☄ Hartley 2 103P/Hartley 2

How to follow comet Hartley 2 live

The panel above recomputes the position of Hartley 2 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 Hartley 2 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 Hartley 2 with no math at all.

Comet fact sheet

About Hartley 2

103P/Hartley 2 is a short-period comet also classified as hyperactive: its gas and water production rate far exceeds what would be expected for a nucleus only 2.25 km long. The EPOXI flyby in November 2010 revealed a bilobed, peanut-shaped nucleus -- two distinct lobes connected by a narrower waist -- and showed that the main jets were driven by sublimating CO2 (carbon dioxide) rather than water, a finding that reshaped models of cometary activity.

Hartley 2 also entered the record books for the deuterium-to-hydrogen ratio in its water, nearly identical to Earth's oceans -- a data point that fuelled an ongoing debate about the origin of terrestrial water. And it was the scientific target of a spacecraft originally built for an entirely different mission: a prime example of smart reuse of space hardware.

History and discovery

Malcolm Hartley discovered the comet on March 15, 1986, at Siding Spring Observatory in Australia during photographic surveys with the United Kingdom Schmidt Telescope. Hartley, who worked as a calculator and astrometrist, discovered several comets and asteroids using photographic plates -- and found 103P on the same plate where he was searching for asteroids, without the comet being the intended target.

The comet was assigned the designation 103P as the 103rd confirmed periodic comet. The orbital period was calculated at roughly 6.46 years. In the decades after discovery 103P was observed at multiple returns without much scientific or public attention -- until NASA chose it as the destination of a spacecraft that had completed its primary mission and still had fuel and working instruments.

Orbit and returns

The orbit of 103P/Hartley 2 has an eccentricity of 0.6942 and an inclination of 13.6 degrees. Perihelion falls at 1.059 AU, just beyond Earth's orbit, and aphelion sits at 5.88 AU between Jupiter and Saturn. The orbital period is about 6.47 years.

The 2010 return was exceptional: the comet closed to just 0.123 AU from Earth on October 20 -- the smallest Earth distance on record for 103P. Combined with the EPOXI mission timing, that return became the most thoroughly studied in the comet's history.

Nucleus, coma and tail

The nucleus of Hartley 2 is bilobed: two distinct lobes of different sizes connected by a narrow waist. Total length is 2.33 km, with the smaller (more active) lobe at the more sunlit end. The surface is extremely dark -- albedo around 0.028 -- and shows rough texture at the ends and smoother terrain at the central waist.

The coma of Hartley 2 is notably green in long-exposure photographs, a color caused by C2 (molecular carbon) and CN (cyanide). In the 2010 return the coma grew to tens of thousands of kilometres in diameter. The ion tail was faint but detectable in long-exposure images.

The most unusual feature of Hartley 2 is its hyperproductivity: the water production rate varies by a factor of 100 between aphelion and perihelion -- one of the largest variations recorded for a short-period comet. Near perihelion the comet produces far more water than its surface area should allow for an object of its size.

How to observe

In the 2010 return 103P/Hartley 2 reached magnitude 4.5 to 5 in Perseus, marginally naked-eye under exceptional conditions and easily seen in binoculars. That was the most favourable return on record. In less favourable returns (such as 2017) it stays at magnitude 11 to 12 and requires a telescope.

For the 2023 return the comet reached magnitude 9 to 10 -- accessible with 100 to 150 mm aperture telescopes. The next favourable return to monitor is 2030. To calculate the current position, use JPL Horizons or astronomy apps with an updated comet database.

Missions and notable observations

The EPOXI mission was an extension of the Deep Impact spacecraft, which in 2005 had fired an impactor at comet Tempel 1. After that mission the spacecraft -- still fully operational -- was reprogrammed and renamed EPOXI (Extrasolar Planet Observations and Characterization / Deep Impact Extended Investigation). The table below summarises flyby milestones:

The most important result was the identification that jets from the smaller lobe were driven by CO2, not water. The CO2 carried macroscopic water-ice chunks -- snowflakes from 1 mm to 20 cm, the first ever confirmed on any comet. The larger lobe released water passively without prominent jets. The deuterium-to-hydrogen (D/H) ratio measured in Hartley 2 water was 1.61 parts per ten thousand -- nearly identical to Earth's ocean water (1.56 per ten thousand) and to the ratio measured by Rosetta at comet 67P.

Trivia and records

• The EPOXI mission was created by repurposing the Deep Impact spacecraft after its original Tempel 1 mission -- a rare example of complete reuse of interplanetary space hardware.
• The snow chunks observed in Hartley 2's jets were the first macroscopic snowflakes ever confirmed on any comet; estimates place grain sizes from 1 mm to 20 cm in diameter.
• The bilobed nucleus of Hartley 2 displays complex tumbling rotation with precession, making it difficult to precisely predict jet orientation from one orbit to the next.
• Hartley 2's water production rate varies by a factor of 100 between aphelion and perihelion -- one of the largest variations ever recorded for a short-period comet.
• Malcolm Hartley found the comet by accident: he was searching for asteroids on the same photographic plate, and the comet was not the intended target.
• The November 2010 flyby was the fifth time in history that a spacecraft had approached a cometary nucleus, after Halley (Giotto, 1986), Borrelly (Deep Space 1, 2001), Wild 2 (Stardust, 2004), and Tempel 1 (Deep Impact, 2005).
• The D/H ratio of Hartley 2 matches Earth's ocean water, suggesting Kuiper Belt comets may have contributed some of Earth's water -- but the Rosetta mission at 67P found a different value, indicating the story is more complex than a single comet type.

Other comets

See the full comet catalogue.