103P/Hartley

• 1. Introduction

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  103P/Hartley, known as Comet Hartley 2, is a comet so small that a spacecraft once flew within 700 km of it and scientists compared its shape to a cross between a bowling pin and a pickle. It measures somewhere between 1.2 and 1.6 km across. Yet this diminutive object has handed researchers some of the most unexpected findings in modern cometary science. Why does it produce more water vapor than other comets its size? What do the glittering, blocky objects covering its rough ends tell us about the earliest days of the Solar System? And what does the ratio of heavy water locked inside this little comet suggest about where Earth's oceans came from?

• 2. Discovery and Orbit

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  Malcolm Hartley found this comet in 1986 while working at the Schmidt Telescope Unit at Siding Spring Observatory in Australia. The comet belongs to a class called Jupiter-family comets, meaning Jupiter's gravity has shaped its path into a relatively short loop around the Sun. Its orbital period runs about 6.46 to 6.48 years, and its closest point to the Sun, known as perihelion, sits near 1.05 AU, almost exactly at the distance of Earth's own orbit.

  In October 2010, Hartley 2 passed within 0.12 AU of Earth, just eight days before reaching perihelion on the 28th of October 2010. From northern latitudes during early November that year, the comet became dimly visible to the naked eye, reaching an apparent magnitude of about 5. The comet made another close pass on the 26th of September 2023, coming within roughly 57 million km of Earth. It reached perihelion that time on the 12th of October 2023, brightening to about magnitude 8.

  Despite passing near Earth's orbit repeatedly, Hartley 2 is not yet a known source of meteor showers. Dust trails from its recent returns do move in and out of Earth's orbit, however, and predictions suggest the 1979 dust trail is expected to intersect Earth's path in both 2062 and 2068.

• 3. Portrait of a Nucleus

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  Observations by the Spitzer Space Telescope in August 2008 measured the comet's nucleus at a radius of 0.57 plus or minus 0.08 km, with a very low albedo of 0.028, meaning it reflects only about 2.8 percent of the light that hits it. The mass of the comet is estimated at around 300 megatons.

  Radar observations by the Arecibo Observatory during the 2010 apparition revealed that the nucleus is highly elongated and rotates over an 18-hour period. The EPOXI mission project manager described the shape as a cross between a bowling pin and a pickle. The Deep Impact flyby later confirmed that the comet stretches 2.25 km in length and takes on a peanut shape, spinning around one axis while also tumbling around a different axis.

  At its rough, larger ends, the surface is studded with blocky objects roughly 165 feet high and 260 feet wide, about the size of a 16-story building. These objects appear two to three times more reflective than the average of the surrounding surface. The smooth, relatively inactive middle section of the peanut shape has probably been re-deposited there over time, as material ejected from the ends moves slowly enough to be captured by even the comet's weak gravity and falls back into its lowest point.

• 4. The EPOXI Mission

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  The Deep Impact spacecraft had already made history by photographing Comet Tempel 1 on the 4th of July 2005. When NASA sought a target for an extended mission, the original candidate was Comet Boethin. Boethin had not been observed since 1986, and its orbit could not be calculated with enough precision to allow a safe flyby, so NASA redirected the spacecraft toward Hartley 2.

  On the 4th of November 2010, the spacecraft closed to within 700 km of the nucleus while moving at 44,300 km/h. Data from the encounter were sent back to Earth through NASA's Deep Space Network. The flyby made Hartley 2 the fifth comet ever visited by a spacecraft and, at its size, the smallest comet ever visited.

  Mission scientist Don Yeomans captured the team's reaction when he described the comet as hyperactive, small, and feisty. Michael A'Hearn, the science team leader for the EPOXI mission and a University of Maryland astronomer, stated that early observations showed that, for the first time, researchers might be able to connect specific activity to individual features on the nucleus. A University of Maryland-led study was later published in the 17th of June issue of the journal Science, drawing on the mission's full dataset.

• 5. Carbon Dioxide, Ice, and Water

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  One of the most striking findings from the flyby concerned how the comet manages to be so active despite its small size. NASA scientists reported that jets of carbon dioxide, rather than water sublimating directly from the nucleus surface, are driving material off the rough ends of the comet. These jets drag water ice along with them, flinging hundreds of tons of fluffy ice and dust chunks into space, with the largest particles ranging from golf ball to basketball size.

  Michael A'Hearn stated plainly that Hartley 2 spews out more water than other comets its size, and that when warmed by the Sun, dry ice deep in the comet's body turns to gas and drags water ice with it. Scientists noted this was the first time that comet activity powered by sublimation of frozen carbon dioxide had been directly observed as a comet neared the Sun. The ice must be primordial, dating from the beginnings of the Solar System.

  The balance between carbon monoxide ice and carbon dioxide ice in Hartley 2 also yielded a broader insight. After reexamination, scientists found that the abundances of these two ices suggest short-period comets formed under warmer conditions than long-period comets, placing their origin closer to the Sun. This aligns with what the heavy water measurements also pointed toward.

• 6. Heavy Water and Earth's Oceans

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  In 2011, the Herschel Space Observatory detected the signature of vaporized water in the comet's coma. The ratio of heavy water to regular water in Hartley 2 turned out to be the same as the ratio found in Earth's oceans. Hartley 2 contains half as much heavy water as other comets previously analyzed, making it an outlier among its peers.

  For many years, scientists knew that few comets produced more water vapor than the simple sublimation of water ice from the nucleus should explain. The flyby of Hartley 2 showed that icy grains in the coma are largely driven out by carbon dioxide outgassing rather than by direct water sublimation. Barring a catastrophic breakup or major splitting event, the comet is estimated to survive up to another 100 apparitions, which works out to roughly 700 years at its current rate of mass loss.