# How long would an occultation by a TNO last?

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A trans-Neptunian object, 1 million km from the observer, with an angular diameter of 0.126° occults the Sun (angular diameter of 0.004°) and the TNO and the observer are moving in the same direction, in the same plane. The TNO is moving 8m/sec with respect to the observer, how long would the occultation last?

Assume the observer is 18 billion km from the Sun.

From the observer's point of view 1 million km away, the TNO's apparent angular motion is

$$mathrm{frac{8~m/s}{10^9~m} = 8 imes 10^{-9}~rad/s = 0.00165~^circ/h}.$$

Assuming that the observer at 18 billion km = 120 au is in a circular orbit around the Sun, the orbital period is 1203/2 = 1320 years, making the Sun appear to move the other way at

$$mathrm{frac{360~^circ}{1320~y} = 0.27~^circ/y = 3.1 imes 10^{-5~circ}/h},$$

so the TNO's apparent motion relative to the Sun is

$$mathrm{0.00165~^circ/h + 3.1 imes 10^{-5~circ}/h = 0.00168~^circ/h}.$$

The Sun would be totally occulted for

$$mathrm{frac{0.126~^circ - 0.004~^circ}{0.00168~^circ/h} = 72.6~h}$$

and partially occulted for

$$mathrm{frac{0.004~^circ}{0.00168~^circ/h} = 2.4~h}$$

at each end.

## Discovery of close binary trans-Neptunian object

A new study authored by Southwest Research Institute scientists Rodrigo Leiva and Marc Buie reveals the binary nature of a trans-Neptunian object (TNO). Leiva and Buie utilized data obtained by the Research and Education Collaborative Occultation Network (RECON), a citizen science research net-work dedicated to observing the outer solar system. The study was published this month in The Astrophysical Journal.

Trans-Neptunian objects (TNOs) are small icy bodies that orbit the Sun beyond Neptune. Binary TNOs occur when two of these objects orbit each other while together or-biting the Sun. Leiva and Buie discovered two objects in a particularly close gravitational configuration. The pair was detected using a stellar occultation, which occurs when an object passes between Earth and a distant star which hides, or "occults," the star from view. Observers located in the path of the object's shadow can record the star blinking out and reappearing. The length of time that the object blocks the starlight can be used to determine its size.

"In this instance, the occulted star also turned out to be a binary system. Binary stars are not unusual and binary objects are not unusual," Buie said. "But it is unusual that we had a binary TNO occulting a binary star."

"What's also interesting and unusual is this object's characteristics," Leiva said. "The two components are quite close, only 350 kilometers apart. Most binary TNOs are very separated, usually 1,000 kilometers or more. This closeness makes this type of binary TNO difficult to detect with other methods, which is what RECON was designed to accomplish."

The discovery of the new TNO was made possible by RECON, a collection of 56 observation stations stretching from Yuma, Arizona, to Orville, Washington. The NSF-funded project provides each station with an array of observation equipment, including 11-inch telescopes. High school teachers are trained by Leiva, Buie and Fiske Planetarium Director Dr. John Keller to operate the stations and observe occultations so they can then teach students how to make the same observations. RECON has seen several students go on to do research related to their observations in college.

"To me this project is citizen science at its best," Buie said. "They're learning as well as making observations and helping to collect data. If they didn't do this, we wouldn't learn about these objects."

RECON stations are commonly placed in small communities along an ideal line, from the southern to the northern border of the United States, for observation of stellar occultations. Eight additional stations were established in Canada in 2018 by colleagues of Leiva and Buie.

Going forward, Leiva and Buie will continue to search for previously unobserved TNOs, with the aim of discovering whether close binaries are common or unusual in our Solar System.

"Most models of the Solar System indicate that binaries are very common, particularly close binaries like this one," Leiva said. "If you have an accurate measurement of how common they are, you can fine tune these models."

"Our overarching aim is to know how common close binary TNOs are," Buie said. "Is this object one in a million or just like 90% of them? This is fueling our knowledge for building better models of how the Solar System formed."

