☄ Holmes 17P/Holmes

How to follow comet Holmes live

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

Comet fact sheet

About Holmes

17P/Holmes is a periodic comet with a nearly circular orbit by cometary standards: its perihelion lies at 2.05 AU, in the asteroid belt between Mars and Jupiter, and it never comes close to the Sun on a cometary scale. This made its two extreme brightness outbursts all the more puzzling: the first in November 1892, shortly after discovery, and the far more spectacular second in October 2007, when the expanding coma made the comet temporarily the largest object in the Solar System by volume -- surpassing even the Sun.

The 2007 outburst was, according to available historical records, the greatest spontaneous brightness increase ever documented for any comet. The amplitude: from magnitude 17 (visible only with professional telescopes) to magnitude 2.8 (comparable to the brightest winter stars) in 42 hours. And the comet was moving away from the Sun, not toward it, ruling out increasing solar heating as the cause.

History and discovery

Edwin Holmes discovered the comet on November 6, 1892, from London while routinely observing the Andromeda Galaxy through a 30-cm telescope. At the time the comet was in outburst, reaching magnitude 4 to 5 and visible to the naked eye. Holmes initially thought he had discovered a new nebula in Andromeda -- the position and diffuse appearance were consistent with this. Comparing with observations from previous nights that showed no such "nebula," he realised the object was new and moving.

Just days after discovery the comet faded rapidly. Its periodic orbital nature was confirmed through subsequent calculations, and the period was estimated at about 6.9 years. The comet was recovered at several oppositions through the twentieth century. No return between 1892 and 2007 produced notable visibility -- until the night of October 23-24, 2007, changed everything.

Orbit and returns

The orbit of 17P/Holmes is unusual for a comet: eccentricity of only 0.4324 -- much closer to a circle than a typical cometary ellipse -- and inclination of 19.1 degrees. Perihelion falls at 2.053 AU, in the asteroid belt between Mars and Jupiter, while aphelion reaches 5.183 AU just beyond Jupiter. This means the comet spends its entire life in regions of the Solar System where other objects would show no visible activity.

The orbital period is approximately 6.88 years. The two documented outbursts -- 1892 and 2007 -- occurred 115 years apart, with many uneventful returns between them. This suggests outbursts are rare even for this comet, and their cause remains without consensus explanation.

Nucleus, coma and tail

The nucleus of 17P/Holmes is small: estimates based on residual brightness outside outburst periods point to a diameter of 3.4 to 3.6 km, with an albedo of about 0.04. Under normal conditions (outside outburst), the comet is invisible without large-aperture telescopes, at magnitude 14 to 17.

During the October 2007 outburst the coma expanded at a phenomenal rate. Measurements indicate:

• October 24, 2007 (one day after outburst onset): coma diameter roughly 100,000 km
• November 9, 2007: coma linear diameter 1.4 million km -- larger than the Sun's diameter (1,392,000 km)
• November--December 2007: angular diameter as seen from Earth exceeding 1.4 degrees, equivalent to nearly three full moons side by side

The expansion was not uniform: high-resolution images revealed concentric rings and spiral structures, suggesting multiple pulses of ejected material. The ion tail was nearly non-existent during the outburst -- most ejected material was dust and solid grains, not ionised gas.

How to observe

In its normal state (outside outburst), 17P/Holmes is invisible without a telescope of at least 300 mm aperture, at magnitude 14 to 17. There is no practical way to predict when an outburst will occur -- the two documented events happened 115 years apart, and intermediate returns produced no notable events.

In the event of a new outburst, Holmes would be visible to the naked eye in the constellation Perseus (where it was in 2007 and where it returns in 2028). To monitor potential future outbursts, follow comet alert lists such as the Comets Mailing List or amateur alert networks like COBS (Comet Observation Database). The comet is normally in Perseus or neighbouring regions at its 2028 return.

The 2007 outburst: what we know and what we do not

The October 2007 outburst was independently detected by two amateur observers -- Juan Antonio Henriquez Santana in the Canary Islands and Ramon Naves in Barcelona -- on the night of October 23-24, 2007. The comet had been photographed two days earlier without anything unusual. In under 42 hours brightness increased by a factor of roughly one million (about 15 magnitudes).

Three main hypotheses have been proposed to explain the outburst:

None of the hypotheses has been conclusively confirmed. The 2007 outburst remains one of the best-documented and least-understood cometary events in the history of astronomy.

Trivia and records

• The October 2007 outburst was the largest spontaneous brightness increase documented for any comet: from magnitude 17 to 2.8 in 42 hours, an amplification factor of roughly one million.
• At peak expansion, Holmes's coma had a linear diameter larger than the Sun's -- making it temporarily the largest object in the Solar System by volume, though with mass and density absolutely negligible compared to the Sun.
• The comet remained naked-eye visible for more than three months after the 2007 outburst, an exceptional period for a comet that never comes closer to the Sun than 2 AU.
• In 1892 Holmes thought he had discovered a nebula in Andromeda -- the position and diffuse appearance were misleading. Only by comparing with photographic plates from previous nights did he realise the object was new and moving.
• Retrospective calculations found no gravitational perturbation by Jupiter or any other planet that could have triggered the 2007 event.
• The 1892 and 2007 outbursts both occurred approximately five months after perihelion -- a temporal coincidence that reinforces the delayed thermal trigger hypothesis without proving it.
• Tens of thousands of observers worldwide followed Holmes in October and November 2007, making it the most simultaneously observed comet by amateurs since Hale-Bopp in 1997.

Other comets

See the full comet catalogue.