Comet S1 ATLAS from October 19th. Credit: Michael Jaeger and Gerald Rhemann.
Sungrazer C/2024 S1 ATLAS broke apart at perihelion.
Alas, a ‘Great Halloween Comet’ was not to be. The Universe teased us just a bit this month, with the potential promise of a second naked eye comet in October: C/2024 S1 ATLAS. Discovered on the night of September 27th by the Asteroid Terrestrial Last-alert impact System (ATLAS) all-sky survey, this inbound comet was surprisingly bright and active for its relative distance from the Sun at the time of discovery. This gave the comet the potential to do what few sungrazers have done: survive a blisteringly close perihelion passage near the Sun.
A fine sungrazer nears its doom as seen via SOHOs LASCO C2 camera. Image credit: NASA/ESA/SOHO/NRLSungrazers
It’s a discovery that could come any day now.
The Solar Heliospheric Observatory spacecraft known as SOHO is set to cross the 3,000 comet discovery threshold this month. Launched atop an Atlas II rocket on December 2nd, 1995, SOHO is a joint NASA/ESA mission, and has observed the Sun now for almost 20 years from the sunward L1 lagrange point. That fact is amazing enough, as SOHO has already followed the goings on of our tempestuous host star for nearly two full solar cycles.
And though SOHO wasn’t initially designed as a comet hunter extraordinaire, it has gone on to discover far more comets than anyone—human or robotic.
SOHO on Earth. Image credit: NASA/ESA/SOHO
The U.S. Naval Research Laboratory’s (NRL) sungrazer website lists the discovery count as 2,987 as of July 31, 2015, with more comets awaiting verification daily. “In the past, SOHO has often discovered as many as four or five comets in a single day,” Karl Battams, a solar scientist at the NRL told Universe Today. “Suffice to say, it really could be any day now, given how close we are to 3,000! I actually expected it to be a month ago, so I’m surprised it’s dragging out like this. Predicting comets is fraught with uncertainty!”
Part of what gives SOHO an edge is its LASCO (the Large Angle and Spectrometric Coronagraph) C2 and C3 coronagraphs. With a field of view about 15 degrees wide, the C3 imager monitors the faint corona of the Sun, while blocking its dazzling disk. The corona is the pearly white outer atmosphere of the Sun, and is about half as bright as a Full Moon. On Earth, we only see the corona briefly during a total solar eclipse. SOHO routinely sees sungrazing comets ‘photobomb’ the view of its LASCO C3 camera, sometimes to the tune of more than 200 a year.
Comet NEAT makes its way through the field of view of SOHO’s LASCO C2 camera in 2003. Image credit: NASA/ESA/NRL/Sungrazers
SOHO has rewritten the history of sungrazers. How far we’ve come: flashback to 1979, and less than a dozen sungrazers were known, one being the famous Comet Ikeya-Seki in 1965. Early space-based platforms such as Solwind and SMM sported early coronagraphs, and paved the way for SOHO. Think about that for a moment; a vast majority of the cometary population of the solar system was simply sliding by, unobserved from the ground. And this was only a generation ago.
Most of what SOHO sees are what’s termed as Kreutz group sungrazers. First theorized by astronomer Heinrich Kreutz in 1888, SOHO has given researchers the ability to classify and characterize the orbits of these doomed comets. These sungrazers nearly always incinerate during their perihelion passage. C/2011 W3 Lovejoy was a famous exception, which passed about 140,000 kilometers from the surface of the Sun on December 16th, 2011 and went on to become a fine southern hemisphere comet.
“We knew little of the Kreutz population, other than that it seemed there were ‘a few’ objects on the Kreutz path,” Battams said. “I would say that probably when the Sungrazer Project was launched in late 2000 was the point at which the team realized that this was something more than just seeing an occasional comet.”
The typical track of a Kreutz-group comet. Click here for the full diagram of C2/C3 tracks throughout the year. Image Credit: NASA/ESA/SOHO
Kreutz comets also have seasons and predictable directions of approach along the ecliptic as seen from SOHO’s point of view. Some periodic comets, such as 96P Machholz, — which orbits the Sun once every six years — have become old friends. To date, SOHO has observed 96P Machholz four times.
Upping the Comet Hunting Game
But here’s the amazing second half of the tale. Legions of dedicated amateurs make these discoveries, patiently combing over daily images sent back by SOHO. In many ways, SOHO has grown up with the rise of the internet. Think about it: what was your internet surfing experience like way back in 1995? Karl Battams at NRL relays these discoveries to the Central Bureau for Astronomical Telegrams, the clearing house for potential comet discoveries. Founded in 1882 and based at Harvard College Observatory since 1965, CBAT actually received its last ‘telegram’ announcing the possible discovery that would become Comet Hale-Bopp in 1995.
The rise of automated surveys and satellites such as SOHO has definitely upped the game. To date, the all-time human champ amongst comet hunters is Robert H. McNaught, with the discovery of 44 long-period and 26 short-period comets.
And I think we can all remember where we were on U.S. Thanksgiving Day 2013, as SOHO gave us a front row seat to the demise of Comet ISON. It’s been a roller coaster ride for sure, and it’s hard to imagine a time now when we didn’t have SOHO as a daily resource. Heck, it’s just fun to watch planets transit the field of view of SOHO, as they move from the dawn to dusk sky and back again.