The last positive occultation captured in North America in 2015 was the occultation of the star 2UCAC 39956822 by the asteroid (96) Aegle. Overall, there were 8 observers who observed the event. There were 7 positive observations and one miss observation. Three of the positives were recorded by RECON team members, Chris Patrick, Steve Bock, and Tony George. This blog will document the data contributed by the RECON members.

Here is the path map for the event. You can see the path crosses through central Arizona and southern Nevada, where three RECON members are located. Other observers in Arizona and central California were also able to observe the event.

Light Curve Analysis

All videos were uploaded via DropBox to an IOTA repository. DropBox is a tool used by IOTA for capturing and exchanging video files. Each of the video files were then analyzed by Tony George using Limovie. Limovie is a tool for commonly used by IOTA for video light curve analysis. Here are the three light curves that were captured by RECON members:

These three light curves have some interesting characteristics that should be noted.

The Chris Patrick light curve is very erratic and during the event, a very flat and uniform bottom. These are characteristics of the camera settings, which did not match the standard RECON guidelines. Whenever observing asteroidal occultations, RECON guidelines should be used for camera settings unless otherwise directed by the campaign organizer.

The Steve Bock light curve looks very ‘skimpy’. Again, this is the result of camera settings, most significantly, the degree of sense-up that was set. Steve used a sense-up of 64x. This integrates 32 frames which means that each block of 32 video frames looks essentially the same. Integration also cuts down on the amount of variation between individual frames and blocks of integrated frames. A sense-up of 64x is a common RECON guideline for TNO occultation observations, however it may be too much integration for use in brighter asteroid occultations.

When compared to Tony George’s light curve, you can see the degree to which a high sense-up can affect the look of the light curve. Tony used a sense-up of 2x. This integrates two video fields into one frame, so there are 30 frames per second that are all independent of adjacent frames.

While Tony’s light curve might not look good, it actually is the best of the three light curves for determining the correct D (disappearance) and R (reappearance) times from the video. That is because integration (sense-up) averages the video data and the D or R might occur somewhere within the integration block and the actual time must be estimated from the brightness value of the block of frames. With Tony’s video, the times can be derived to times less than 1-frame accuracy.

Occultation Disappearance and Reapperance Analysis

Once each video file was analyzed in Limovie, a data file is created in a comma separated variable format (csv). That csv file can be analyzed by different software applications to search for hard-to-find occultations, however this occultation was easy to see. The software programs can also be used on easy-to-see occultations, particularly those with video integration, so that more accurate estimates of the D and R times can be derived. Tony George used the program R-OTE (R-code Occultation Timing Extractor) to analyze all three light curves. The D and R times were determined and provided to the observers so they could send in their reports on a standard Excel spreadsheet provided by IOTA. For those that have OccultWatcher on their computers, the Excel form is easily available through a reporting app in OccultWatcher.

Asteroid Profi le An alysis

Once the various observation D and R times are sent to IOTA for analysis, a powerful program called Occult4 is used to combine the times and geographic coordinates to determine the size and shape of the asteroid. For each observer, a chord across the earth is developed indicating where and when the star was visible. Where the star disappears, there is a gap in the chord equivalent to the length of time of the disappearance. When multiple chords are combined, the gaps in the chords trace out the size and shape of the asteroid. Here is the profile developed for the (96) Aegle event:

Note: Click on the above image to get a high-resolution view of the plot

The chords of observations by the various observers are shown in different colors. The chords of the three RECON observers are Chord 1: Chris Patrick Chord 2: Steve Bock and, Chord 5: Tony George. You can see from this plot the rough outline of the size and shape of the asteroid. Superimposed on the plot of the chords is the ellipse of best fit to the openings in the chords. The size of the asteroid as determined by this analysis is and ellipse with major axis of 169.6 km and minor axis of 163.0 km. This is one of the best observations ever obtained of the asteroid (96) Aegle. While this is only one snapshot of the asteroid on the date of the observation, future observations can determine the size and shape from other perspectives and hence the volume and density of the asteroid can also be determined from amateur astronomer observations. This type of data is very helpful to astronomers and space scientists as they continue to characterize the main belt asteroids and try and made decisions on asteroids to visit on future space flights.

