Occultations of Triton

Occultations of Triton

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I've been listening to an episode of the "NASA in Silicon Valley" podcast about the SOFIA mission.

One of the techniques they use at SOFIA is stellar occultations to study different objects.

My impression was that the occultations by Pluto happen much more often than by Triton. Is this true and why is that?

Pluto is actually smaller in diameter than Triton, and is also farther away, meaning that Triton covers roughly 1.4x (according to WA) the angle that Pluto does, making occultation that much more probable apriori, ignoring their actual orbits.

In addition to the above, New Horizons recently observed Pluto's atmosphere with far more detail than what we can do from here on Earth, so it makes sense to measure a different target.

Occultation Techniques

Cooray, A. R., J. L. Elliot, Scintillation caustics in planetary occultation light curves, Astrophysical Journal Letter, 587, 121-124, 2003.

Elliot, J. L., E. W. Dunham, C. B. Olkin, Exploring small bodies in the outer solar system with stellar occultations, In Proceedings of the Airborne Astronomy Symposium on the Galactic Ecosystem: From Gas to Stars to Dust (M. R. Haas, J. A. Davidson and E. F. Erickson, Ed.), pp. 285-296, ASP, San Francisco, 1995.

Elliot, J. L., C. B. Olkin, Probing Planetary Atmospheres with Stellar Occultations, In Annual Review of Earth and Planetary Sciences (G. W. Wetherill, A. L. Albee and K. C. Burke, Ed.), pp. 89-123, Annual Reviews Inc., Palo Alto, 1996.

Olkin, C. B., J. L. Elliot, Occultation astrometry: Predictions and post-event results, In Galactic and Solar System Optical Astrometry (L. V. Morrison and G. F. Gilmore, Ed.), pp. 286-290, Cambridge Univ. Press, Cambridge, 1994.

Stone, R. C., S. W. McDonald, J. L. Elliot, 5145 Pholus stellar occultation candidates: 1999-2005, Astronomical Journal, 118, 591-599, 1999.

Stone, R. C., S. W. McDonald, J. L. Elliot, E. Bowell, 10199 (Chariklo) stellar occultation candidates: 1999-2005, Astronomical Journal, 119, 2008-2017, 2000

Neptune's Moon Triton: Summer sky of methane and carbon monoxide

According to the first ever infrared analysis of the atmosphere of Neptune's moon Triton, summer is in full swing in its southern hemisphere. The European observing team used ESO's Very Large Telescope and discovered carbon monoxide and made the first ground-based detection of methane in Triton's thin atmosphere. These observations revealed that the thin atmosphere varies seasonally, thickening when warmed.

"We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away. This icy moon actually has seasons just as we do on Earth, but they change far more slowly," says Emmanuel Lellouch, the lead author of the paper reporting these results in Astronomy & Astrophysics.

On Triton, where the average surface temperature is about minus 235 degrees Celsius, it is currently summer in the southern hemisphere and winter in the northern. As Triton's southern hemisphere warms up, a thin layer of frozen nitrogen, methane, and carbon monoxide on Triton's surface sublimates into gas, thickening the icy atmosphere as the season progresses during Neptune's 165-year orbit around the Sun. A season on Triton lasts a little over 40 years, and Triton passed the southern summer solstice in 2000.

Based on the amount of gas measured, Lellouch and his colleagues estimate that Triton's atmospheric pressure may have risen by a factor of four compared to the measurements made by Voyager 2 in 1989, when it was still spring on the giant moon. The atmospheric pressure on Triton is now between 40 and 65 microbars -- 20 000 times less than on Earth.

Carbon monoxide was known to be present as ice on the surface, but Lellouch and his team discovered that Triton's upper surface layer is enriched with carbon monoxide ice by about a factor of ten compared to the deeper layers, and that it is this upper "film" that feeds the atmosphere. While the majority of Triton's atmosphere is nitrogen (much like on Earth), the methane in the atmosphere, first detected by Voyager 2, and only now confirmed in this study from Earth, plays an important role as well. "Climate and atmospheric models of Triton have to be revisited now, now that we have found carbon monoxide and re-measured the methane," says co-author Catherine de Bergh.

