In the early hours of December 10, 2021, volunteers across Japan were invited to point small telescopes at one particular star and record the moment it dimmed. At about 3:08 a.m. Japan time, a Trojan asteroid called Eurybates was due to pass in front of it. The shadow was the whole point. NASA's Lucy spacecraft is scheduled to fly past Eurybates in August 2027, and every measurement of that shadow helps scientists know the target better before it arrives.
An event like this is called a stellar occultation, from a Latin word meaning to hide. A nearer body slides in front of a more distant star, and the star's light is blocked for a short time. Seen from the right spot on Earth, the star blinks out and returns. That simple blink can reveal the size, shape, rings and even the atmosphere of a body too small and too far away to show details in any telescope image.
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Why a shadow beats a picture
Stars are so far away that, even through large telescopes, they look like points of light. The same is true for a small body in the outer Solar System. It may be only a fraction of a pixel wide in the best images. A telescope cannot see its outline directly.
But the body moves in front of the star, and the star is a perfect pinpoint light source. For a moment, Earth passes through the body's shadow, a narrow strip that sweeps across the planet. Anyone inside that strip sees the star disappear. The length of time the star stays hidden, multiplied by how fast the shadow travels, gives the width of the body along that line. The shadow's speed comes from the body's known orbit, which is why predictions rest on well-measured orbits. The guide to how to read an orbit explains the numbers behind that.
What matters in this method is timing, not mirror size. A small telescope with an accurate clock can record an event that a big observatory misses by being in the wrong place.
What the shadow reveals
Different patterns in the starlight mean different things.
| What you see | What it means | Example |
|---|---|---|
| Star vanishes sharply and returns sharply | A solid body with little or no atmosphere; the time gives its width along that line | A small asteroid crossing a star |
| Star fades gradually before disappearing | A thin atmosphere bends and dims the light first | Bodies such as Pluto, whose thin atmosphere NASA says is mostly nitrogen |
| Brief dips just before or after the main event | A ring or a moon is passing in front of the star | The rings of Haumea and of the centaur Chariklo |
| Event seen at some sites and missed at others | The shadow's edges fall between the observers | Any campaign with several stations |
Rings. In 2017, scientists announced that Haumea, a dwarf planet beyond Neptune, has a ring. NASA calls it the first known Kuiper Belt object to have rings, and says the discovery came from watching Haumea pass in front of a star. Haumea is an oval, football-shaped world with an equatorial diameter of about 1,740 km (1,080 miles). It spins once about every 4 hours. NASA also notes that it has two moons, Namaka and Hiʻiaka. More about the planet appears in the guide to Haumea, the fast-spinning dwarf planet. The centaur Chariklo, a small body orbiting among the giant planets, also has thin rings. NASA says that Webb captured the shadows of starlight cast by those rings and found a crystalline-ice signature there for the first time. Webb's role is outlined in the guide to the telescopes that find and study small bodies.
Atmospheres. NASA describes Pluto's atmosphere as thin and tenuous. It expands when Pluto is nearer the Sun and collapses as Pluto moves away, and it is mostly molecular nitrogen with methane and carbon monoxide. An atmosphere like that changes how the star fades, which is why an occultation can probe air that no picture could show.
Moons. A second, smaller dip can reveal a companion. Eurybates, for example, has a known moon called Queta, and Lucy is scheduled to visit both.
One chord is a line; many chords are a shape
Each observer on the path records one line across the body, called a chord. A single chord gives only a width at one place. Put many together, and the outline appears. Unistellar, a company whose telescopes many volunteers use, wrote in a 2021 blog post that detecting the shadow helps NASA improve Lucy's guidance system.
A campaign for Lucy shows the scale. According to a report the South Plains Astronomy Club posted in August 2024, club members were among 68 volunteers on 36 teams who observed Patroclus and Menoetius, a pair of Trojan asteroids, as they crossed in front of a star. The report says the campaign refined the pair's orbit and shape to help plan Lucy's encounter, and that the size and shape data came out accurate to 1 to 2 km (0.6 to 1.2 miles) on bodies 300 to 700 million miles (480 million to 1.1 billion km) away. NASA lists Lucy's visit to the pair for March 3, 2033.
