140 years of observations, in order.

In August 1883, astronomer José Árbol y Bonilla was observing the Sun from Zacatecas Observatory in Mexico. Looking at the Sun through a telescope turns anything passing between the telescope and the solar surface into a dark silhouette.

Across August 12 and 13, Bonilla reported 447 separate objects crossing the Sun. He photographed some of them and recorded their passage. The original glass plates are no longer available, but reproduced images and his written account were published in the French astronomy journal L'Astronomie in 1886.

The photographs show that dark forms crossed the telescope's view of the solar disc. They do not reveal the objects' distance. A small object nearby and a much larger object far away can cover the same apparent area in a telescope.

Bonilla proposed bodies travelling inside Mercury's orbit or groups of meteoroids. An editorial published with his report suggested birds or insects passing closer to the telescope, an explanation Bonilla rejected.

In 2011, Héctor Javier Durand Manterola and colleagues re-examined the published record. Because the event was reported from Zacatecas but not from observatories farther away, they proposed that Bonilla had seen fragments of a comet passing very close to Earth. Their model uses the limited geographic visibility to estimate distance and size.

That modern interpretation is an arXiv preprint, not a peer-reviewed confirmation, and it cannot be checked against the lost original plates. The historical observation is real. The distance, size, and nature of what crossed the Sun remain unresolved.

Before digital cameras, astronomers recorded the sky on glass plates coated with light-sensitive chemicals. A telescope tracked one region of sky while the plate collected light, sometimes for nearly an hour. When developed, stars appeared as small points on the glass.

The first Palomar Observatory Sky Survey used this method to photograph large parts of the northern sky. Because its plates survive, researchers can scan them at high resolution and compare the recorded positions with other Palomar exposures and with modern sky surveys.

On April 12, 1950, one fifty-minute Palomar exposure contained nine star-like points that were not present on a plate taken about thirty minutes earlier. They were also absent six days later and have never been recovered in modern observations. All nine appeared on the same piece of glass during the same exposure.

Beatriz Villarroel and an international team identified the group while visually examining POSS-I transient candidates. They published the analysis in the peer-reviewed journal Scientific Reports in 2021. The researchers checked the shapes and positions of the points and compared the plate with exposures taken before and after it. Deep follow-up with the 10.4-meter Gran Telescopio Canarias covered six of the nine positions and found no convincing persistent counterpart linked to the original transients.

The plate therefore records nine features that looked like stars for one exposure and were absent before, afterward, and seventy years later. The observation does not by itself reveal whether the features were brief events in the sky or marks produced within the photographic material. It establishes the event that any explanation must account for.

The first Palomar Observatory Sky Survey sometimes photographed the same region more than once. Each glass plate was a long exposure of one patch of sky, so different plates can be placed beside one another like frames separated in time.

On July 19, 1952, one POSS-I exposure recorded three bright, star-like points crowded into a very small area of sky. On the next exposure, taken about fifty minutes later, all three were gone. Ordinary stars do not switch off together on that timescale.

Enrique Solano, Geoffrey Marcy, Beatriz Villarroel, and their collaborators examined the plates, measured the three images, and checked modern catalogues for anything at the same positions. They published the result in the peer-reviewed Monthly Notices of the Royal Astronomical Society in 2024.

In 2023, the 10.4-meter Gran Telescopio Canarias looked much more deeply at the same location. It found no persistent source at any of the three positions. Anything still there would have to be more than ten thousand times fainter than the points recorded in 1952.

The paper proposes gravitational lensing as one possible natural explanation. In such an event, the gravity of an object passing in front of a more distant source briefly magnifies its light. No lens, repeating event, or remaining source has yet confirmed that explanation.

Roughly nineteen hours after the Palomar exposure, radar operators around Washington, D.C. began recording the first events of the well-documented July 1952 radar episode. The observations were not simultaneous. They occurred on the same American calendar date.

For generations, Harvard College Observatory photographed the night sky on glass plates. Each exposure preserved the brightness and position of stars at a particular time. Taken together, the plates became a record that later astronomers could search for changes nobody had known to look for when the photographs were made.