Of Neptune's 13 moons, Triton is by far the largest, and, at 2700 kilometres in diameter (or three quarters the Earth's Moon), is the seventh largest moon in the whole Solar System. Since its discovery in 1846, Triton has fascinated astronomers thanks to its geologic activity, the many different types of surface ices, such as frozen nitrogen as well as water and dry ice (frozen carbon dioxide), and its unique retrograde motion*.

Observing the atmosphere of Triton, which is roughly 30 times further from the Sun than Earth, is not easy. In the 1980s, astronomers theorised that the atmosphere on Neptune's moon might be as thick as that of Mars (7 millibars). It wasn't until Voyager 2 passed the planet in 1989 that the atmosphere of nitrogen and methane, at an actual pressure of 14 microbars, 70 000 times less dense than the atmosphere on Earth, was measured. Since then, ground-based observations have been limited. Observations of stellar occultations (a phenomenon that occurs when a Solar System body passes in front of a star and blocks its light) indicated that Triton's surface pressure was increasing in the 1990's. It took the development of the Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES) at the Very Large Telescope (VLT) to provide the team the chance to perform a far more detailed study of Triton's atmosphere. "We needed the sensitivity and capability of CRIRES to take very detailed spectra to look at the very tenuous atmosphere," says co-author Ulli Käufl. The observations are part of a campaign that also includes a study of Pluto [eso0908].

Pluto, often considered a cousin of Triton and with similar conditions, is receiving renewed interest in the light of the carbon monoxide discovery, and astronomers are racing to find this chemical on the even more distant dwarf planet.

This is just the first step for astronomers using CRIRES to understand the physics of distant bodies in the Solar System. "We can now start monitoring the atmosphere and learn a lot about the seasonal evolution of Triton over decades," Lellouch says.

*Triton is the only large moon in the Solar System with a retrograde motion, which is a motion in the opposite direction to its planet's rotation. This is one of the reasons why Triton is thought to have been captured from the Kuiper Belt, and thus shares many features with the dwarf planets, such as Pluto.

The team is composed of E. Lellouch, C. de Bergh, B. Sicardy (LESIA, Observatoire de Paris, France), S. Ferron (ACRI-ST, Sophia-Antipolis, France), and H.-U. Käufl (ESO).

Story Source:

Materials provided by ESO. Note: Content may be edited for style and length.

A flash from Triton

Like, how can you learn more about the extremely tenuous atmosphere around an icy moon orbiting a planet 4.5 billion kilometers away?

The fun thing is, we can answer it: Wait until the moon passes directly in front of star, and watch the atmosphere flash at you*.

Triton is the largest moon of Neptune, the farthest planet from the Sun. It's a big moon, 2,700 kilometers across, and because it's so cold out there — about 40 K, which is -230°C or -390°F — its surface is covered in ices like frozen nitrogen and methane.

However, it just barely gets warm enough that during local summer the nitrogen thaws, and turns into a gas. A ridiculously thin one, covering the moon in an atmosphere only about 0.000015 as dense as Earth's. That's practically a vacuum. On our planet you'd have to go up practically into space to get to where our air is that attenuated.

But it's there. In fact, there's even wind, such as it is when Voyager 2 passed Neptune and Triton in 1989 it saw aligned black streaks on the moon's surface, evidence of vents of nitrogen and carbon-rich dust erupting out of the surface and getting blown downwind.

Voyager 2 flew past at high speed, and couldn't study Triton's surprising atmosphere very much. But there's another way to do that, and it doesn't even involve leaving Earth.

