Comet Hale-Bopp from The Perfect Silence


A general term for icy bodies whose orbits take them inside the "snow line" - roughly 2.5 AU, where their atmophile solid material sublimates into an atmosphere that interacts with the solar wind and IMF. Often conspicuous in the sky, therefore known to humans long before their true nature was understood.

Short-period cometary orbits from Abovetopsecret

Short-period comets:

Those that have orbital periods short enough that they appear repeatedly on the time scale of human history - typically 200 years or less. Their orbits may be very elliptical (in some cases extending to aphelia in the Kuiper Belt) but are otherwise tame:

Comet Halley in 1986 from The Daily Galaxy
Halley's comet: A typical short-period comet. Edmund Halley (1656-1742) first proposed that comets occur on long, highly elliptical orbits. After observing the comet shown at right in 1682, he noted that its appearance fit a pattern of historical sightings of a comet that went back at least as far as 1066. He predicted its return in 1757 (it returned in 1758). Halley's comet has an orbital period of 75.3 years.

Cometary structure:

Most of the time, comets are solid bodies with icy and silicate components. When they enter the inner Solar System, they become "active." Their ices begin to sublimate, yielding a fragile atmosphere that interacts with solar radiation and the solar wind, and kicking off dust particles that follow their own trajectory. The most intense activity occurs at perihelion passage when the comet makes its closest approach to the Sun.

We see the following typical features:

Because charged ions and electrically neutral dust interact differently with the solar wind, they are driven in slightly different directions, yielding the two distinct tails.

Cometary Exploration:

Active nucleus of Comet Halley from The Solar Space Station
Comet Halley - 1986: A total of six spacecraft from ESA, the Soviet Union, and Japan studied Comet Halley. ESA's Giotto spacecraft returned the first images of an active comet's nucleus. Its longest dimension is 15 km. Many active jets of sublimating gas are visible: Very little of this material freezes back onto the comet as it retreats into the outer Solar System. Halley loses about 6 m of its outer surface with each pass. When the Earth later passes through this debris, it becomes visible as the Orionid meteor shower.

Geomorphology of comet Borrelly from The Thunderbolts Project
Deep Space 1 to Comet Borrelly - 2001: The Deep Space 1 craft flew past Comet Borrelly, taking very detailed images of the nucleus of Comet Borrelly (Maximum length 8 km.) Borrelly's surface includes mesas, ridges, and hills, resembling terrestrial surface features. The mission also demonstrated the feasibility of sending a spacecraft near the nucleus of an active comet. This mission revealed that the comet's surface was dark and dry. Reservoirs of water and CO2 supplying active jets were apparently concealed beneath a non-volatile layer (JPL, 2002). On its surface, at least, Borrelly is more like an icy dirtball than a dirty snowball. This concentration of dust may, however, be a surface feature. Through the process of deflation, after many perihelion passages, dust that is left behind on a comet's surface may form a coating that shields underlying ice from the sun.

On Earth we see something similar when the wind creates a deflation surface by removing small grains from a sedimentary deposit and leaves the large ones.

Comet Tempel 1 from World News Network
The NASA Deep Impact mission targeted comet Tempel 1 - 2005. The idea was to photograph the comet, then smash a copper projectile into it, hopefully penetrating the dry deflation surface. The impact could reveal chemical composition of material form the comet's interior and assess its physical strength from the type of crater that formed upon impact.

Comet Tempel 1 just prior to impact from Astrnonomy Picture of the Day
The surface of Tempel 1 was similar to that of the (few) other comet surfaces that have been imaged. It has a complex surface with (apparent) craters and some smooth regions too. This photo was taken just before impact.

Deep Impact strikes Comet Tempel 1 from Wikipedia
The impact occurred on July 4, 2005. It kicked up a dust cloud that unfortunately did not dissipate while the flyby Deep Impact spacecraft could photograph the surface, however the crater was viewed later by the Stardust spacecraft.

