☄ Grigg-Skjellerup 26P/Grigg-Skjellerup

How to follow comet Grigg-Skjellerup live

The panel above recomputes the position of Grigg-Skjellerup 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 Grigg-Skjellerup 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 Grigg-Skjellerup with no math at all.

Comet fact sheet

About Grigg-Skjellerup

26P/Grigg-Skjellerup has a discovery story that spans two countries and three decades. New Zealand astronomer John Grigg detected it on 23 July 1902, but it was promptly lost. Only in 1922 did Norwegian-born South African astronomer Frank Skjellerup recover it and confirm it was the same object, establishing the hyphenated name it carries today.

With a period of about 5.11 years, it is a short-period Jupiter-family comet. What might have remained just another modest object on the JPL list became a landmark in space exploration history when ESA's Giotto probe, after its 1986 Halley mission, was redirected in the Giotto Extended Mission (GEM) to fly within a few kilometres of its nucleus in 1992.

History and discovery

John Grigg, from Thames, New Zealand, discovered the comet in July 1902 with a 5-inch refractor. Early observations were enough to compute a preliminary orbit, but the comet faded from view before additional data could be gathered and remained unconfirmed for two decades.

In May 1922, Frank Skjellerup, a trained meteorologist and amateur astronomer working at the Cape Observatory in South Africa, independently rediscovered it. Orbital calculations showed it was Grigg's object, and the IAU convention established the combined name 26P/Grigg-Skjellerup. The original 5-inch amateur instrument makes this one of the few Solar System objects visited by a spacecraft that was first spotted with non-professional equipment.

Orbit and returns

26P/Grigg-Skjellerup's perihelion lies at about 0.99 AU from the Sun, almost exactly at Earth's orbital distance, while aphelion reaches 4.93 AU. Its period of 5.11 years is among the shortest in the Jupiter family. The Tisserand parameter T_J is approximately 2.9.

The orbit has been perturbed by Jupiter on several occasions throughout the twentieth century, shifting perihelion between 0.89 and 1.12 AU across different apparitions. On the most favourable returns the comet approaches Earth to within 0.3 AU, pushing apparent magnitude to around 8 to 9 and placing it within binocular reach. The most recent perihelion occurred in October 2023. The next is expected around 2028-2029.

Nucleus and dynamic family

The nucleus of 26P/Grigg-Skjellerup is estimated at only 2.6 km in diameter, roughly four times smaller than Halley's (about 11 km along the major axis). That small size was confirmed indirectly by Giotto's data: the gas and plasma coma measured in 1992 was less abundant than Halley's in 1986, consistent with a smaller, less active nucleus.

The comet belongs to the Jupiter family, the dynamical group that gathers objects with T_J between 2 and 3 whose orbits were shaped by encounters with Jupiter. Like other members of this group, Grigg-Skjellerup likely migrated from the Kuiper Belt. The relatively low orbital inclination (22.4 degrees) is typical of younger group members that have not yet been heavily perturbed in inclination.

How to observe

On the most favourable returns, 26P/Grigg-Skjellerup reaches magnitude 8 to 9, within reach of 10x50 binoculars under a dark sky. At median returns it sits between magnitude 10 and 12, requiring a 100 to 200 mm telescope. The coma is diffuse and low-condensation, without a sharp central nucleus.

Its short period of 5.11 years guarantees frequent returns. For the 2028-2029 cycle, check JPL Horizons ephemerides to pinpoint the peak brightness window. Southern hemisphere observers have a geometric advantage on several returns due to the 22.4-degree orbital inclination.

• Minimum aperture (good return): 10x50 binoculars
• Minimum aperture (median return): 100 mm
• Peak magnitude (good return): 8 to 9
• Next estimated perihelion: 2028-2029

The Giotto mission and the science of the 1992 encounter

ESA's Giotto probe was launched in July 1985 to intercept Comet Halley, which it did in March 1986, passing within 596 km of the nucleus. After the primary mission, ESA performed an Earth gravity-assist manoeuvre in July 1990 to redirect the spacecraft toward 26P/Grigg-Skjellerup, calling this second phase the Giotto Extended Mission (GEM).

On 10 July 1992, Giotto flew within 200 km of the Grigg-Skjellerup nucleus at a relative velocity of 13.99 km/s. Although the onboard camera (HMC) was switched off to conserve power, other instruments recorded fundamental data. The EPONA energetic particle detector captured the first cometary ion signatures 440,000 km from the nucleus, 12 hours before closest approach. The magnetometer detected plasma waves with a period of about 70 seconds, generated by water ions picked up by the solar wind. Data from the encounter also raised the hypothesis that a smaller companion object, possibly a nucleus fragment, was accompanying Grigg-Skjellerup at the time.

Facts worth knowing

• The GEM (Giotto Extended Mission) to Grigg-Skjellerup ran on residual propellant, costing a fraction of a new mission's budget.
• The nucleus is estimated at 2.6 km in diameter, roughly four times smaller than Halley's.
• Frank Skjellerup, who recovered the comet in 1922, was a trained meteorologist and an astronomer by passion.
• Giotto's flyby velocity was 13.99 km/s: at that speed it would travel from London to Edinburgh in about 45 seconds.
• Giotto's EPONA detector sensed the first cometary ions 440,000 km out, 12 full hours before closest approach.
• Analysis of the Giotto data raised the hypothesis that a smaller fragment was accompanying the main nucleus, though the disabled camera could not confirm this visually.
• The comet is one of the few Solar System objects visited by a spacecraft whose original discovery was made with amateur-grade equipment, a 5-inch aperture telescope.

Other comets

See the full comet catalogue.