RECON Observations Were Critical to Getting Size and Shape of (96) Aegle

The determination of the size and shape of an asteroid by occultations requires a good spread of observers across the path. In the case of the (96) Aegle event, the two Chords collected by RECON members Chris Patrick and Steve Bock were critical in setting the northern limb of the asteroid. Without those two chords, the true size and shape of (96) Aegle could not be determined from the other chords. This shows that occultation astronomy is a team sport. It takes a variety of observations to get the size and shape of an asteroid.

Opportunity to Participate in Future Main Belt Asteroid Events

While not a focus of the RECON project, RECON members can participate in the observation of main belt asteroids to sharpen their skill in doing occultations. The probability of getting a positive with a main belt asteroid event is higher for those within the path, since the orbits of main belt asteroids are better known than TNO’s. In the future, Tony George will be sending out alerts for main belt asteroid occultations predicted to cross the observing sites of RECON members. Typically, for a large main belt asteroid, the path may go over 3 or 4 RECON observers in the network. Watch for postings of favorable main belt asteroid events on the tnorecon email list. With luck, we will see your groups chord on a future profile plot.

## Summer Interns Capture Asteroid Occultation

The SETI Institute's summer internship research program, which this year includes 13 Research Experience for Undergraduates (REU) students, and two STEM Teacher and Researcher (STAR) students, is underway at the SETI Institute and, in a couple of cases, virtually. And two students, Yuki Matsumura from CalPoly and Peter Santana-Rodriguez from the University of Puerto Rico, have already had the opportunity to observe an asteroid occultation.

An occultation is when an object, such as a planet, moon or asteroid, passes in front of another object (such as a star), blocking the view of whatever is behind it. On June 14, 2021, the main-belt asteroid 2426 Simonov was predicted to occult a 10-mag star and cast its shadow above the Bay Area of California. Shortly before 10 pm PDT, the students and their mentor, Dr. Franck Marchis, traveled throughout the Bay Area to capture the nearly 1.5-second occultation. While observing, they saw this event live, and after processing their data, a positive detection was confirmed!

The REU program at the SETI Institute has been running since 2006 and is for highly motivated students interested in astronomy, astrobiology and planetary science. They work with scientists at the SETI Institute and NASA Ames Research Center on microbiology, planetary geology, observational astronomy and SETI.

This summer, Marchis and his students will be conducting scientific investigations of comets and active asteroids, and studying exoplanets by transit with the Unistellar’s eVscope network. The network consists of approximately 5,000 digital telescopes that allow citizen astronomers to observe the universe from almost any place on Earth, including areas such as light-polluted cities. The students process and analyze the data collected by network members worldwide and work with them to facilitate the experiments they are conducting.

“As usual with occultation, this observation was an adventure. We quickly realized that we would not be able to observe the event from the SETI Institute parking lot because of the limited visibility, so we drove toward Cupertino and tried to find a good spot,” said Marchis. “We got kicked out from the spot we had identified on the map, so we improvised and found a spot at McClellan Ranch Preserve a half hour before the event. If a few minutes, so students had set up the eVscope, and we were ready. We saw the star disappearing almost at the predicted time, so we quickly realized that we had been successful. Despite the long day, the spirit on the way back home was really positive, and I am sure the students will remember this scientific adventure.”

The next step will consist of gathering any additional observations from other telescopes and estimate the size and shape of this

50km main-belt asteroid. The Unistellar SETI Institute team is already planning more occultation events in the Bay Area to involve REU students and citizen astronomers who have an eVscope.

The students will participate in several observation events throughout the summer and collaborate with Marchis on any papers that announce their discoveries.

## How long would an occultation by a TNO last? - Astronomy

Regional Coordinators for Asteroidal Occultations

Statement from John Talbot: "I am member and past Chairman of Wellington Astronomical Society (WAS) and a member of the Royal Astronomical Society of NEW Zealand (RASNZ). I was the webmaster for a while. I have been an observer of Occultations for about 8 years and for a while I was the RASNZ coordinator and collected reports and did some analysis before forwarding to IOTA. That job got passed to Steve Kerr when I retired. I also wrote a few Excel macros that make the loading of reports quicker. I suspect this was a primary reason for the award. Since my stroke in 2014, I have not managed to get my 12-inch rig working again. I hope to get my 4.5 Meade to working near future but that will restrict me to mag 4 and brighter events. :-(