Exposed ice on Comet Tempel 1 from NASA
However, we did learn something about its composition (Kadono et al., 2010):

Comet Wild 2 from NASA - JPL
Stardust and Wild 2 - 2004. Stardust flew by comet Wild 2 and collected samples from its cometary coma using an aerogel collector. Its sample capsule returned to Earth in January 2006. One of the reasons Wild 2 was chosen is that it had not passed near the Sun many times and should be in a more "pristine" state.

The Stardust samples yielded a big surprise. The particles from Wild 2 contain bits of minerals that form at a much wider range of temperatures than expected, including some very high-temperature minerals like olivine that can only have crystallized in the inner Solar System. (Matzel et al., 2010.) How this material got into the Kuiper Belt where the comet formed is the topic of intense speculation.

Active nucleus of Comet Hartley 2 from Astronomy Picture of the Day
EPOXI and Hartley 2 - 2010. EPOXI is the mission of the repurposed Deep Impact observer module, which flew past the active nucleus of Comet Hartley 2 in 2010. Here, the big surprise was that the active jets consisted of CO2 rather than water vapor. (A'Hearn et al., 2011)

Comet 67P/Churyumov- Gerasimenko from Wikipedia
The headline-maker: ESA's Rosetta launched in March 2004. Entered orbit around Comet 67P/Churyumov- Gerasimenko in August 2014, and has accompanied it through its perihelion passage.

On November 12, 2014, the Philae lander was released onto the comet's nucleus. The lander was intended to characterize the nucleus surface and study the comet's activities over time. Alas, harpoons designed to secure it to the comet's surface failed, and the lander bounced several times before coming to rest on its side in a place of permanent shade. It returned data for several hours until its batteries failed. Rosetta continued to function until 2016, and has returned major scientific results:

Long-period comet orbit from Chiemgau Impact

Comet populations:

In terms of orbits, there are two distinct groups of comets.

The nature of the orbits reveals two different sources for comets.

Oort cloud schematic from Biblioteca Pleyades
The Oort Cloud was first proposed in 1950 by Jan Oort, a Dutch astronomer, based on the observation that long period comets can enter the Solar System from any direction, and from calculations of their apoapses. Based on the frequency with which we see long-period comets, it is estimated that there are trillions of such comets in the Oort cloud. It extends from ~1000 AU to nearly a light-year from the Sun (maybe). The cloud is roughly spherical, with a denser core near the ecliptic plane of the solar system.

So far our discussions of the formation of the Solar System have always stressed that it originates as a roughly disk shaped protoplanetary nebula. How, then, did long-period comets come to occupy a spherical space and so far from the sun? Unlikely that they actually formed there. Different methods have been proposed, but all require that:

Fernandez et al., 2004 performed simulations showing that scattered disk objects whose periapses brought them near the orbit of Neptune had significant chances of being flung into the inner Solar System (as centaurs) or into the Oort cloud.

But once out there in the vasty deep, their orbits could be further altered by:

But now for the creepy aspect:

(Wait for it)

Aside from the presence of long-period comets, there is no direct observational evidence for the existence of the Oort Cloud. (Maybe detached objects like Sedna represent the inner-reaches of the Oort cloud.)

When comets die:

Sometimes this happens spectacularly, when a comet collides with an object like Jupiter or doesn't survive a perihelion passage. More often, as successive perihelion passages drain a comet of ices, it eventually fragments and disintegrates into interplanetary dust. Indeed, comet 332P/Ikeya-Murakami was observed in 2016 to be propelling fragments of itself into solar orbit during perihelion passage.

Where do short-period comets come from? Comets that enter the inner Solar System can't last long. Their life spans are measured in thousands or tens of thousands of years. So why do we still see them? The inner Solar System's "supply" of short-period comets seems to be continuously replenished through the perturbation of the orbits of Kuiper Belt objects.

Key concepts and vocabulary:
Additional reading: