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Active vents of comet Halley (lower middle) release jets of gas and dust that are seen as white streaks in reflected sunlight. The largest nucleus measured was of Hale-Bopp (70 kilometers; about 6 times larger than Halley’s nucleus).
Comets and asteroids formed during collapse of the natal molecular cloud core, and their compositions depend greatly on the time and place of their formation. As remnants of the early Solar System, comets and asteroids are central to understanding its origin and formation. Many asteroids appear to be the shattered remnants of primitive rocks.
LARGE ASTEROIDS ARE DWARF PLANETS: Small asteroids are rubble piles, but large asteroids like Ceres and Vesta may have a layered internal structure, or even a core. Ceres (the largest known asteroid) is 950 kilometers (km) in diameter. Vesta is 530 km across. For comparison, Earth is 12,742 km across, Mars is 6,779 km, and Pluto is 2,390 km.
NASA spacecraft visited several comets and asteroids, and even brought material from comet Wild-2 back to Earth. Goddard scientists found glycine, a building block of life, in samples of Wild-2 collected by the Stardust spacecraft.
The iconic object above is a scale model of the nucleus of comet 103P/Hartley-2. Small bodies, such as comets and asteroids, have very irregular shapes due to their small masses (and thus, low gravity). Shape model courtesy of the EPOXI team and the PDS Small Bodies Node/P. Thomas & T. Farnham.
COMET HARTLEY-2: This false-color image of the nucleus of Hartley-2 shows jets of water vapor (blue), CO2 (green) and dust (yellow). The ices of the measured gases are not mixed uniformly in the nucleus of this comet.
Comet nuclei are about 50-50 composed of ices (water, dry ice and others) and rocky minerals, while asteroids are richer in minerals. Goddard scientists investigate cometary ices by measuring the gases released from the nucleus. They find that comets are highly diverse in their organic (for example: methanol,ethane, methane) and nitrogen- containing ices (hydrogen cyanide, ammonia), whether the comets reside in the Oort Cloud or the Kuiper Belt.
Comets and asteroids delivered much of the water, and building blocks of life, to our planet. The fraction of each delivered in this way is under intense study. Importantly, the isotopic compositions of water are the same in comet Hartley-2and in Earth’s oceans, but other comets are different. Did comets just like Hartley-2 deliver Earth’s oceans?
A comet nucleus could serve as a rich resource for a future space colony. A comet of 1 kilometer diameter contains about 500 million cubic meters of material, or about 250 million metric tons of ice and mineral-rich dust, enough to supply a colony with fuel, water, metals and food for a very long time.