Statement from Steve Kerr:

About Eric Frappa: "Eric Frappa is a French amateur astronomer and, since 2003 through his website Euraster, the regional coordinator of IOTA asteroidal observations for Europe. As an active observer himself, he has been involved to date in more than 140 positive reports using his own mobile equipment on the field and two robotic telescopes (TAROT) in France and Chile provided by A. Klotz (CNRS-IRAP). He also participated in several international campaigns for TNO and large satellite occultations with the Paris Observatory team, including a pre-New Horizons accurate measurement of Charon's radius. He was honored in 2005 by having an asteroid, 20246 Frappa, named after him.

Statement from Tsutomu Hayamizu: I've never imagined that the big award would be given to me. Perhaps this will be the heaviest honour for my life. The reason of this prize is Leadership in Regional Coordination of IOTA Asteroidal Observations and Continuing Contributions of Occultation Measurements. Then I would like to thank Japanese occultation observers for this award. And, I say congratulations to Brad, Eric, John and Steve, who win the prize together. The observation of occultations stands out as a field where amateurs contribute to astronomy. Especially Japan is a small country, but it is a good region for the observation of occultations, because, Japan has long area from north to south and have many excellent observers. Japan is also located at an important position because the USA, Europe and Japan can cooperate with each other to cover the whole sky of the northern hemisphere. Also in the future, Japan should continue to play an important part. I would like to make an effort to play that part, too. I wish I could receive this award with Takashi Setoguchi, who died young two years ago, because he provided predictions of many important occultation events for us and he taught me how to analyze occultation observations. I would like to respect him for his achievement and share the thanks of winning the prize.

About Brad Timerson: I am very surprised and deeply appreciative at having been named a DaBoll Award recipient. And I am very glad to see the other coordinators recognized with this award. It is certainly well deserved. Thank you to all who voted for myself and this dedicated group of coordinators. I began observing lunar occultations in the late 1960's and received yearly printouts of predictions from the USNO. Soon I was helping David Dunham with the predicting of lunar occultations and sending out printed copies to IOTA members. Then, along came asteroidal occultations and the ability to compute and deliver all predictions via email. It was soon after I recorded an occultation by Phocaea that I relaized IOTA needed a better way to report occultations. Too many times, crucial information was missing from emailed reports. The Excel report Form was born and, with the help of John Talbot, became a useful tool for reporting occultations. A lunar form was also created, but the ability to report these occultations from within Occult Jphn soon came up with a macro to read the details in the Excel Form into the format required for Occult. No more transcription errors! Now in its 11th year, the Form remains a useful tool for reporting asteroidal occultations with accuracy and with all the required observational information.

## TNO occultations pages

Since 2010 I have been involved in the observation of occultations of TNOs (Trans Neptunian objects).

Very little is known on these far away asteroids, their orbits, for some which have satellite(s) their mass, but they are so far away that their diameters can not be determined directly.

The observations of occultations of very small objects (like the ones in the main belt between Mars and Jupiter) require a high density of telescopes on the ground. So while I did observe MBA's occultations while I was in France, I have never intended to observe any from Chile because there are almost no amateur astronomers able to provide other chords on the asteroid's shadow and you can not get observing time on professional telescopes for such events concerning "ordinary" asteroids..

Under the impultion of Bruno Sicardy, who maintains a web page for such events, I have observed successfully several of these TNO occultations. Felipe Braga Ribas of the same team, also has a similar page. One of the member of the group, Mario Assafin from Rio de Janeiro has published an exhaustive study of the future occultations involving the Pluto system in Assafin, M., Camargo, J.I.B., Vieira Martins, R., Andrei, A.H., Sicardy, B., Young, L., da Silva Neto, D.N., and Braga-Ribas, F., Precise predictions of stellar occultations by Pluto, Charon, Nix, and Hydra for 2008-2015, Astronomy and Astrophysics, Volume 515, id.A32, 2010.