Every now and again, an object in the solar system will pass directly in front of a star as seen from Earth. We call these events occultations, and they're relatively rare. For small and/or distant bodies they're even more rare, so Triton doesn't do this terribly often. But astronomers predicted that on October 5, 2017, the frigid moon would occult the star UCAC4 410-143659 (or, if you prefer, it's also called 2MASS 22541840-0800082 and/or USNO B 0819-0776968). The star isn't terribly bright, but it's brighter than Triton at nearly all wavelengths, making this event easier to see if you observe it you'd see the star suddenly blink out, and only the light from Triton would remain. A little less than three minutes later, the star would pop out from the other side of Triton, appearing much as it did before.

The event was visible across Europe and the United States, and nearly 80 different observatories were pointed Neptuneward on that evening. And they caught it!

Neptune is the bright blob, and Triton + the star just to its upper left. The star suddenly gets fainter when Triton blocks it, then reappears moments later (in the animation several minutes later in real time).

But… did you see that bright flash of light right in the middle of the observation? The real-time plot helps rewatch it if you need to.

That event is called a central flash, and it's due to the starlight being bent by Triton's thin atmosphere! Air can act like a lens, bending the path the light takes. In this case, when the star was directly behind Triton (that is, the Earth, Triton, and the star made an almost exact straight line) the air all around the limb of the moon bends the light toward us, and we see a flash of brightness from it. This happened with Saturn's moon Titan in late 2001, and I wrote an article about that describing how it works. The European Space Agency (ESA) also put together an explainer video:

Astronomers are still analyzing the data, but expect to learn a something about Triton's thin nitrogen atmosphere from the event. The total brightness of the flash and how it rises and fades depends on the vertical structure and layering of the gas, so they can use this occultation to probe the atmosphere.

That's very cool. But it gets cooler.

From Earth, Triton's disk is incredibly small, only about 0.1 arcseconds across. For comparison, our own Moon in the sky is about 18,000 times bigger! The star is so far away it's essentially a point source, a dot. The alignment, therefore, had to be extremely precise to get this right. Like a solar eclipse, you have to be in the right place at the right time to see it it you are too far north or south, the moon would miss the star.

Making this harder is that extremely precise measurements of a star's position are hard to get, so using older positional data for the star meant the path of the occultation's visibility wasn't as accurate as it could be. So astronomers turned to an incredibly precise machine: Gaia.

This is an ESA mission that is mapping the positions of a billion stars in the sky. Yes, a billion. The Gaia team had already released some data, but the star in question wasn't due to have its info released until after the occultation! So astronomers made a special plea to the team to help them, and the Gaia folks jumped in, measuring the star's position in the sky (as well as a bunch of other stars near it) and giving it to the people observing the occultation.

Not only that, they had to compensate for the star's motion! All stars in our galaxy orbit the galactic center, and slowly move across the sky. We call this proper motion, and because stars are so far away the motion is small, but this event was so strict that the star's velocity made a difference. All together, the star's position using Gaia was found to be 0.0142 arcseconds different from the old position, moving the path of visibility about 300 kilometers south.

central line, and the red the actual center line measured using observation. The light blue was the predicted southern limit and the dark blue the actual one. Yellow pins are observatories. Credit: Google, INEGI, ORION-ME annotation: ERC Lucky Star project

And it worked! They nailed it. In the diagram above, the pink line was the predicted center of the path using the Gaia data, and the red line is the path determined after the fact using the occultation data. The Gaia data were off by a mere 0.002 arcseconds, an incredible achievement.

I know that part of it may not sound as sexy as the observations themselves, but the Gaia data allowed a far more precise measurement of the event, which is critical to the science itself. Knowing whether the star passed exactly through Triton's center or instead made a shallow chord across the disk is essential for understanding the atmospheric structure, and the new data allows all the observations to be combined in way to greatly enhance what was learned.

