight converts water ice to gas that escapes from the nucleus in the form of jets. So the activity signals that the tail might not be made of water. In fact, observations by Spitzer indicated the presence of carbon monoxide and carbon dioxide gas, which could be driving the creation of the tail and jets seen on the Jupiter-orbiting comet. These volatiles do not need much sunlight to heat their frozen form and convert them to gas.

Once the comet gets kicked out of Jupiter’s orbit and continues its journey, it may meet up with the giant planet again. “Short-period comets like LD2 meet their fate by being thrown into the Sun and totally disintegrating, hitting a planet, or venturing too close to Jupiter once again and getting thrown out of the solar system, which is the usual fate,” Lisse said. “Simulations show that in about 500,000 years, there’s a 90% probability that this object will be ejected from the solar system and become an interstellar comet.”

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, managed the Spitzer mission for NASA’s Science Mission Directorate in Washington, D.C. Science operations were conducted at the Spitzer Science Center at IPAC at Caltech. Spitzer’s entire science catalogue is available via the Spitzer data archive, housed at the Infrared Science Archive at IPAC. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado.

Credits: NASAESA, and B. Bolin (Caltech)

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February 26, 2021 2:40 am

Wikipedia has a good illustration showing the Trojans and the Greeks.

Tom Abbott
Reply to  commieBob
February 26, 2021 2:02 pm

They hardly ever mention the Greeks anymore.

Dudley Horscroft(@dudleyhorscroft)
February 26, 2021 2:41 am

OK, it comes in from outer space, and passes close to Jupiter. The gravity pull of Jupiter accelerates it and alters its orbit, and has apparently moved it to the group of Trojans. But since the Trojans remain in the same position relative to Jupiter, when the comet reached the Trojans it must have been travelling rather faster in its orbit than them So it it is staying in the Trojan Group for some time – as this item appears to indicate, what has slowed it down?

It can hardly be the [pull of Jupiter, although this will have slowed it as it moved away. Remarkable if J’s pull has managed to slow it to exactly the amount to keep it within the Trojans?

Reply to  Dudley Horscroft
February 26, 2021 3:29 am

Most likely flew into one of two Sun- Jupiter Lagrange points, and if so it will stay there forever.
At the Lagrange points the gravitational  forces of the two large bodies cancel out in such a way that a small object placed in orbit there is in equilibrium between the large bodies.
NASA should land a space probe and investigate for existence of microbes that could prove the life on the Earth is of the extraterrestrial origin.

Leonard Weinstein
Reply to  Vuk
February 26, 2021 5:45 am

It would only stay there if it had the correct velocity and direction, which it can’t have except if it has a collision with another object to cancel it’s entry velocity. A stable point does not capture entering objects unless those that enter and collide and lose escape velocity from it can be captured. It is stable due to being at the balance point of competing Sun and Jupiter gravity.

Kevin kilty
Reply to  Leonard Weinstein
February 26, 2021 7:36 am

Sorry, Leonard, I didn’t see your comment before posting mine, which says the same thing in a different way. Good comment.

Reply to  Leonard Weinstein
February 26, 2021 7:37 am

Right. The article is behind a paywall, but I was able to read it through my university’s access. It is indeed in the *region* of the Trojans (the Trojan “point” is not exactly a point, but a region itself), but apparently this comet has some velocity relative to true Trojans which makes its orbit unstable.

Jim Whelan
Reply to  mcswell
February 26, 2021 8:47 am

And the region has a quite shallow energy profile.Objects can easily escape undert the influence of outside gravitation or collisions inside the region. they aren’t necessarily there “permanantly”.

Reply to  Leonard Weinstein
February 26, 2021 12:39 pm


You make a good point,
but you ignoring the initial condition, for the given velocity(due to acceleration) and the given direction acquired.

A countering condition that can nullify the initial condition role, can “correct” the velocity and direction for a permanent stable orbit of this comet around Jupiter.

What I am trying a say here is that you and Vuk are equally correct as this stands from this point of view,
both in a 50/50.