Looking at the “SOHO Bump” and the rise of automated comet hunters in the early 21st century. Image credit: Dave Dickinson
Comet hunting via SOHO is fun and easy to do, though yes, there are lots of eyeballs out there looking, so you have some pretty dedicated competition. Patience is key, and there’s also a dedicated message board describing the latest discoveries and known objects entering the field of view that have already been identified.
“What’s the future of SOHO? “December is SOHO’s 20th anniversary, so that’s another milestone,” Battams said. “Beyond that, who knows? Engineers designed SOHO to operate for two years, and with no intention of comet discovery; it has lasted 20 years and re-written the history books for comets. It remains the only coronagraph we have along the Sun-Earth line, so for space weather forecasting it remains a unique and valuable asset.”
Congrats, and be sure to follow Karl Battam’s @SungrazerComets account on Twitter… number 3,000 could be discovered any day now!
This artist’s impression shows exocomets orbiting the star Beta Pictoris. Credit:
ESO/L. Cacada
Between the years 2003 and 2011, the High Accuracy Radial velocity Planet Searcher – better known as HARPS – made more than a thousand observations of nearby star, Beta Pictoris. On board the ESO 3.6-metre telescope at the La Silla Observatory in Chile, the sensitive instrument normally combs the sky nightly in search of exoplanets, but lately it has contributed to another astounding discovery… exocomets!
Located about 63 light-years from the Sun, Beta Pictoris is a youthful star, estimated to be only around 20 million years old. Keeping it company in space is a vast disc of material. This swarm of gas and dust is the beginnings of an active planetary system and was likely created by the destruction of comets and collisions of rocky bodies like asteroids. Now a French team using HARPS has been able to create the most complete catalog of comets to date from this system. Researchers have found no less than five hundred comets belonging to Beta Pictoris and they divide in two unique branches of exocomets. Split into both old and new, these two active flows behave much like our own cometary groups… They have either made many trips around the parent star or are the product of a recent breakup of one or more objects.
Flavien Kiefer (IAP/CNRS/UPMC), lead author of the new study, sets the scene: “Beta Pictoris is a very exciting target! The detailed observations of its exocomets give us clues to help understand what processes occur in this kind of young planetary system.”
Beta Pictoris is located about 60 light-years away towards the constellation of Pictor (the Painter’s Easel) and is one of the best-known examples of a star surrounded by a dusty debris disc. Earlier observations showed a warp of the disc, a secondary inclined disc, and comets falling onto the star, all indirect, but tell-tale signs that strongly suggested the presence of a massive planet. Observations done with the NACO instrument on ESO’s Very Large Telescope in 2003, 2008, and 2009, have proven the presence of a planet around Beta Pictoris. It is located at a distance between 8 and 15 times the Earth-Sun separation — or Astronomical Units — which is about the distance Saturn is from the Sun. The planet has a mass of about nine Jupiter masses and the right mass and location to explain the observed warp in the inner parts of the disc. This image, based on data from the Digitized Sky Survey 2, shows a region of approximately 1.7 x 2.3 degrees around Beta Pictoris. Credit: ESO/Sky Survey II
Just like discovering planets through the transit method, astronomers believe exocomets can cause a disturbance in the amount of light we can see from a given star. When these icy travelers exhaust themselves, their gas and dust tails could absorb a portion of the star light passing through them. For nearly three decades scientists had been aware of minute changes in the light from Beta Pictoris, but attributing it to comets was next to impossible to prove. Their tiny light was simply overpowered by the light of the star and could not be imaged from Earth.
Enter HARPS…
Using more than a thousand observations taken by this sensitive equipment, astronomers chose a sample of 493 exocomets unrelated to each other, but sharing in the Beta Pictoris system. Of these, some were dutifully followed for hours at several different times. The size and speed of the gas clouds produced were carefully measured. Researchers were even able to document the orbital properties of some of these exocomets – the size and shape of their passage paths in relation to the parent star allowing scientists to infer their distances.
Knowing that comets exist around other stars is very exciting – and knowing that solar systems around other stars work much like our own is downright rewarding. Through this study, we’re able to take a unique look at what might be several hundreds of exocomets connected to a solitary exo-planet system. What the research has revealed is two distinct branches of the comet family tree. One of these is old comets – their orbit dictated by a single, massive planet. The other half of the family fork belongs to comets that might have arisen from the destruction of a larger object.
The older group behaves in a predictable manner. These exocomets have differing orbital patterns, and their gas and dust production is greatly reduced. If they follow the same rules as the ones in our solar system, it’s typical behavior for a comet which has exhausted its volatiles during multiple trips around the parent star and is also being controlled by the system’s massive planet. This is exciting because it confirms the planet’s presence and distance!
“Moreover, the orbits of these comets (eccentricity and orientation) are exactly as predicted for comets trapped in orbital resonance with a massive planet.” says the science team. “The properties of the comets of the first family show that this planet in resonance must be at about 700 million kilometres from the star – close to where the planet Beta Pictoris b was discovered.”
The second group also behaves in a predictable manner. These exocomets have nearly identical orbits and their emissions are active and radical. Observations of this cometary type tell us they more than likely originated from the destruction of a larger body and the rubble is caught in a orbit which allows the fragments to graze Beta Pictoris. According to the research team: “This makes them similar to the comets of the Kreutz family in the Solar System, or the fragments of Comet Shoemaker-Levy 9, which impacted Jupiter in July 1994.”
Flavien Kiefer concludes: “For the first time a statistical study has determined the physics and orbits for a large number of exocomets. This work provides a remarkable look at the mechanisms that were at work in the Solar System just after its formation 4.5 billion years ago.”