And the precision! I'm still stunned by that. Take a tennis ball, which is about 6.7 centimeters across. The difference from the predicted and actual position of the star from the Gaia data would be the difference between the left side of that tennis ball and the right… if it were nearly 7,000 kilometers away. That's far wider than the continental US.

I glow with pride hearing things like this. We're really good at astronomy, folks, and we keep getting better. This is amazing stuff, and just barely scratching the surface of what Gaia will do. A revolution in astronomical measurement is coming, and our capabilities will expand tremendously because of it.


A variety of CCD astrometric data was used to predict the location of the path for the occultation of the star we have denoted “Tr176” by Triton, which occurred on 1997 July 18, and was visible from locations in northern Australia and southern North America. A network of fixed and portable telescopes equipped with high-speed photometric equipment was set up to observe the event, with the following observational goals: (i) mapping the central flash (to establish the global shape of Triton's atmosphere at about 20-km altitude by modeling the detailed shape of the central flash), (ii) obtaining one or more light curves of high signal-to-noise ratio from a large telescope (to accurately determine the thermal structure of Triton's atmosphere), and (iii) obtaining light curves distributed across Triton's disk (to probe the thermal structure of Triton's atmosphere above different areas and to establish the shape of the atmosphereat about 100-km altitude by modeling the half-light surface). Although the large, fixed telescopes proved to be outside of the occultation shadow and observations with some of the portable telescopes were foiled by clouds, light curves were successfully recorded from Brownsville, Texas, and Chillagoe, Queensland. These were combined with data from another group to determine the radius and shape of the half-light surface in Triton's atmosphere and the equivalent-isothermal temperatures at the sub-occultation latitudes on Triton. A circular solution for the half-light surface (projected into Triton's shadow) yielded a radius of 1439±10 km. However, the data are indicative of a global shape more complex than a sphere. Such a figure is most likely caused by strong winds. Light-curve models corresponding to the best fitting circular and elliptical atmospheres were fit to the data. The mean pressure at 1400-km radius (48-km altitude) derived from all of the data was 2.23±0.28 μbar for the circular model and 2.45±0.32 μbar for the elliptical model. These values suggest a global pressure increase at this level since a previous Triton occultation in 1995 August. The mean equivalent-isothermal temperature at 1400 km was 43.6±3.7 K for the circular model and 42.0±3.6 K for the elliptical model. Within their (sometimes large) uncertainties, the equivalent-isothermal temperatures agree for all Triton latitudes probed.

Pinpoints of Light: Triton Occultation Prediction

Our telescope operator is sprinting between the telescope and auxiliary buildings, leaping over small clumps of grass as gracefully as one of the local pronghorn antelope. Heidi Larson knows that we are pressed for time, and is going through all of the startup checks at the 4.3-meter Discovery Channel Telescope as quickly as she can, while still being complete.

We are preparing for a stellar occultation by Triton, to be observed on SOFIA (the Stratospheric Observatory for Infrared Astronomy). Minutes count, as we have a hard deadline by which the flight planners need to receive information. Where will SOFIA be flying to catch this occultation?

By their nature, stellar occultation observations are usually large collaboration events this is even more true when SOFIA is in the mix. We are attempting to predict the location of the central flash of Triton, some 100 km wide at a distance of 4.5 billion km. In angular terms, this is about 1 milli-arcsecond. To achieve this, we work with collaborators to refine the astrometry of Triton and the occultation star, and thus refine the prediction. We are hoping our prediction will be accurate enough to navigate SOFIA into this tiny central flash zone, thereby providing data that will probe the lower levels of Triton's atmosphere that are not accessible with a non-central occultation.

To achieve this level of accuracy, we had the benefit of ultra-precise positions and proper motions for the occultation star AND the other stars nearby. These positions were from the Gaia Data Release 2 catalog, made public by the Gaia team for use by those planning to observe this occultation.