Of course they typicallly involve rather faint stars, therefore not of the utmost astrometric accuracy, and the same is true for most of these TNOs, which have relatively short arcs (compared to long numbered MBAs), so there is an important work of high quality astrometry in order to predict the reality of a probable occultation. For the November 4th 2010 event for example, predictions for the shadow were somewhere between Alaska and the middle of Chile. Then the object is 14 billions of kilometers away, so ten milli arc second at this distance is 68 kilometers on the ground. The group working in collaboration with Bruno are observers from Brazil (Mario Assafin et al. ) and Spain (Jose Luis Ortiz and his team). While the observations here are normally performed by one or two persons, it is the result of a pretty large collaboration (the last letter to Nature on the Eris occultation has 65 coauthors).

The observations have been performed in the past with a 40cm Ritchey Chrétien belonging to Campo Catino (occultation of Charon in 2006 ?), but mostly with Caisey Harlingten's 50cm Planewave telescope, which is equipped with a thinned CCD Apogee U42 camera which we normally used in binning 2x2 with a subframe of 100x100 pixels. When I can, I do the observations, when I can't (mainly during the beginning of the night when I work doing tours), somebody else is replacing me (thank you Sebastian and Nicolas).

There were 6 occultations of TNOs observed in the world in recent times and all 6 were observed successfully from our observatory.

Caisey's scope is the black one in the foreground on the left of the image, 2 domes further back, the ASH2 40cm ASA telescope, normally used to search for large transneptunian objects in the southern hemisphere.

136472 Makemake, April 23rd 2011

50000 Quaoar, May 4th 2011

All these events were or are going to be published, first in IAU circulars or electronic circulars, the Eris event paper has been submitted by Bruno Sicardy to Nature, the others will take time before all the reduction and analysis can be done (with these astronomers, one year is typical :) ). At this rate, a lot more data than previously available on TNO physical properties will be available in a few years, we live exciting times.

## IOTA-VTI v3

This is the latest version of our IOTA-sponsored video time inserter. It offers improved GPS sensitivity: more satellites will be acquired in less time. A coin cell has been added to give the GPS non-volatile memory. In addition to speeding initial satellite acquisition, the IOTA-VTI v3 will retain knowledge of leap seconds when they are added. (The most recent leap second addition was December 31, 2016.) The IOTA-VTI v3 is compatible with both NTSC (30 frames/second) and PAL (25 frames/second) video formats. Selection of format is via an internal switch that will be set to the buyer’s preference, but which can easily be changed.

Every so often, a Trojan asteroid, orbiting roughly 800 million km (500 million miles) from the Sun, briefly passes in front of a star on the order of 30,000 trillion km, or 20,000 trillion miles, away. For a few seconds, as the Earth, asteroid and star perfectly align, the asteroid casts a shadow on the Earth. This phenomenon is called an occultation. Occultations are particularly important for the Lucy Mission, since finding and measuring shadows cast by asteroids means pinning down the positions of the asteroids before the spacecraft approaches them. In few other contexts can astronomers measure the positions of astronomical objects with this degree of precision from Earth, which is why teams of astronomers travel all over the planet for the chance to stand in the shadows of asteroids.

A diagram of an asteroid occultation, not to scale, though the shadow of the asteroid on Earth is the same size as the asteroid itself. Credit: IOTA

One of these astronomers is Brian Keeney, an Observing and Logistics Specialist for the Lucy Mission. Keeney has been a professional astronomer for 20 years, spending the last three of those years observing occultations. “I think that being part of these [occultation] campaigns has been the most satisfying and adventurous thing I’ve ever [done] as a professional astronomer,” he explains. Working with teams that run the gamut of astronomy skill levels, from professional astronomers to high school students, Keeney has chased the shadows of space rocks around the world, including to Argentina, Australia, and Senegal.

During an occultation, the asteroid blocks a star so distant that the shadow cast on the Earth’s surface is extremely close to the size of the asteroid itself. We cannot choose where the shadow will fall, so teams of astronomers often have to travel to the locations where the shadow is predicted to be. The goal is for the observing teams to spread out in the predicted shadow region, then aim their telescopes at the star being occulted. If all goes according to plan, they will record the moment when the asteroid passes in front of the star from their perspective, this will look like the star has briefly disappeared, as though someone turned it off for a few seconds. They can then measure exactly when and for how long the star is hidden from view. Not only does this data help astronomers determine the asteroid’s location with incredible precision, but they can also use it to estimate the size and shape of the asteroid’s shadow, and thus the size and shape of the asteroid itself.

