Comet Hartley 2: the icy heart scrutinized by Deep Impact

Comet Hartley 2 photographed by the Deep Impact probe on November 4, 2010 — Comet Hartley 2 seen by the Deep Impact probe during its EPOXI mission.
Image source: NASA/JPL-Caltech/UMD (new window)

Scientific summary

The Astronoo article presents comet 103P/Hartley 2, a small icy body from the Jupiter family, flown by the Deep Impact probe in 2010 as part of the EPOXI mission. The observations reveal a bilobed, dumbbell-shaped nucleus about 2.2 km long, whose activity is powered by jets rich in carbon dioxide. This particular volatile composition explains intense sublimation and projects aggregates of water ice. The article also explores the implications of the water's isotopic composition for the origin of Earth's oceans and places Hartley 2 in the history of cometary flybys.

What makes comet Hartley 2 so special for the study of comets and the origin of Earth's water?

This article is devoted to comet 103P/Hartley 2, a periodic object from the Jupiter family discovered in 1986. But what makes this comet so special? First, its nucleus has an exceptional bilobed shape, resembling a peanut, which probably reflects the gentle merging of two icy bodies. Then, during the close flyby of the Deep Impact probe in November 2010, the cameras revealed particularly active jets of carbon dioxide, capable of projecting grains of water ice. This activity, powered by internal CO\(_2\) reservoirs, differs from that of water-dominated comets. Finally, the isotopic composition of Hartley 2's water proves compatible with that of Earth's oceans, which strengthens the hypothesis that Jupiter Family Comets may have contributed to the delivery of water to the young Earth. The article thus details the discovery, orbit, structure and scientific lessons of this historic flyby.

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An icy rendezvous in space

A close flyby of the comet

On November 4, 2010, the Deep Impact probe, reassigned to the EPOXI mission, flew past comet 103P/Hartley 2 at a distance of only 700 km. This small celestial body, about 2.2 km long and 0.4 km wide, offered an unexpected spectacle: a dumbbell-shaped nucleus, from which jets of gas and dust escape. Hartley 2, although modest in size, reveals surprisingly intense activity.

Structure and composition of the nucleus

A bilobed nucleus rich in ices

The high-resolution images show a nucleus with two dense lobes connected by a narrower and less massive central region. This configuration suggests a bilobed structure, potentially formed by the gentle accretion of two distinct objects. The comet contains a high proportion of CO\(_2\) ice, volatile at low temperature, which explains persistent activity even far from the Sun.

Unexpected jets of carbon dioxide

Jets of CO\(_2\) and water ice

Unlike typical comets that release water as they approach the Sun, Hartley 2 massively releases CO\(_2\) from its extremities. These jets are capable of projecting centimeter-sized aggregates of water ice at high speed. This behavior reveals a deep degassing mechanism, where solar heat reactivates internal pockets of CO\(_2\) ice, stimulating sublimation and erosion processes.

A mission with multiple objectives

EPOXI and cometary exploration

The EPOXI mission combined two objectives: the study of exoplanets (Epoxi) and the exploration of comets (DIXI). Hartley 2 was thus the fifth comet to be closely flown by a space probe, after Halley, Borrelly, Wild 2 and Tempel 1. The detailed analysis of its surface and dynamics allows a better understanding of the first materials of the Solar System, preserved in the cometary nucleus.

Implication for the origin of Earth's water

An isotopic signature close to that of the oceans

The isotopic composition of Hartley 2's water, measured indirectly, is compatible with that of the Earth's oceans (\(D/H \sim 1.61 \times 10^{-4}\)), which fuels the hypothesis that comets of the Jupiter Family Comets (JFC) type may have played a major role in delivering water to Earth.

Main characteristics of comet Hartley 2

Orbital and physical data

The table below gathers the main orbital and physical characteristics of comet Hartley 2.

Main data of comet Hartley 2
Parameter	Value	Unit	Remarks
Official name	103P/Hartley	–	Jupiter-family periodic comet
Orbital period	6.47	years	Strongly influenced by Jupiter
Nucleus dimensions	2.2 × 0.4	km	Elongated bilobed shape
Perihelion distance	1.05	AU	Just beyond Earth's orbit
Volatile composition	CO\(_2\), H\(_2\)O	–	CO\(_2\)-rich jets
Flyby date	November 4, 2010	–	700 km from the nucleus

References

FAQ: Everything you need to know about comet Hartley 2

What is comet Hartley 2?

Comet Hartley 2, officially designated 103P/Hartley 2, is a periodic comet from the Jupiter family. It was discovered in 1986 by astronomer Malcolm Hartley. Its nucleus measures about 2.2 km long and orbits the Sun in 6.47 years.

Why is comet Hartley 2 famous?

It is famous for the close flyby of the Deep Impact probe in November 2010, as part of the EPOXI mission. The images obtained revealed a bilobed nucleus and particularly active carbon dioxide jets, offering an unprecedented glimpse of cometary activity.

What is the shape of its nucleus?

Its nucleus has an elongated bilobed shape, often compared to a peanut or a dumbbell. It consists of two dense lobes connected by a narrower central region, suggesting a possible accretion of two distinct bodies.

What did the EPOXI mission discover during the 2010 flyby?

The EPOXI mission discovered that Hartley 2 massively releases CO\(_2\) from its extremities, unlike water-dominated comets. These jets project aggregates of water ice and reveal a deep degassing mechanism.

Can comet Hartley 2 explain the origin of Earth's water?

The isotopic composition of Hartley 2's water is compatible with that of Earth's oceans. This strengthens the hypothesis that comets of the Jupiter Family Comets type may have contributed to the delivery of water to the young Earth, without however being the sole source.

How can one observe or obtain data on Hartley 2?

As Hartley 2 is a periodic comet, it becomes observable again during its passages near the Sun. The scientific data from the flyby are archived by NASA and JPL. Images and results are available via the public databases of the EPOXI mission.