A major effort at Cabrillo Astronomy has been introducing students to the adventure, fun, and science of getting asteroid occultation data from relatively affordable amateur astronomy accessible equipment. The science of asteroids is a rapidly growing part of professional astronomy research for many reasons - collaborations with the Planetary Defense experts in how to deal with potential impacts, interest from long term thinkers on mining asteroids, from pure researchers on the origin and evolution of these ancient bodies, and what their study can tell us about the formation and evolution of the solar system. The advent of high precision astrometry of stars by the HIPPARCOS Mission in the 1990's and then the GAIA mission of the early 2000's, has led to a rapid expansion in the discovery of small asteriods, and a need to characterize their orbits. The much higher precision possible on stars has allowed the calculation of the paths of asteroid shadows across Earth, with the prospect of placing astronomers and amateur astronomers into those paths and getting occultation timing data which has the ability to map the shapes, the sizes, and in lucky cases, to discover moonlets and hence the size, mass, density of these objects. Together with reflection spectroscopy, we can learn much about the nature of these objects. Realize that 99.999% of asteroids are only visible as specks of light on even the largest telescopes. To get the data described above, requires occultation data, and that data cannot be gathered by the big telescopes because observes must be within the narrow shadow tracks, nor by big space missions because of high expense. This has left a niche for mobile astronomy efforts to get into the paths of these events to get the data. The Cabrillo College Astronomy Department has been one of the most prolific and successful groups to aim at these efforts. Department Chair Rick Nolthenius was recently awarded a major prize for his part in the discovery and confirmation of the first binary asteroid by the occultation method.
This talk is for the intelligent layman and astronomy enthusiast, and hopefully will stimulate more observers in the South Bay area to take up this work. Together with the "Team Cabrillo" observers, this has the hope of even better mappings of asteroids and discovery of more moonlets of these objects.
Example: Occultation by The Earth Hazard Comet Nucleus Phaethon - July 29, 2019, w/ former student Kirk Bender, joining a 60 astronomer collaboration. and International Academy of Astronautics 2021 "Planetary Defense Conference" paper: D. Dunham's Abstract which highlights our work.
At my assigned station track, right on the centerline of the most recent JPL Orbit calculation path. These events can get you out into wonderful dark sky countryside if you're lucky and prepared. |
This asteroid will some day hit the Earth, causing a global catastrophe unless we keep track of its orbit and take Planetary Defense action. This object is the source of the strongest meteor shower of the year; the Geminids. So it was high priority, and we enlisted 60 observers to man the 60 purple evenly spaced tracks on this map. The path crossed one of my favorite places, and an excellent area for asteroid events - Carrizo Plain National Monument. |
My data - sharp and with tight confidence limits. That's what you can do when you have a bright 7th magnitude star and 1/60s time resolution. |
Skyplane plot of our data, centered on the data from the observers who got positive occultations. Kirk Bender's track just missed the asteroid shadow.. |
Later in the morning - Kirk Bender and I, at Kirk's observing site where we camped - happy with our successful data. |
Example: Collaboration with the Monterey Institute for Research in Astronomy (MIRA) for the Occultation of a faint star by the mysterious rings of the "Plutino" Object Quaoar May 13, 2023
I put my excellent student Kirk Bender on the choice instrument - the big 36" f/4.7. As backup and confirmation of difficult S/N events, I set up separately on the 14" Planewaver SCT instrument mounted underneath the main telescope. We both successfully got good data on this very challenging target. |
Our team - Dan Cotton and Jean Perkins of MIRA, and myself and Kirk Bender of Cabrillo College, under the night-safe red light and dark sky just after our occultation event. |
Here's the predicted path of MIRA's telescope through the two mysterious rings of Quaoar. These rings are the only known rings around any astronomical object which are NOT inside the Roche Limit, and therefore are of high astronomical interest. What's their structure? Why are they there? How did they form when tidal disruption cannot be the explanation? |
My recording of the faint star. Similar to Kirk Bender's data, they show no clear evidence of the rings. This means the opacity of the rings at the locations sampled by our chord, are much less than tau=1. |
That's me, monitoring data taking on my videocam mounted on the 14" Planewave, beneath the tangle of wires connecting the amazing array of instrumentation carried by this telescope. |
Example: Aug 15, 2018; Collaboration with Fremont Peak Observatory Association, to get the Occultation of a Star by Pluto, and to characterize Pluto's Atmosphere.
Adjusting the video camera on the 30" Newtonian telescope |
The beautiful recording of the occultation. Amazing that this technique can probe sensitive density and temperature profiles in the extremely thin atmosphere of Pluto, 3 billion miles away, using the changing brightness of a star as it passes into the atmosphere. The point-to-point spikes are Poisson noise, but the more obvious correlated ups and downs are real, and due to Pluto's atmosphere, now 3 years after the New Horizon spacecraft had left Pluto, and the late Pluto summer transitions to a century long fall into winter and freezing of its nitrogen atmosphere onto the ground. |
Our team: I'm flanked by my former students Chris Angelos, now a Fremont Peak Observatory astronomer, and Kirk Bender, who has become an extremely competent and prolific observer of occultation events. |
Fremont Peak was well place to get an extended view of the atmospheric immersion of the star, being near the northern limit of the predicted shadow path. Note the red curve, which marks the 1-sigma limit of the predicted shadow. We do not know the orbit of Pluto well enough to predict this event better than those confidence limits. |
The sky plane plot of our data and that of several other astronomers who found clear skies and a large enough telescope to do this challenging event. |
Example: My growing team of students and former students - chasing after the shadow of the new binary asteroid Martschmidt; Feb 23, 2025.
I spent many many hours GoogleEarth scouting sites for our team members, first in Santa Cruz, then to Watsonville east, then to Mercey Hot Springs, then to West Panoche Rd, and finally to near Helm, CA in the center of the Central Valley east of Fresno, to escape the clouds. This was a high-value science event, and worth the effort. It was actually rather exciting, and Bernard's great success was an occasion for excited celebration (last slide), at the meet-up afterwards. This asteroid is the subject of the Special Studies 80s course created for Bernard's energetic work doing astroid photometry for months, at home, and for this event, which will result in a paper to be submitted to the Minor Planet Center's publishing arm. |
I took this shot after my own recording, from an orchard west of Helm, CA. The first clouds are in the background. My recording shows a very brief 0.1s long occultation at the moment of Kirk Bender's higher confidence event, arguing that it was real. This almost certainly is due to the first and longer occultation seen by Bernhard another 1/2 km south of Kirk (see at right). . |
The real hero was Bernard H, who got a beautiful double occultation recording by both components of this new binary asteroid. The reference star on top, and the target star dropped twice to zero, within a second of each other. This was quite similar to the discovery observation of mine last Nov 17 of a different occultation. We were extremely lucky to have this 2nd occultation by this asteroid trace a narrow shadow path across Central California just 3 months after the discovery. |
Clouds! My recording showed the clear-ish sky was in fact still plagued by thin cloud. That's what the wavy brightness curves show. Buried near the middle, though, is a brief two-integration drop to zero by the target star in green. It matches perfectly with Kirk Benders longer and more solid brief occultation a km south of my chord.
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Our growing team. Sandy A, me, Bernard H, and Kirk Bender. We had HOPED to record this shadow from Santa Cruz, as the narrow path actually did cross Santa Cruz, but an advancing cloud cold front forced us to load cars and race southeast to try to get ahead of the advancing clouds. We succeeded, but just barely, literally by just a couple of miles, or we'd have all been clouded out. It took a mad drive all the way to the middle of the Central Valley west of Fresno, to get into clear skies and on the shadow path. |