Map of the Leucus occultation on November 18, 2018. The grey band represents the predicted track (after the mistake was found) and the red band represents the observed occultation track. Blue dots represent teams with good data and no occultation red diamonds represent good data with a positive occultation detections grey squares indicate sites unable to collect useful data. Credit Buie and Keeney et al.

Even with the most accurate predictions, however, unforeseen obstacles can still arise. On November 18, 2018, the Lucy team was in Texas preparing to observe an occultation by asteroid Leucus that would occur that night. They had been using the Southwest Research Institute’s San Antonio campus as a place to meet up, hand out equipment (such as telescopes, laptops, and astronomical cameras), and hold practice sessions. Each of the 23 observation teams made sure to choose a location within the predicted path before the big day. That morning, however, Brian Keeney and Marc Buie (the mission’s Satellites and Rings Working Group Lead) noticed a glitch in their prediction software that had gone undetected for the entire campaign. This glitch shifted the timing of the occultation prediction by about 30 seconds, which erroneously put the predicted path south of San Antonio. The two realized that if they didn’t travel north, they would miss the occultation. The weather in the San Antonio area was getting worse, however, so if they traveled directly north or east, they would be caught in a rainstorm! “We talked to the teams right after breakfast that morning, right after we convinced ourselves we were right,” Keeney remembers. “We just said, ‘Hey. Guess what, guys? Sorry, but we need to go north. And not just north, because the weather’s going to be bad here. We need to go north and as far west as possible.’” Each team scouted locations within the new predicted path, found areas with the least cloud cover, and pointed their telescopes at the star when the time came. Due to everyone’s quick thinking and adaptability, nine teams were able to see the occultation, and almost all teams walked away with amazing data.

Data from the Leucus occultation in November 18, 2018. The lines each represent the observation track of a single observer (the observer is stationary while the asteroid passes overhead). In turquoise is the best fit ellipse to the data, and in black is an example of what Leucus' shape might be based on the data points in orange (although many other shapes fit the data within the uncertainties). Credit Buie, Keeney et al shape outline added by David Dezell Turner.

One of the most exciting aspects of occultations is the fact that with practice and the right equipment, anyone can collect data for a space mission, regardless of experience level. IOTA, the International Occultation Timing Association, is an organization dedicated to supporting the occultation observation efforts of both professionals and amateurs, encouraging “[a]mateur astronomers at all levels of experience… to become ‘citizen-scientists.’” One project supported by IOTA is led by Marc Buie and fellow planetary scientist John Keller. This project, called RECON (Research and Education Collaborative Occultation Network), is a network of amateur astronomers, teachers, high school students, and other community members spread out across the Western United States. RECON aided in the Lucy team’s observation of a Leucus occultation on October 2, 2019. Though RECON was intended to be a stationary network, many teams were more than willing to travel long distances for the event. Some teams helped transport equipment and other observers from their hometowns, and some, like one team from Sisters High School in Sisters, OR, even stayed in the occultation area overnight.

The COVID-19 pandemic has presented new challenges for the occultation team. On September 24, 2020, the team planned to observe an occultation by Polymele in Senegal. Learning about Polymele is especially important for the Lucy team because, as it is the smallest Trojan target, it will be the most challenging to observe during the high speed encounter. The more the team knows about it ahead of time the better. Unfortunately, due to the pandemic, travelling to Senegal was impossible. Fortunately, sending equipment was still feasible. This equipment was lent to a group of Senegalese astronomers with whom Keeney has worked before on August 4, 2018, they aided NASA’s New Horizons mission in observing an occultation by the Kuiper Belt object Arrokoth, which helped the New Horizons team to better understand the object before the spacecraft’s flyby. Organizing an occultation campaign on the other side of the world without being able to travel there was fairly new territory for the Lucy team, but this is the culmination of lessons learned over several occultation campaigns. During their earliest campaigns, the Lucy team relied on the same 40-cm (16-inch) diameter telescopes New Horizons used for the Arrokoth occultation, which are heavy and require two or three people to set up. Now, the team deploys much lighter 20-cm (8-inch) telescopes whenever possible. For that Senegal occultation, the team sent a set of these smaller telescopes, since the star being occulted is so bright that the larger telescopes are not necessary. On the logistical side, the team is also drew from their experience planning a campaign in Phoenix, Arizona on December 29, 2019, for which Keeney himself drove equipment from Boulder, Colorado and lent it to local astronomers. That event involved teams from both IOTA and RECON, and though it had less direct oversight from the Lucy team than most, nearly all of the teams involved were able to collect good data. “The Phoenix event was kind of a scaled down version of what we did for Senegal in that for the most part, teams were much more independent and on their own until the very last minute…. It was new, and it’s different, but I expect that if we hadn’t tried the event in Phoenix in December, we wouldn’t have had the confidence to do the Senegal event.”

