☄ Swift-Tuttle 109P/Swift-Tuttle

How to follow comet Swift-Tuttle live

The panel above recomputes the position of Swift-Tuttle every second in your browser: its distance from the Sun and from Earth, its position in the sky (right ascension and declination), and a live countdown to the next perihelion. It runs on the same kind of engine observatories use, a Kepler solver applied to the JPL osculating orbital elements, so the numbers are not a static snapshot, they keep ticking.

Just below, the top-down map of the Solar System shows exactly where Swift-Tuttle is right now among the planets. You can fast-forward time with the day slider, zoom and pan, compare its distance to another body with a click, and press "Next event" to jump straight to perihelion. It is the most direct way to grasp the orbit of Swift-Tuttle with no math at all.

Comet fact sheet

About Swift-Tuttle

Comet 109P/Swift-Tuttle, independently discovered by Lewis Swift and Horace Parnell Tuttle in July 1862, is the parent body of the Perseids - the August meteor shower that delights observers in the northern hemisphere every year. With an orbital period of roughly 130 years and a nucleus approximately 26 km in diameter, Swift-Tuttle is one of the largest known relatively short-period comets, with enough mass to cause catastrophic destruction if it were to collide with Earth - though current orbital calculations rule out any collision for the next several millennia.

The next perihelion of Swift-Tuttle is predicted for July 12, 2126, when the comet should reach naked-eye magnitude 0.7 and pass just 22.9 million km from Earth - close enough to be a memorable spectacle, but with no collision risk. The Perseids will continue lighting up every August long before that.

History and discovery

Lewis Swift spotted the comet on July 16, 1862, from Marathon, New York, and Horace Tuttle independently observed it three days later, July 19, from the U.S. Naval Observatory in Washington. Dual discoveries in such short intervals were common in the pre-telegraph era, when communication between observers took days. The comet was visible to the naked eye for several weeks, reaching magnitude 2 at its peak.

After the 1862 pass, the comet was lost and initial orbital calculations were imprecise enough to create uncertainty about its exact period. Brian Marsden of the Smithsonian Astrophysical Observatory used historical Japanese and European records to trace previous apparitions and predicted the comet would be rediscovered around 1981 or, more likely, around 1992. Rediscovery came on September 26, 1992, by Tsuruhiko Kiuchi of Japan, confirming Marsden's predictions. Historical records traced by Marsden include an apparition in 188 BC documented in Chinese sources, and possibly the 69 BC apparition described by Roman sources.

Orbit and returns

Swift-Tuttle has a highly elliptical orbit, with perihelion about 0.96 AU from the Sun (slightly inside Earth's orbit) and aphelion at roughly 51 AU, beyond Pluto's orbit. The orbital period is approximately 130 years: the last perihelion occurred on December 12, 1992, and the next is predicted for July 12, 2126. At that next pass, the comet is expected to reach naked-eye magnitude 0.7 and pass just 22.9 million km from Earth - far more favorable than the 1992 return.

In 1992, Brian Marsden initially calculated the comet might collide with Earth in August 2126. Subsequent refined calculations ruled out any collision - the predicted minimum approach in 2126 is about 0.15 AU (22.9 million km, well beyond the Moon's orbit). Long-term analyses also rule out collision in 2261. The orbit is inclined about 113 degrees to the ecliptic plane, which means Perseid meteors enter Earth's atmosphere almost head-on against the direction of Earth's orbital motion - hence the very high entry speed of 59 km/s.

Nucleus, coma and tail

Swift-Tuttle's nucleus is approximately 26 km in diameter, making it one of the largest known relatively short-period comets. For comparison, the asteroid that caused the mass extinction of the dinosaurs 66 million years ago was estimated at 10 to 15 km across - Swift-Tuttle is about twice as large. Estimated nuclear mass is roughly 1 to 2 x 10¹⁷ kg, and the kinetic energy of a hypothetical impact would be on the order of 10²⁸ joules - enough to trigger global mass extinction.