That record is not continuous. Harvard stopped taking astronomical plates in 1953 and did not resume until 1968. The fifteen-year interruption is now identified in material from the Digital Access to a Sky Century at Harvard project as the Menzel Gap.

The consequence is simple. If no plate was taken, no later instrument can recover what crossed that part of the sky on that night. The sky continued. Its photographic record did not.

Donald H. Menzel was acting director and then director of Harvard College Observatory across the relevant period. The stated institutional reason for ending plate production was financial pressure and the continuing cost of the program.

The gap is not the only documented loss. In her 2002 memoir, Harvard astronomer Dorrit Hoffleit wrote that Menzel instructed his secretary to destroy part of the plate collection without first inspecting it. Later committees approved the destruction of additional plate series. Those decisions permanently removed observations that could otherwise have been examined with modern methods.

Menzel also performed wartime and postwar military work, became a prominent public advocate of natural explanations for UFO reports, and promoted atmospheric inversion as an explanation for the 1952 Washington radar events. An atmospheric inversion occurs when layers of air at different temperatures bend radar signals in unusual ways.

Between 1949 and 1957, the first Palomar Observatory Sky Survey photographed the northern sky on glass plates.

These plates were the astronomical photographs of their time. Instead of a digital camera, the telescope focused light onto glass coated with a light-sensitive chemical layer. When a plate was developed, each star appeared as a small point. Because the plates survive, researchers can scan them with modern instruments and compare that earlier sky with the sky we see today.

The VASCO research project assembled a catalogue of more than 106,000 transient candidates from those plates. A transient candidate is a point that appears on an old exposure but cannot be found in modern surveys. It may represent a brief event in the sky. It may also be a defect or contamination on the photographic plate. Distinguishing between those possibilities is the central dispute.

An international research team led by astronomer Beatriz Villarroel then asked where those points appeared in relation to Earth's shadow.

Earth does not cast its shadow only across the ground. On the night side of the planet, the shadow continues far into space as a long cone. Objects orbiting high above Earth can still be illuminated by the Sun even while the ground below them is dark. But when an object enters the shadow cone, Earth blocks the sunlight from reaching it.

Many things can produce a point on an astronomical photograph. A distant star produces its own light. It will remain visible whether or not the region of near-Earth space in front of it lies inside Earth's shadow. A defect on the photographic plate should also have no relationship to where that shadow falls.

A nearby object seen only because it reflects sunlight is different. When it enters Earth's shadow, the sunlight is cut off. The object may remain in exactly the same place, but its reflected flash disappears. For every Palomar plate, the researchers used its date, exposure time, and position in the sky to calculate where Earth's shadow would have appeared at different possible distances from the planet.

They could then ask whether the recorded points were distributed randomly, or whether they became rarer where reflected sunlight should become impossible.

That is the pattern the researchers reported. Far fewer transients appeared inside the calculated shadow than the amount of photographed sky would predict. The strongest match occurred at approximately 36,000 kilometers above Earth, the region where geosynchronous communication satellites orbit today.

The team published the result in the peer-reviewed journal Publications of the Astronomical Society of the Pacific in 2025.

The analysis does not measure the distance to any individual flash. It finds that, across eight years of pre-Sputnik observations, the reported population behaved less like distant stars or randomly distributed marks on glass, and more like sunlight reflecting from sources at satellite altitude.

Before satellites, a brief reflection high above Earth was an unusual kind of observation. After satellites, human-made objects began producing flashes of sunlight in the same region of space.

That does not mean continuous space observation began in 1957. It means the channel changed. The category of light being examined was no longer free of objects humanity had placed there itself.

The observing record changed at the same time. The first Palomar Observatory Sky Survey ended in April 1957. Sputnik launched in October. Harvard had already stopped taking plates in 1953 and would not resume until 1968. Later programs used different telescopes, photographic emulsions, parts of the sky, exposure lengths, and observing schedules.

