Richard A Lovett
Occultations are like eclipses, but on a smaller scale. They occur when a distant body in our Solar System moves between us and a star. As seen from Earth, the star winks out…then reappears as the occulting body moves away. How long the starlight stays blocked, and from where this can be seen, depends on the size of the occulting body. The smaller it is, the narrower the path. (For a 30-kilometer-wide asteroid, for example, the path will be only 30 kilometers wide.)
Most occultations involve stars too faint to be seen by the naked eye, but that doesn’t mean they aren’t important, because any astronomer with a telescope large enough to see the star, and a clock good enough to time the occultation precisely, can use it to help determine the size and shape of the occulting body.
Western Australia, with its dark skies and cloud-free nights is an occultation chaser’s paradise – as the Southwest Research Institute in Boulder, US’ Marc Buie, Leslie Young and Eliot Young discovered.
Western Australia is an occultation chaser’s paradise.
“We spent six weeks in Australia chasing occultations,” said Buie earlier this month at a meeting of the American Astronomical Society’s Division for Planetary Sciences (DPS) in London, Ontario. “We basically drove all over Western Australia. I’m not sure how many tens of thousands of kilometers we covered, but we got some great data.”
The goal, he says, is to set up an array of portable telescopes across each occultation track, so that each sees a different portion of the asteroid passing in front of the star. (Leslie Young describes this as creating a “picket fence” of telescopes across the path.) “Once you get enough of these you can figure out the shape of the object,” Buie says.
Buie has been studying occultations for years, but since 2018, his focus has been on the five asteroids (and three asteroid moons) to be visited by NASA’s Lucy spacecraft, starting in 2027. “Think of it as a report from a reconnaissance team,” he says. “I’m trying to figure out as much as I can about Lucy’s targets.”
These asteroids lie in a loose swarm known as the Trojan asteroids, held together by the interplay of Jupiter’s giant gravity field and the more distant Sun’s. “At the start of the project, we just had rough guidelines on what the objects’ rough sizes and compositions are,” he says. “I’m trying to learn more and to build that into the mission plan so we can not waste our time trying to figure out the simple stuff.”
“I’m not sure how many tens of thousands of kilometers we covered, but we got some great data.”
A single occultation only gives a single picture of the asteroid: basically, its silhouette at the moment it passes between the star and us. But there are a lot of stars, and each new occultation allows us to see the asteroid in a different profile. Over time, that adds up—enough that it’s possible even on 20-kilometer-wide asteroid to spot mountains, craters, and other topographic features. “We’re building up a 3D view,” Buie says.
His 2022 Australian campaign involved six occultations of various Lucy-target Trojans, all between 30 June and 28 July.
Some of the asteroids look fairly normal, but one, Polymele, is very odd. “We worked out the geometry from a couple of events, and it’s actually a hamburger shape,” he says.
Each new occultation allows us to see the asteroid in a different profile.
Two of Buie’s observers also saw a “secondary blip,” indicating that Polymele has a previously
unknown moonlet. “Now the challenge is on to see if we can find this again,” he says.
Such moonlets are important, he says, because “any time you find a moon around an object, you get to measure the mass of that system.”
Once the mass is known, the asteroid’s shape can be used to calculate its volume, and from that, its density—a possible clue to how it was formed.
But the occultation data has another use, because once a 3D image of an asteroid is assembled, it should also reveal large craters: critical clues to planetary scientists trying to figure out how the Solar System was assembled.
Once a 3D image of an asteroid is assembled, it should also reveal large craters: critical clues to planetary scientists trying to figure out how the Solar System was assembled.
Buie hopes his work with Lucy’s asteroids will pave the way to applying his methods on a larger scale. If his occultation-based topographic maps (and their all-important crater counts) match what Lucy’s cameras find once the spacecraft gets there, he says, it opens the door to using occultation data to count craters on other asteroids, without the need to send an expensive spacecraft.
“I can survey thousands of asteroids, whether it’s a near-Earth asteroid or all the way out to the Kuiper Belt, [and] independent of the distance, I tell you this object has never been hit, or, this thing has been beat up like crazy,” he says.
Leslie Young’s and Eliot Young’s interest is a bit different. They are studying Pluto, and aren’t interested in finding mountains or counting craters. Not only has NASA’s New Horizons spacecraft done a far better job of that than they possibly could, but Pluto has an atmosphere. A tenuous one, to be sure, but enough that in an occultation the starlight doesn’t simply wink out, but fades away as it is gradually obscured. “Nobody has yet seen the effect of Pluto’s surface on the occultation,” Leslie Young said at the DPS meeting.
When the team conducted an occultation study of Pluto in Western Australia last June, their telescopes were scattered as far north as Indonesia.
Pluto is also big, meaning that when the team conducted an occultation study of it in Western Australia last June, their telescopes were scattered as far north as Indonesia.
Of particular interest was observing the “central flash,” which occurs when the center of Pluto passes directly in front of the star. For a brief moment, Pluto’s atmosphere bends the starlight from all sides into a brief burst that can be far brighter than the star itself.
New Horizons, of course, studied Pluto’s atmosphere in extreme detail. But that was a single snapshot. By using occultations and examining the starlight in multiple wavelengths during the central flash, it’s possible to track changes in the atmosphere over time, looking to see how its temperature, surface pressure, and haze layers are evolving not only in response to Pluto’s changing distance from the Sun, but in response to such things as changes in solar flare activity during the Sun’s 11-year solar cycle.
It really is like storm chasing, but here the storm is a tiny wink of starlight, and the critical moments are there and gone in a matter of seconds.
Not that any of this is easy. Leslie Young’s and Eliot Young’s team lost some observing stations (particularly in Indonesia) due to bad weather. One observer in Australia was forced to relocate when his RV got infested with ants.
It can be grueling work. Buie notes that he’s done 13 international expeditions in the past 12 months, visiting (or soon planning to visit) locations not only in Australia, but in Spain, Senegal, the U.S., and the Middle East. And even that hectic schedule was a compromise. “I probably could have chased 100 different events all over the world,” he says.
It really is like storm chasing, but here the storm is a tiny wink of starlight, and the critical moments are there and gone in a matter of seconds. Perhaps it’s best thought of as astronomy for adrenaline junkies.