Three amateur astronomers discovered C/2025 F2 SWAN, which later developed a two-degree tail

On March 30, 2025, a steadily brightening object was spotted in publicly accessible SWAN data.

At the time, the object was little more than a steadily brightening point of light in low-resolution sky maps.

Following many weeks, and after much new observational data became available, the object would grow significantly brighter.

In fact, in just a few short weeks, the object would be named Comet C/2025 F2 SWAN.

The identification of Comet C/2025 F2 SWAN came from ultraviolet observations collected by the SWAN instrument aboard the SOHO spacecraft.

After several weeks, the very same comet would become a target for skywatchers and produce a tail stretching more than two degrees across the sky.

How three comet hunters found a visitor from deep space

Vladimir Bezugly, Michael Mattiazzo, and Rob Matson independently discovered the comet while separately examining publicly accessible SWAN data.

Since the comet was faint at discovery, its existence can be dated back to March 22, 2025.

It grew brighter each of the following days as it moved through the sky.

Finding possible comets in SWAN data is slow and laborious because the maps are low resolution and contain numerous false positives.

For example, when objects come close to the Sun, they tend to brighten up and therefore become easier to find.

Why confirming the object became much harder

Identifying a possible comet in SWAN data was only the beginning.

The public images will have some type of lag (approximately three days) before being released.

Observers needed to use their best judgment as to where in the sky the object may now be visible.

To make matters worse, the object was approximately thirty degrees from the Sun, which limited the ability of many telescopes to observe that area of the sky.

Poor weather conditions added another obstacle to making further observations.

On April 1, Mattiazzo contacted the international comet observing community and requested help locating the object, a process he later described through Southern Comets.

That help arrived on April 3.

Using a 40-millimeter refractor, Quicheng Zhang was able to visually locate the object from Arizona.

Additional observations of the comet and its appearance were later featured by NASA Astronomy Picture of the Day, where astronomers highlighted its green coma and extended tail.

What happened after the orbit became clear

With sufficient measurement data obtained, astronomers were able to determine a preliminary orbit for the comet.

That preliminary orbit allowed astronomers to look for previous sightings of the comet.

The search successfully uncovered earlier pre-discovery images dating back to September 2024.

The orbit determined that the comet would reach perihelion on May 1, 2025.

At that time, the comet would pass only .33 astronomical units away from the Sun.

It would be in Mercury’s orbital region.

One day later, the comet would pass approximately 0.96 astronomical units from Earth.

This combination of events suggested that the comet should provide an opportunity for amateur astronomers to observe with binoculars.

Predictions remained uncertain because the comet underwent an outburst on April 6.

How additional observations changed the picture

Following a number of additional observations, astronomers reported that a distinctively colored green coma surrounded the comet.

The green coloration was due to glowing carbon molecules that were illuminated by sunlight.

The comet had a weakly ionized tail extending over almost two degrees in length.

Further analysis of orbital elements showed that Comet C/2025 F2 SWAN was originally in the distant Oort Cloud.

The comet’s origin placed it in a vast reservoir of icy bodies far beyond the known planets.

What was initially a faint object in spacecraft observations became a newly discovered visitor to our solar system from one of its most remote areas.