Mike Kretlow had planned carefully and traveled for days—flying from Germany to Namibia, then driving into South Africa toward the Kalahari Desert—all in search of clear skies for a brief appointment with a rare celestial alignment only visible from that decidedly off-grid vantage point. A blanket of clouds nearly scuttled his trip, but luck prevailed: With scant time to spare, Kretlow broke into open sky, convinced some locals to let him set up his telescope on their property and watched as a small world called Quaoar at the edge of the solar system passed in front of a star, dipping the star’s light for barely two minutes. “A couple of hours before the event I found a farm and good weather,” he says. “Finally it worked at the last minute.”
Kretlow is a computer scientist turned amateur astronomer, and one of a growing number of people across the globe involved in studying stellar occultations. These are the moments when objects in our solar system, from asteroids to comets to planets, eclipse a background star to cast a shadow down to certain locations on Earth. Unlike a solar or lunar eclipse, these more modest events are usually noticeable solely via telescope, appearing as a star briefly blinking out of view. Depending on the size of the eclipsing object the occultation can last from seconds to minutes, but the information gleaned is of enduring value. Save for sending a spacecraft for direct reconnaissance, no other technique can so precisely determine the size and shape of small objects in the solar system’s far reaches. Occasionally occultation-chasing astronomers have discovered true wonders such as ring systems around the minor planet Chariklo and Uranus as well as dwarf planet Pluto’s tenuous atmosphere.
Occultation science is not new, with the technique’s usage dating back to the late 1950s. But it has suddenly blossomed in the past few years, thanks to advancements in camera technology, cheaper consumer telescopes and, most importantly, the European Space Agency’s star-mapping Gaia spacecraft. Launched in 2013, Gaia is busy monitoring the positions and motions of more than one billion stars in the Milky Way, creating a treasure trove of data for occultation science that has increased the number of predicted events from dozens to hundreds per year. “We are in the best moment for occultation science,” says Paolo Tanga from the Côte d’Azur Observatory in France. “And people are realizing this.”
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The science on offer is vast. Occultations can be used to study asteroids between Mars and Jupiter as well as Kuiper Belt Objects (KBOs), which drift in the outer solar system beyond Neptune’s orbit. They predominantly reveal the finer details of known objects, pinning down their size, shape and brightness with such exactitude an observer might spot the silhouettes of mountains and craters on bodies well past Pluto. But, now guided by Gaia’s data, astronomers are increasingly using occultations to discover new objects, with a team of observers from the National Astronomical Observatory of Japan announcing in January a 60-hour survey of 2,000 stars had yielded a KBO just 1.3 kilometers across—the smallest ever found.
The field has seen a wonderful partnership between professional and amateur astronomers alike via organizations such as the International Occultation Timing Association (IOTA), which boasts hundreds of members. A global network of observers is crucial for catching occultation shadows, which pass all too quickly across tens to thousands of square kilometers of terrain. Any given object might exhibit as many as three occultations per year if it traverses a background rich with stars—or as few as one per century if its orbit places it outside of the Milky Way’s star-studded galactic plane. “Before Gaia we were limited by predictions,” says Bruno Sicardy from Paris Observatory who runs the European Research Council project 'Lucky Star' to observe occultations, drawing on the help of more than 200 individuals, including Kretlow. “Now we are limited by manpower. There are so many [occultations]!”
The technique’s most spectacular success arguably occurred in 2017, when amateur and professional astronomers combined to deliver remarkably accurate measurements of the size and shape of 2014 MU69, the KBO NASA’s New Horizons spacecraft flew past on New Year’s Day 2019. Dozens of telescopes monitored the events, including NASA’s SOFIA flying observatory, a 2.7-meter telescope housed in a modified Boeing 747 jet. The occultation itself lasted just two seconds but set New Horizons’ course for the coming years—without it, the spacecraft would not have risked such a close flyby of MU69 for fear of colliding with any undiscovered moons, rings or other debris. “MU69 was the most challenging flight for SOFIA,” says Kimberly Ennico Smith, an astronomer at NASA Ames Research Center who previously served as SOFIA’s project scientist. “To get the measurement, we needed to fly to a particular point on Earth within 10 kilometers [of the shadow] and within one second of the target point.”
Today the field is progressing faster than ever before, with exciting new capabilities on the horizon. Franck Marchis from the SETI Institute, for instance, has helped develop an automated telescope called the eVscope that amateurs can use to easily observe occultations when alerts are sent out. And astronomers like Sicardy and Marc Buie of the Southwest Research Institute, who led the occultation campaign for New Horizons, are using amateur teams of “citizen scientists” to address lingering mysteries about the solar system. Are rings around objects like Chariklo the norm, for example? And do most small objects orbit our sun in isolation or as part of a binary pair? Answering such questions could lead to fresh insights about how our solar system came together more than 4.5 billion years ago. “I am very interested in how many objects out there are binary,” Buie says. “In a certain size range, around 100 to 150 kilometers in size, it’s possible that they are all binary.”
This dawning golden age of occultations may have an expiration date, unfortunately. Within 20 years, as the stars in Gaia’s catalogue continue their journeys through the galaxy, the accuracy of their known positions will slowly degrade, reducing their efficacy for predicting when and where celestial shadows shall fall. “Eventually we’ll be back to where we were before Gaia, and then we won’t be able to do these kinds of predictions anymore,” Buie says. Gaia may, however, be but the first of many future missions to make ever-more expansive maps of the Milky Way’s stars. “There are people already dreaming about Gaia 2,” Tanga says. Until that time comes astronomers are making the best use they can of Gaia’s riches.
Already this year astronomers have identified a number of occultation events that look particularly promising. In March an occultation by the asteroid 433 Eros will be visible across North America, and an October occultation by the asteroid 3200 Phaethon is also generating excitement. As for Kretlow, he is busy preparing his next trip to see yet another shadow of Quaoar—this time in Chile in July, and once again for no personal gain beyond the sheer frisson of glimpsing a fleeting shadow from a far-off lucky star.