This prevents a clean before-and-after experiment. Researchers do not have one unchanged instrument watching the same sky in the same way across the satellite boundary. A lower density of the earlier signature after 1957 could mean that a physical population changed. It could also mean that the instruments changed, the coverage changed, and human satellite glints began filling the same observational category.

The factual boundary is therefore stronger than either interpretation. POSS-I closed, the satellite era began, and the optical channel became different. The existing record cannot determine whether the earlier source population withdrew or whether its signal became harder to distinguish from our own reflections.

Ohio State University's Big Ear was a stationary radio telescope. It did not track a target across the sky. Instead, Earth's rotation carried each part of the sky through the telescope's narrow listening beam.

That created a built-in test. A signal from a fixed position in the sky should gradually strengthen as the source entered the beam, peak near the center, and weaken as it moved out again. The complete passage would take seventy-two seconds.

On August 15, 1977, during a search for possible extraterrestrial radio signals, Big Ear recorded a powerful signal concentrated in a very narrow range of frequencies. That means most of its radio energy arrived inside one small part of the spectrum, the kind of concentrated signal SETI surveys were designed to notice. It rose and fell across the full seventy-two seconds in the shape expected from something fixed in the sky.

The telescope's computer encoded the changing strength as the characters 6EQUJ5. The sequence was not a message. It was a compact way of printing six successive intensity measurements. When astronomer Jerry Ehman later reviewed the paper output, he circled the sequence and wrote "Wow!" beside it.

The telescope's observing method made the event look consistent with a source passing through its beam. But the signal was never detected again. Without a second observation, astronomers could not refine its position, compare its behavior, or test an explanation against a repeat.

Proposals have included terrestrial interference, reflected human transmissions, and natural emission associated with hydrogen. None has become a generally accepted identification of the original signal.

Radar works by sending out radio energy and measuring what returns. The timing and direction of the echo can indicate that something is present and moving, but radar does not automatically reveal what produced the return. Aircraft, weather, the way radio waves bend through the atmosphere, equipment behavior, and processing can all affect what an operator sees.

On the night of March 30 to 31, 1990, Belgian ground radar registered unusual contacts during a wider period of public reports. Controllers directed two F-16 fighters to investigate. The aircraft obtained intermittent radar locks of their own, but the pilots did not visually identify a corresponding craft.

That distinction matters. The event was not merely an eyewitness report. Ground systems and aircraft systems produced an instrument record. But the different observations did not resolve into an identified object that could be inspected afterward.

The Belgian Air Force did something unusual with the uncertainty. Colonel Wilfried De Brouwer presented its account and radar traces publicly, and a military technical report by Lieutenant Colonel Salmon examined the recordings. The Air Force documented the interception without claiming that it had established what the target was.

The episode occurred within the larger Belgian wave of reports that began in November 1989. A famous photograph associated with that wave was later admitted to be a hoax. That photograph is not the source of the March radar record.

ʻOumuamua was discovered in October 2017 after it had passed closest to the Sun. Its speed and path showed that it was not bound to the Sun. It had entered from interstellar space and would leave again. Astronomers followed it by repeatedly measuring its position against the background stars. Gravity predicts where an object should appear next. ʻOumuamua did not remain exactly on that predicted path. As it moved away from the Sun, it received a small additional push directed outward.

Comets often receive this kind of push when sunlight heats their ice. Escaping gas acts like a weak jet and alters the orbit. Usually that process also produces a visible coma, the cloud of gas and dust surrounding an active comet.

No resolved coma or ordinary outgassing signature was detected around ʻOumuamua. Marco Micheli and collaborators nevertheless found that a comet-like non-gravitational force provided the best fit to the measured trajectory. They published the acceleration in the peer-reviewed journal Nature in 2018.

The acceleration is measured. Its cause is not. Researchers have proposed several natural mechanisms, including unusual volatile ices and forms of outgassing that would have been difficult to detect. Shmuel Bialy and Avi Loeb proposed that sunlight itself could push an extremely thin object through radiation pressure, opening the artificial-sail interpretation.

No telescope resolved ʻOumuamua into an image. Its shape, composition, and the mechanism behind the acceleration are reconstructed from limited light and motion data rather than seen directly.