The 10 8-inch occultation telescopes being checked out prior to use at SwRI (one of the 16-inch telescopes is visible in the back right). Credit: SwRI/Kretke

On September 24, 2020, Polymele racing by at 50,000 kph (31,000 mph) — crossed in front of a star. Unfortunately, at that precise moment, the skies over much of Sengal was covered with patchy clouds. One lucky observer had a fortuitous break in the clouds and was able to observe a single star wink out for less than two seconds. While this single observation was not enough to really pin down the dimensions of this small asteroids, it did add new constraints to the size and showed the scientists the location of the asteroid with unprecedented precision.

Fortunately that was not the last opportunity to observe Polymele. This fall, observers in Spain will have a chance to see Polymele is cross in front of another star. Again, we don’t know if COVID-19 will allow international travel, but based on these previous experiences the team is confident that at least the Lucy telescopes will make their way across the Atlantic ocean to attempt to view this occultation. With the data gathered from that single observation last year, Buie and Keeney can predict where this occultation track will fall with incredible accuracy, allowing the observers to be more tightly clustered on the ground. If the weather cooperates, this will give amazing data on the size, and shape of this small asteroid. With these precise predictions and a little luck, a group of astronomers will be fortunate enough to find themselves standing in this asteroid’s shadow.

You can see a list of upcoming occultations of Lucy Targets here. If you’d like to learn more or get involved, the best way is to reach out to your local IOTA chapter.

Banner Image: Julien Salmon during a dress rehearsal for the Orus occultation. Credit: Mike Grusin

## How long would an occultation by a TNO last? - Astronomy

This listing of other reported/postulated asteroid/TNO companions may be incomplete. It includes 310 asteroids and TNOs: 291 suggested binaries, plus 18 cases where subsequent analysis and/or observations have refuted the existence of a companion, plus one object suggested to have rings. Note there are two distinct reports each for (2) Pallas and (146) Lucina. Some diameter figures are triaxial dimensions from shape models which supercede the interpretations suggesting the presence of a companion. Additional data for these asteroids/TNOs are available on this page. (Note that the delineation between the most likely candidates on this list and the least likely candidates on the previous page is ambiguous corrections are welcome.)

• Cellino, A., R. Pannunzio, V. Zappala', P. Farinella, and P. Paolicchi, "Do we observe light curves of binary asteroids?," 1984, Astronomy and Astrophysics, 144:355-362.
• Denissenko, Denis, 15 Feb 2002, List of double minor planets.
• Dunham, David, 27 Feb 2002, Observed minor planet occultation events.
• Higgins, David, 2006, Minor planet lightcurves.
• JPL, 2003, JPL DASTCOM Database.
• "A list of suspected or known binary asteroids," 2 April 2002, http://www.bdl.fr/observateur/binast/binary_ast.php.
• Peebles, Curtis, 2000, Asteroids: A History, Smithsonian Institution Press, Washington, DC.
• Sonnett, S., A. Mainzer, T. Grav, J. Masiero, and J. Bauer, 2014, Binary candidates in the Jovian Trojan and Hilda populations from NEOWISE lightcurves, arXiv (1412.1853).
• Tedesco, E., "Binary asteroids: evidence for their existence from lightcurves," 1979, Science, 203:905-907.
• Tedesco, E., et ali, Mar 1998, IRAS Minor Planet Survey, version 4, IMPS Derived Diameters and Albedos.
• Weidenschilling, Stuart J., Paolo Paolicchi, and Vincenzo Zappala', 1989, "Do asteroids have satellites?," pp. 643-658 in Asteroids, ed. by R. P. Binzel, T. Gehrels, and M. S. Matthews, Univ. of Arizona Press, Tucson, AZ.