If this thing that may be called “gravity friction” of Jupiter versus this given comet can decelerate or nullify the gained acceleration of this comet by the given initial condition,
then the chance of a permanent orbital is possible.
Which if it happens will mean that there you have a comet being
not anymore a comet, if or when this “correction” established.

The correction is still possible, even as in this given, only at 50/50.

But the other point there;
whether this comet being captured permanently or not by the Jupiter, at that end point it will be not anymore a comet… but more like an ice asteroid…
with no any more mass shedding…
or otherwise known as the tail.


John Tillman
Reply to  Vuk
February 26, 2021 5:52 am

Assuming that the microbes use terrestrial systems of replication and metabolism, ie DNA, RNA, proteins and AMP/ADP/ATP. Phosphorus is more plentiful farther out in the solar system, so odds are good.

A bilayer lipid membrane is probably common in the universe, at least where water rather than a liquid hydrocarbon is the solvent.

Pat Frank
Reply to  John Tillman
February 26, 2021 8:50 am

I was at a Planetary Society meeting once, where the kinetic expulsion of Earth rocks toward the Kuiper belt was discussed. Such events can happen following bolide impacts.

Swarms of terrestrial bolide impacts occur whenever a passing star disturbs our Oort cloud, sending comets into the inner solar system.

The speaker observed it was a good way for Earth bacteria to seed the galaxy. So, if some bacteria are ever discovered in the Kuiper belt, they may well have come from Earth.

Reply to  Vuk
February 26, 2021 8:34 am

extraterrestrial origin?…..that would only lead to another question….where did the extraterrestrial come from?

Reply to  T. C. Clark
February 26, 2021 9:23 am

All kind of possible and impossible ideas are put forward by various people

Reply to  Vuk
February 26, 2021 1:45 pm


If I may share a thought with you about ideas.

In the main body or realm of ideas, all ideas are innocent or neutral, either when viewed as
possible-impossible or smart-crazy.

Simply, ideas are ideas, paths of thought exploration.

The problem with an idea is when it grows and matures outside the means and platform of logic and rationale.
But simply grows in potential by feeding and nourishing in other possible ideas… leading to “abortion” of other ideas.

Humanity has got to a point of evolution that offers a consolidated
logical rationale platform for analyzing-validating ideas.
It is called science, with its firm scientific method.
Where an idea fails-collapses or gains potential,
by it’s main merit.

Any idea that grows in potential outside the realm of
logical-rationale, will do so only by vamping in other ideas and creating a realm of ideology.
Where one idea overrules and crushes many other possible ideas, just because of certainty of some belief,
or/and because of a force of a given outside special interest authority.

Full growth and maturity from such as is known otherwise, as delusion or a delusional path,
which in full blossom leads to either;
reality dysfunction psychosis or worse,
reality disconnect syndrome,
otherwise known as forms of insanity.

Well maybe a bit out of line here,
but only just saying…
the problem is not the ideas, how many or how possible impossible,
but the way we decide to go and implement or treat
and tend such thought exploration points.


Reply to  T. C. Clark
February 26, 2021 9:38 am

My favourite and at same time most unlikely is Radiopanspermia
“In 1903, Svante Arrhenius published in his article The Distribution of Life in Space, the hypothesis now called radiopanspermia, that microscopic forms of life can be propagated in space, driven by the radiation pressure from stars. Arrhenius argued that particles at a critical size below 1.5 μm would be propagated at high speed by radiation pressure of the Sun. However, because its effectiveness decreases with increasing size of the particle, this mechanism holds for very tiny particles only, such as single bacterial spores”
Remember him, the ‘Global warming’ fame man.

Last edited 10 months ago by Vuk
Kevin kilty
Reply to  Dudley Horscroft
February 26, 2021 7:00 am

If a smaller object approaches a larger body closely, a collision without impact in other words, it is possible during the collision for the small body to absorb some of the other bodies KE or to have some of its energy absorbed by the other body. It can work either way. The new orbit it has after collision will reflect that change in KE as a new orbit — voyager I was boosted out of the solar system; these centaurs settle into an orbit closer in. The range of orbital parameters that allow Jupiter, say, to absorb energy from the smaller body is perhaps quite broad, leading to Jupiter “capturing” many of these, at least temporarily and making them an object in the bucket brigade.