3I/ATLAS is the third object confirmed to have entered the Solar System from interstellar space. Unlike ʻOumuamua, it developed a clearly active coma as sunlight heated its surface and released frozen material.

Scientists can identify the gases in a coma without collecting them. A spectrograph separates the object's light into wavelengths. Different molecules leave different infrared fingerprints, allowing researchers to estimate how much of each gas the comet is releasing.

Before ATLAS reached its closest point to the Sun, the James Webb Space Telescope measured an exceptionally carbon-dioxide-rich state. At that observing epoch, the coma was producing about 7.6 units of carbon dioxide for every unit of water. That was among the highest carbon-dioxide-to-water ratios reported for a comet.

Martin Cordiner and collaborators published the measurement in the peer-reviewed Astrophysical Journal Letters. Later observations, including additional work with JWST, found different mixtures of gas as ATLAS travelled through the Solar System. The ratio did not remain fixed.

The initial measurement was therefore not the object's permanent composition. It was a real state observed at a particular time. That changing chemistry also carries information: different ices can begin escaping as sunlight reaches new layers, so the evolution may reflect where the object formed, how it was altered before arrival, and how heat moved through it during passage.

A comet's coma forms when sunlight releases material from its surface. Water, carbon dioxide, and other volatile substances can become gas at low temperatures. Solid metal should be much harder to lift into the coma because the comet is far too cold for bare nickel or iron to evaporate in the same way. Spectroscopy nevertheless detects the fingerprints of individual metal atoms in cometary gas. Jean Manfroid and collaborators reported neutral iron and nickel across a broad sample of Solar System comets. Piotr Guzik and Michał Drahus then detected gaseous nickel in the coma of 2I/Borisov, the second known interstellar object. Both findings were published in the peer-reviewed journal Nature in 2021.

The leading question is therefore not whether nickel has been detected. It is what molecule can carry nickel out of cold cometary material and then release it into the coma. Metal carbonyls are one candidate because they can form volatile compounds containing metal atoms at relatively low temperatures, but no carrier has been confirmed.

For 3I/ATLAS, the narrower anomaly belongs to specific observing epochs. Nickel emission was detected when corresponding iron emission was not. Iron appeared in later observations, so "nickel without iron" does not describe the entire passage.

The measurement shows low-temperature metal chemistry that existing observations have not fully identified.

Geomagnetic storms begin when activity from the Sun disturbs Earth's magnetic field and the charged-particle environment surrounding the planet. The Kp index expresses the strength of that disturbance on a scale running from quiet conditions to the most severe storms.

The transient candidates were points found on one historical photograph but absent from later surveys. Some may be brief flashes from the sky, while others may be defects in the photographic material.

Kevin Cann took the dates and times of Palomar plates containing those candidates and compared them with the geomagnetic conditions during each exposure. He divided the observations into five levels of storm activity and asked whether the rate of transient detections changed as the storms became stronger.

It did. Transients appeared on 17.4 percent of the examined plates during quiet conditions. At the strongest level of geomagnetic activity, they appeared on 2.4 percent. The decline occurred step by step across the five storm levels rather than appearing only at one arbitrary dividing line.

Cann then examined what happened after storms ended. Detection rates briefly rose above their normal level before settling back toward the usual background. He reported the decline, rebound, and recovery in a 2026 preprint series.

There was an obvious alternative explanation. Geomagnetic storms can change airglow in Earth's upper atmosphere. Perhaps the sky became brighter, the photographic plates recorded less detail, and faint marks were simply harder to detect.

A separate control analysis tested that possibility using plates from Harvard and Palomar. It found no matching decline in the faintest light the plates could record. The plates did not become less sensitive in step with the storms.

A random scratch in photographic emulsion has no physical reason to know whether Earth's magnetic environment is quiet or disturbed. A population of material inside that environment could respond. The same research proposes charged dust interacting with plasma near geosynchronous altitude as a natural explanation.

That natural model changes what the sources might be. It does not remove the reported connection between the transient population, space weather, and the region where modern satellites orbit.