☄ Borrelly 19P/Borrelly

How to follow comet Borrelly live

The panel above recomputes the position of Borrelly 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 Borrelly 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 Borrelly with no math at all.

Comet fact sheet

About Borrelly

19P/Borrelly is a short-period comet with an elongated, irregular nucleus measuring roughly 8 km long by 4 km wide. Its perihelion at 1.35 AU brings it inside Mars's orbit at every return, making it a worthwhile target in favourable years. The Deep Space 1 visit in September 2001 provided the highest-resolution cometary nucleus images obtained to that date -- and revealed a surface with two radically different terrain types, with gas and dust jets emerging from the smoother, more unexpected region.

The mission was even more remarkable in context: Deep Space 1 had been launched as a testbed for advanced technologies including ion propulsion and autonomous navigation. The Borrelly flyby was an opportunistic scientific objective added after the primary mission succeeded -- and it turned out to be the most remembered chapter of the spacecraft's history.

History and discovery

Alphonse Louis Nicolas Borrelly discovered the comet on December 28, 1904, at Marseille Observatory in France. Borrelly was one of the most prolific astronomers of his era: between 1874 and 1904 he found 19 comets, cementing his place among the great names of 19th-century French observational astronomy. 19P was the last comet of his career. The periodic designation 19P indicates it is the nineteenth confirmed periodic comet.

In the decades after discovery 19P/Borrelly was regularly observed at its 6.8-year returns. The orbit is influenced by Jupiter, which caused minor perturbations through the twentieth century without fundamentally changing perihelion. At several returns the comet reached magnitude 6 to 9, requiring binoculars or a small telescope. The 2001 return coincided with the opportunity window for Deep Space 1, and the comet gained international recognition it could not have achieved on its own.

Orbit and returns

The orbit of 19P/Borrelly has an eccentricity of 0.6241 and an inclination of 30.32 degrees -- relatively high for a Jupiter-family comet. Perihelion sits at 1.354 AU, placing the comet inside Mars's orbit at each passage, while aphelion reaches 5.835 AU just beyond Jupiter. The orbital period is 6.84 years.

Nucleus, coma and tail

The nucleus of Borrelly has a bowling-pin shape: elongated, with two lobes and a pronounced waist. The sharpest image, obtained by Deep Space 1 at 45 to 58 metres per pixel, revealed an object roughly 8 km long by 3 to 4 km wide at the ends. The albedo is only 0.03 -- the lowest measured for any Solar System object at that time, darker than any C-type asteroid or Uranian ring.

The surface shows two clearly distinct terrain types: a wide, relatively smooth central region (surprisingly brighter), with mesa-like structures and shallow depressions; and rough, dark ends with ridges and parallel grooves. The surprise: the main gas and dust jets emerged from the smooth region, not the rough dark areas. This inversion challenged the assumption that active regions would necessarily be rough and well-sunlit.

Borrelly's coma in favourable returns shows subtle greenish colour. The dust tail is moderate and the ion tail is more prominent than in other similarly bright Jupiter-family comets, possibly related to high CO production.

How to observe

Under favourable conditions 19P/Borrelly reaches magnitude 7 to 8, accessible with 10x50 binoculars under a dark sky. In less favourable returns it stays between magnitude 9 and 12. The comet is not suitable for urban observers, but is regularly tracked by enthusiasts with modest equipment (80-mm refractor or 150-mm reflector) at dark sites.

The 2022 return was particularly well placed for the southern hemisphere. The next perihelion is expected in February 2029. To locate the comet, consult JPL Horizons ephemerides near perihelion: Borrelly moves noticeably against the star background within a few days when close to the Sun.

Missions and notable observations

Deep Space 1 (DS1) was launched on October 24, 1998, as a technology demonstration mission under NASA's New Millennium program. Its primary goal was to test 12 advanced technologies in deep space, including ion propulsion, autonomous navigation, and miniaturised communication systems. After completing the primary phase successfully, the spacecraft was redirected to Borrelly as a scientific extension mission.

During the flyby the comet caused an unexpected problem: the coma temporarily blinded the spacecraft's star tracker (the instrument that determines orientation in space). Engineers on the ground had to reprogram the spacecraft to use the ion engine tracker as a substitute -- and even so obtained the best comet images to that date, at 47 metres per pixel.

Trivia and records

• Deep Space 1 was originally a testbed for 12 advanced technologies including ion propulsion and autonomous navigation. The Borrelly flyby was added as a secondary scientific objective after the primary mission succeeded -- and turned out to be the most-remembered chapter of the spacecraft's history.
• The ion propulsion tested by DS1 (ionised xenon engine) delivered only 90 mN of thrust -- less than the weight of a sheet of paper -- but operated continuously for months, accumulating speed over time. It became the basis for the Dawn (Vesta and Ceres) and Hayabusa (asteroid) missions.
• The nucleus of Borrelly has an albedo of only 0.03 -- it reflects 3% of sunlight. It is darker than charcoal (albedo ~0.04) and was, at the time of the flyby, among the darkest objects in the Solar System ever measured.
• Alphonse Borrelly died on February 28, 1926, just a few years before the telescope technology of the era reached its natural limits. He never knew the comet bearing his name would be visited by a spacecraft.
• During the flyby the DS1 star tracker was temporarily blinded by the coma; the spacecraft operated with the ion engine tracker as backup -- a contingency plan that worked in practice but had never been tested before.
• The gas and dust jets from Borrelly emerged from the smooth central region, not the rough dark ends -- contradicting the intuition that more sunlit and exposed regions should be the most active.
• The 2022 return produced some of the best amateur images ever obtained of Borrelly, with 300-mm telescopes detecting internal coma structure.

Other comets

See the full comet catalogue.