Lucy's schedule is part of the timeline of missions to small bodies. Eurybates and Queta come first, on August 11, 2027, followed by Polymele and its satellite on September 14, 2027.
How amateurs take part
The International Occultation Timing Association, or IOTA, is a volunteer science group founded in 1983. It publishes predictions for star occultations by asteroids and the Moon, offers guidance on equipment and technique, and publishes data in scientific journals. IOTA credits amateur timings with helping refine asteroid shapes and orbits, and encourages amateurs of all experience levels to take part.
The Eurybates call in 2021 shows what a volunteer does. The organizers asked observers to go to an occultation-prediction page, pick their country, select 3548 Eurybates, and zoom in on a map of the path to find an exact observing spot. The page then shows the timing and the length of the event.
- [ ] Choose an event from a prediction list and check that the path passes near enough to travel to
- [ ] Set up well before the event, with the telescope tracking the star
- [ ] Use a clock tied to UTC, because the timing is the data
- [ ] Record video or photos through the event, and a bit before and after
- [ ] Report the result to the organizer, even if you saw nothing
A "nothing" result matters. A clear sky and no blink places the edge of the shadow somewhere, which is a measurement too. Never point a telescope or binoculars at the Sun without a proper solar filter made for that purpose. Occultations of stars happen in the dark, so there is no reason to try.
Common mistakes and myths
"An occultation is an eclipse." An eclipse is the specific case of the Sun or Moon being hidden or shadowed. An occultation is the general case of one object hiding another. IOTA's predictions include stars hidden by the Moon, not just by asteroids.
"You need a big telescope." The key tool is a precise clock. A modest telescope in the right spot beats a large one in the wrong spot.
"If you miss the blink, the data is useless." A miss helps pin down where the shadow's edge lies.
"The shadow is as wide as the continent." For a small body, the shadow path is narrow. That is why campaigns put many observers at different positions across the path.
"Volunteers only help with asteroids." The same method found the ring around a dwarf planet, Haumea. Citizen groups such as those described in the guide to citizen science for asteroids and comets include occultation work alongside image-checking projects.
Frequently asked questions
How long does the star stay hidden?
Usually seconds, and sometimes less. The time depends on the body's size and the speed of the shadow across the ground.
Why is the shadow path so narrow?
Because small bodies cast small shadows. Even a large Trojan asteroid is only a tiny dot in Earth's sky, so its shadow is a thin strip.
Can an occultation find a new moon or ring?
Yes. Haumea's ring was announced in 2017 after observers watched it pass in front of a star, and a second dip in a light curve can signal a moon.
Is the Unistellar network the only way to join?
No. IOTA publishes predictions and guidance for observers using their own equipment, and any telescope with a camera and an accurate clock can be used for timing.
Sources
- Haumea. NASA Science. https://science.nasa.gov/dwarf-planets/haumea/
- Exploring Our Solar System with Webb. NASA Science. https://science.nasa.gov/mission/webb/science-overview/science-explainers/exploring-our-solar-system-with-webb/
- Pluto Facts. NASA Science. https://science.nasa.gov/dwarf-planets/pluto/facts/
- Lucy. NASA Science. https://science.nasa.gov/mission/lucy/
- Another chance to detect Trojan asteroid Eurybates for NASA's Lucy mission. Unistellar, December 4, 2021. https://www.unistellar.com/blog/another-chance-to-detect-trojan-asteroid-eurybates-for-nasas-lucy-mission/
- Patroclus Menoetius occultation presentation. South Plains Astronomy Club, August 19, 2024. https://southplainsastronomy.org/2024/08/19/patroclus-menoetius-occultation-presentation
- International Occultation Timing Association (IOTA). https://occultations.org/