Reply to  Dudley Horscroft
February 28, 2021 3:35 pm

Objects already in orbit around the Sun and in Jupiter’s orbit will gather in either the L4 or L5 Lagrange Point, 60 degrees ahead and 60 degrees behind the planet. An object coming into Jupiter’s orbit from elsewhere and being captured in one of those locations would require some kind of change of trajectory, I would think.

February 26, 2021 6:10 am

You know this is not a climate science effort because it does not contain the word unprecedented.

Alan Robertson
Reply to  ResourceGuy
February 26, 2021 8:48 am

But, it could and maybe does. It’s possible that the article behind the paywall might project hypothetical probabilities of viable scenarios with practicable climate worries.
Further study is needed.

Pat Frank
Reply to  ResourceGuy
February 26, 2021 8:52 am

Or ‘worse than we thought.’ 🙂

Reply to  ResourceGuy
February 28, 2021 3:41 pm

Nobody’s been “stunned” either.

Pat Frank
February 26, 2021 8:45 am

NASA, “Simulations show that in about 500,000 years, there’s a 90% probability that this object will be ejected from the solar system and become an interstellar comet.”

Engineering extrapolations indicate a near 100% probability that P/2019/LD2 will become manufacturing raw material within about 1000 years.

Carlo, Monte
February 26, 2021 10:52 am

Visible-light snapshots by NASA’s Hubble Space Telescope reveal

Tech writer alert, level 3…

February 26, 2021 12:51 pm

It’s disappointing that this writer is so insistent that the roaming comet is “near” Jupiter (e.g., “alongside Jupiter”, “near giant Jupiter”). The Lagrange points are 60 degrees ahead of and behind Jupiter in its orbit. That’s 5 times farther from Jupiter than Earth is from the sun.

February 28, 2021 3:29 pm

A compilation of near-Earth events of the last few years, offered without comment:

14 Feb 2014       
The Chelyabinsk meteor was a small asteroid about the size of a six-story building that broke up over the city of Chelyabinsk, Russia, on Feb. 15, 2013. The blast was stronger than a nuclear explosion, triggering detections from monitoring stations as far away as Antarctica.

19 October 2017:   
Oumamua is the first known interstellar object detected passing  through the Solar System. Formally designated 1I/2017 U1, it was spotted by Robert Weryk using the Pan-STARRS at Haleakala Observatory

1 October 2018:      
A meteor caused a massive explosion over China.  It is the second-largest recorded impact in the past century, after the meteor that exploded over the Russian region of Chelyabinsk in 2013. The giant fireball hit at 2350 GMT on 18 December over the Bering Sea, a part of the Pacific Ocean between Russia and Alaska.

16 October 2019:   
Hubble has given astronomers their best look yet at an interstellar visitor comet 2I/Borisov — whose speed and trajectory indicate it has come from beyond our solar system. Comet 2I/Borisov is only the second such interstellar object known to have passed through the solar system.

23 June 2020:      
Earth’s quiet magnetic field was unexpectedly disturbed by a wave of magnetism that rippled around much of the globe. There was no solar
storm or geomagnetic storm to cause the disturbance. So what was it?

25 February 2020   
FBI Confirms Report of ‘Long, Cylindrical’ UFO ‘Moving Really Fast’ Over New Mexico.  “Do you have any targets up here? We just had something go right over the top of us. I hate to say this, but it looked like a long, cylindrical object that almost looked like a cruise missile type of thing—moving really fast right over the top of us.”

26 February 2021   
A comet-like object has been spotted near the Trojan population. After traveling several billion miles toward the Sun, a wayward young comet-like object orbiting among the giant planets has found a object has settled near a family of captured ancient asteroids, called Trojans, that are orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan population.

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