The data from the DCT and another telescope across town at the U.S. Naval Observatory have been reduced, using the Gaia pre-release star positions. The revised prediction has been uploaded to our website for this event astronomers across the eastern U.S., Europe, and northern Africa will get this information and adjust their observing plans, if they're able.

At a few minutes before 3pm EDT today (5 October 2017), the 747SP that is SOFIA will taxi down the runway at the Daytona Beach International Airport in Florida, and will take off for a night of observing the Triton occultation--and maybe the central flash.

Dr. Amanda Bosh is a Senior Lecturer in EAPS, and is the lead of the Astrometry Team. She and EAPS Senior Research Associate Carlos Zuluaga, have just sent the updated prediction to the SOFIA flight planning team, and will now be watching anxiously to see what happens.

EAPS Research Scientist and Wallace Observatory Director Dr. Michael Person is the Principal Investigator for the Triton occultation observations on SOFIA, and will be on SOFIA collecting data during the event.

Update: The observation of the Triton Occultation aboard SOFIA was a success!

Dr. Michael Person reports that the team collected data with three instruments simultaneously, including the HIPO camera developed jointly between Lowell Observatory and MIT , and confirmed that the prediction sent by Dr. Amanda Bosh was correct as the central flash was observed!

Analysis has already begun, with the preliminary light curves suggesting interesting features in Triton’s atmosphere. more updates coming soon!

Story Image: The Stratospheric Observatory for Infrared Astronomy SOFIA courtesy Michael Person

Occultations of Triton - Astronomy

Journal issue: 2017 August
Pages: 194&ndash195

Log in or join the BAA today to view the full article in PDF format.

On 2017 October 05 Neptune’s largest satellite Triton will occult the 12.7 V-mag star UCAC4 410-143659, as seen from the US east coast, Northern Africa, and Europe. After almost 10 years since the last documented Triton occultation (observed on 2008 May 21), this event will be a new opportunity to gather data about Triton’s current atmospheric state and possible changes.

Scientific rationale
The stellar occultation technique is a very powerful tool for probing and monitoring planetary atmospheres. Not only can the atmospheric pressure be measured, but also haze layers can be detected through multi-wavelength observations, and by observing the so-called central flash, wind regimes in the atmosphere can be analysed.
Key questions are:
- What is the current atmospheric state (pressure)?
- Are there any (drastic) changes since the 1990s?
- Are the haze layers seen by Voyager 2 in 1989 still present?
- Can wind regimes be constrained from central flash observations?

Occultation predictions for this event are provided by several sources. For example by theEuropean Research Council (ERC) Lucky Star project group ( and by the author on his website ( See also Figure 2. The main occultation circumstances for the UK are given in Table 1.

Observation campaigns
Further information in addition to practical tips and suggestions will be available from a variety of websites, like the International Occultations Timing Association, European Section (IOTA-ES) (, BAA ARPS ( and dedicated individual webpages such as that by Tim Haymes of the ARPS: Announcements on mailing lists like PLANOCCULT and the release of a BAA e-bulletin are also planned.
It is noteworthy that SOFIA (Stratospheric Observatory for Infrared Astronomy) is also scheduled to observe this occultation.

Mike Kretlow, IOTA-ES

Tim Haymes (BAA ARPS) adds:

Many thanks to Mike for an excellent introduction to this important occultation.

Suggested instrumentation would be of long focal length (e.g. SCT or Newtonian + Barlow lens) and CCD/video at 1 sec exposure (say), shorter if possible. We recommend trial observations of Triton on an earlier night (preferably the night before), or during the lead up to the event itself. The occulted star will be brighter than Triton but a full Moon will also be close by, so it may be worthwhile erecting a temporary 'moonshade' to prevent moonlight entering the telescope directly. Since timing is important, computer clocks need to be sync’ed to UT. For more information on video & CCD monitoring of occultations, see the web pages referenced in Mike Kretlow’s article.