© 2002-2018, 2019 by Wm. Robert Johnston.

## Astronomers study Kuiper Belt object during stellar occultation

Until now, astronomers have used telescopes to find Kuiper Belt objects (KBOs), moon-sized bodies, and obtain their spectra to determine what types of ices are on their surface. They have also used thermal-imaging techniques to get a rough idea of the size of KBOs, but other details have been difficult to glean.

While astronomers think there are about 70,000 KBOs that are larger than 100 kilometers in diameter, the objects' relatively small size and location make it hard to study them in detail. One method that has been has been proposed for studying KBOs is to observe one as it passes briefly in front of a bright star such events, known as stellar occultations, have yielded useful information about other planets in the solar system. By monitoring the changes in starlight that occur during an occultation, astronomers can determine the object's size and temperature, whether it has any companion objects and if it has an atmosphere.

The trick is to know enough about the orbit of a KBO to be able to predict its path and observe it as it passes in front of a star. This was done successfully for the first time last October when a team of 18 astronomy groups led by James Elliot, a professor of planetary astronomy in MIT's Department of Earth, Atmospheric and Planetary Sciences, observed an occultation by an object named "KBO 55636."

As Elliot and his colleagues report in a paper published to be published June 17 in Nature, the occultation provided enough data to determine the KBO's size and albedo, or how strongly it reflects light. The surface of 55636 turns out to be as reflective as snow and ice, which surprised the researchers because ancient objects in space usually have weathered, dull surfaces. The high albedo suggests that the KBO's surface is made of reflective water-ice particles, and that would support a theory about how the KBO formed. Many researchers believe there was a collision that occurred one billion years ago between a dwarf planet in the Kuiper Belt known as Haumea and another object that caused Haumea's icy mantle to break into a dozen or so smaller bodies, including 55636.

More importantly, the research demonstrates that astronomers can predict occultations accurately enough to contribute to a new NASA mission known as the Stratospheric Observatory For Infrared Astronomy (SOFIA) that completed its first in-flight observations in May. A Boeing 747SP aircraft that has a large telescope mounted onto its rear fuselage, SOFIA can record infrared measurements of celestial objects that are not possible from the ground. Elliot hopes his research will help guide future flights of SOFIA to observe stellar occultations in detail.

Elliot, who has been studying 55636's orbit for five years, thought it would most likely pass in front of an unnamed star on Oct. 9, 2009. But the KBO's small size made it difficult to predict exactly where the object would travel, and so, to be on the safe side, he and his colleagues assembled a network of 18 observation stations along a 5,900-kilometer stretch of the Earth's surface that corresponded to the KBO's predicted shadow path. Such a strategy "covered our uncertainty about where the path would go, both to the north and to the south," Elliot explains. "It was our way of hedging our bets."

While some of the stations couldn't observe because of weather, and others simply didn't detect the occultation, two stations in Hawaii captured data on the changes in starlight that occurred during the roughly 10-second occultation. After measuring the exact amount of time that the star was blocked from view, as well as the velocity with which the shadow of 55636 moved across Earth, the researchers calculated that the KBO has a radius of about 143 kilometers. Knowing this, they could then calculate the object's albedo.

The highly reflective surface of 55636 is perplexing because the surfaces of celestial bodies in the outer solar system are supposed to darken over time as a result of dust accumulation and exposure to solar radiation.

Although other highly reflective bodies in the solar system, such as the dwarf planet Pluto and Saturn's moon Enceladus, have their surfaces continuously renewed with fresh ice from the condensation of atmospheric gases or by volcanic activity that spews water instead of lava, 55636 is too small for these mechanisms to be at work, says Elliot. He has no plans to investigate the cause of the high albedo but will continue to collect data about the orbits and positions of the largest KBOs in order to predict future occultations with enough accuracy that he doesn't have to rely on a vast network of observers.

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Materials provided by Massachusetts Institute of Technology. Original written by Morgan Bettex, MIT News Office. Note: Content may be edited for style and length.