At the 1992 perihelion the comet developed a coma hundreds of thousands of kilometers across and a naked-eye dust tail. Cometary activity was substantial, with gas and dust jets visible in amateur telescope images. Spectroscopic composition determined in 1992 is typical of CO and H2O-rich comets, with reasonable abundance of organic compounds. A surface crust, as in other active comets, covers most of the nucleus, with active jets emerging from regions more exposed to sunlight.

How to observe

The most immediate legacy of Swift-Tuttle for the general public is the Perseid meteor shower, active from approximately July 17 to August 24, peaking typically around August 11 to 13. The Perseids are generated as Earth crosses the debris trail Swift-Tuttle deposited across its many past orbits. Fragments enter the atmosphere at roughly 59 km/s, one of the highest entry velocities among major meteor showers, producing fast and often bright meteors with persistent trains.

Zenithal hourly rates (ZHR) in favorable years reach 100 meteors per hour. In 2016 the Perseids produced an exceptional outburst with rates exceeding 200 meteors per hour during brief periods, caused by Earth passing through a particularly dense debris filament. The radiant lies in Perseus, and meteors can be seen in any part of the sky. Best viewing: dark skies, after midnight (when the radiant is high), on nights close to the peak. A waxing gibbous or full moon can significantly impair visibility in some years.

When the next perihelion arrives in 2126, the nucleus itself may be a notable naked-eye spectacle, reaching magnitude 0.7 - comparable to stars like Betelgeuse or Rigel.

Missions and scientific exploration

No spacecraft has visited Swift-Tuttle close-up through 2026. The 130-year orbital period makes rendezvous missions extremely difficult to plan and fund - a probe launched now would reach the comet only around 2030 to 2050 (on the outbound leg from the 1992 perihelion), when it would be more than 20 AU from the Sun and completely inactive.

The main scientific contribution of Swift-Tuttle comes from analysis of Perseid meteors via meteor radars, video cameras and spectroscopy of fragments Earth intercepts annually. Studies published in the 2010s and 2020s mapped the internal structure of the Perseid debris trail, identifying filaments deposited during specific returns - including filaments from 1862, 1079 and even older passes. When Earth crosses these dense filaments, activity rates spike well above normal.

Spectroscopic analysis of Perseid meteors reveals composition rich in magnesium, iron and sodium, with an abundance of organic compounds, consistent with other Jupiter-family comets. The extreme entry velocity (59 km/s) generates very high-temperature plasma, allowing detailed spectroscopy of fragments just millimeters across.

Trivia and records

• At roughly 26 km in diameter, Swift-Tuttle's nucleus is about twice the size of the asteroid that caused the dinosaur extinction 66 million years ago, and travels at roughly 60 km/s relative velocity - making it the most dangerous known natural object in potential Earth-crossing trajectory, though its current orbit rules out any collision for many millennia.
• The Perseids have been called "Tears of Saint Lawrence" in European Christian tradition, as their peak coincides with the feast of Saint Lawrence on August 10, documented at least since the 10th century AD.
• Brian Marsden, who correctly predicted the 1992 return using historical records, initially warned of a possible 2126 collision - generating worldwide headlines. The subsequent correction received far less coverage.
• The Perseid debris trail is so extensive that Earth takes several days to cross the full stream, producing visible meteors for more than a month around the August 12 peak.
• In 2016 the Perseids produced an outburst with rates exceeding 200 meteors per hour during brief periods - more than double the normal rate - caused by Earth passing through a dense debris filament deposited centuries ago.
• The next perihelion on July 12, 2126 falls exactly 264 years after the comet's discovery. People born in 2026 will be 100 years old on that date.
• Perseid meteor shower has one of the best-defined radiants of any annual shower, making it ideal for astronomy outreach and citizen-science programs.

Other comets

See the full comet catalogue.