The ARPS Section would also be pleased to receive visual observations from observers using telescopes of 30cm aperture or larger in good seeing. At 10 arcseconds separation from Neptune, the 12.5-mag star will be a difficult object but do give it a try! Visual observers could make audio recordings (e.g. with a smartphone), adding a UT time marker at the start and end of recording to a precision of ±0.5sec or better.

Tim Haymes, Assistant Director, Occultations (Login or click above to view the full illustrated article in PDF format)

20170523 Pre-release Triton Chariklo - Gaia

On 22 June and 23 July 2017 relatively brights stars will be occulted by the largest known centaur Chariklo. The object is unique due to the ring system around it. By observing the occultation, a better shape of Chariklo and the detailed structure of the rings can be obtained. Knowing these characteristics improves our understanding of the ring stability and formation.

Figure 1: Artist impression of a close-up of the rings around Chariklo
(Image credit: ESO/L. Calçada/M. Kornmesser/ Nick Risinger ( ))

Even more unique is the occultation of a star on 5 October 2017 by Triton, the largest moon of Neptune. Triton occultations of suitable stars are extremely rare and can be used to study its atmosphere. While Gaia DR1 positions for these stars are very accurate, there are no proper motions available. This leads to large uncertainties in planning for the ground-based occultation observing campaigns. In order to help preparations for these unique events, we make the preliminary astrometric solutions for these stars, prepared for Gaia DR2, public.

Figure 2: Image of Triton, Neptune's largest satellite, taken on 22 August 1989 by Voyager 2
(Image credit: NASA/JPL)

In the table below Gaia presents the three stellar positions to the astronomy community, taken from preliminary Gaia DR2 data, for epoch 2015.5 in the ICRF reference frame.

Occultations of Triton - Astronomy




  • [Dec 6] - Minor Planet results for 2020 - Updated
  • [Oct 12] - Journal for Occultation Astronomy 2020 #4
  • [Oct 3] - Minor Planet Predictions for 2021
  • [Jul 16] - Journal for Occultation Astronomy 2020 #3
  • [Apr 15] - Journal for Occultation Astronomy 2020 #2
  • [Jan 26] - Minor Planet results for 2019 - COMPLETE LISTING FINAL
  • [Jan 26] - Journal for Occultation Astronomy 2020 #1



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  • What's an Occultation?
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TTSO14 - The Fourteenth Trans-Tasman Symposium on Occultations
The Services Club, Parkes , New South Wales, Australia - 13 April 2020

Occultations occur when one celestial body passes in front of another.
These frequent and intriguing events are fun to watch, and provide an important way for amateur astronomers to make significant discoveries about objects within our own Solar System as well as the stars beyond.

This website exists to promote and encourage occultation
observing in New Zealand, Australia and the South Pacific.

On this site you will find information about different sorts of occultations, and how to contribute useful scientific results by observing occultations.
The site contains all the background information needed by someone new to occultation observing. It also provides more technical information of use to advanced observers.

The RASNZ Occultation Section is also proud to represent the interests of the International Occultation Timing Association (IOTA) in the Australasian region.

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Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2007 March 4
Triton: Neptune's Largest Moon
Credit: Voyager 2, NASA

Explanation: In October of 1846, William Lassell was observing the newly discovered planet Neptune. He was attempting to confirm his observation, made just the previous week, that Neptune had a ring. But this time he discovered that Neptune had a satellite as well. Lassell soon proved that the ring was a product of his new telescope's distortion, but the satellite Triton remained. The above picture of Triton was taken in 1989 by the only spacecraft ever to pass Triton: Voyager 2. Voyager 2 found fascinating terrain, a thin atmosphere, and even evidence for ice volcanoes on this world of peculiar orbit and spin. Ironically, Voyager 2 also confirmed the existence of complete thin rings around Neptune - but these would have been quite invisible to Lassell!

Watch the video: The TRITON Occultation of 2017Oct05 (August 2022).