The Oort Cloud
A Tour of the Outer Solar System
Discoveries of new planetesmials or small planetoids like Eris in the Oort Cloud led to a raging debate as to the definition of a planet. Pluto has been re-classified from planet to a Trans-Neptunian object, simply one more body orbiting in the concentric shell of rocks, snowballs and potential comets that orbit outside of Neptune's orbit.
Comets and other bodies in the Kuiper Belt and Oort Cloud are not meteoroids; this term refers to objects that drift within the inner solar system like the Leonid meteor shower.
The Kuiper Belt includes all objects orbiting within 30 to 50 astronomical units from the Sun. Objects farther out than 50 AU (astronomical units) are called scattered disk objects. The Oort Cloud is an even larger shell around the solar system.
The Oort Cloud includes everything between 2,000 and 100,000 astronomical units. The outer diameter of the Oort Cloud is one light year across. Astronomers face a serious challenge with the definitions of these different layers of the solar system, because there are numerous objects that do not fit neatly into the definition of an Trans-Neptunian object or Oort Cloud object.
What Is the Oort Cloud?
The Oort Cloud and Kuiper Belt are all that remains of the debris that once encircled the fledgling Sun. The halo of rock and ice coalesced over millions of years to create a planetary disk that eventually formed the inner planets. Unlike the planetary disk, the Oort Cloud and Kuiper Belt remain in orbit around the Sun in all directions, though the Oort Cloud is farther out and more disbursed than the Kuiper Belt.
Bodies in these distant clouds often orbit at a radical angle from the ecliptic defined by the orbits of the planets; this is called an inclination to the ecliptic. For example, Sedna is a dwarf planet with an orbit that takes it up to 975 astronomical units from the Sun every 12,500 years. Given its orbit, astronomers believe that Sedna originated in the Oort Cloud.
Bodies in the Oort Cloud and Kuiper Belt often have very eccentric orbits as well. 2008 KV42, nicknamed Drac but not officially given a name by the IAU, orbits almost perpendicular to the planets of the Solar System. 2008 KV42 is thought to have originated in the inner Oort Cloud, but its orbit does bring it in as close as the orbit of Uranus.
The Oort Cloud is sometimes divided into two layers. The torus shaped inner Oort Cloud orbits at 2,000 to 20,000 astronomical units. This inner Oort Cloud is also called Hills cloud. The torus of the inner Oort Cloud is thickest near the solar system's plane. The outer Oort Cloud is spherical and orbits at 20,000 to 60,000 astronomical units.
Several Oort Cloud bodies are covered by frozen methane residue that has broken down and re-linked into a reddish substance called tholin. The composition of Oort Cloud objects is estimated by its size and rotational speed, giving scientists an idea of its composition. Most Trans-Neptunian objects are comprised of ice or a thick layer of ice over a small, rocky core. Haumea is one of the exceptions of a mostly rocky body with a thin layer of ice.
The Oort Cloud could be considered the edge of the Solar System. It could also be classified as outer space, since it rests past the heliopause, the point where the solar wind slows and equals the pressure (or lack thereof) of deep space. The Oort Cloud does define the limit of the Sun's gravitational influence.
Exploration of the Oort Cloud
According to "Fundamentals of Geophysics, Second Edition", the Oort Cloud is assumed in models as the origination point of comets. This theory originated in the study of the study of known comets' orbits. We don't know much more because the area is mostly unexplored.
Voyager 1 was launched in 1977. While it has passed the Kuiper Belt in the 1990s, it has not yet reached the Oort Cloud.
The New Horizons spacecraft was launched in 2006 to Pluto. If that mission is a success and there is sufficient power, New Horizons may be sent to explore other Kuiper Belt objects. The Pluto Express mission was cancelled due to lack of funding.
The sheer amount of time required to send a probe has to date prevented the launch of an exploration craft specifically to the Oort Cloud. "Frontiers of Propulsion Science" says that it would push modern propulsion technology to its limit to send a probe a tenth of a light year from Earth within 40 years. Probes sent to the Oort Cloud could follow the example of the NEAR-Shoemaker spacecraft that landed on Eros in 1997.
The Oort Cloud is primarily studied by Earth bound telescopes and the orbiting Hubble telescope. Some of the telescopes known to study the Oort Cloud include the Canada-France-Hawaii Telescope that discovered 2008 KV42, the MMT Telescope, the Palomar Observatory and the Gemini telescopes in both the North and South hemispheres. The National Optical Astronomy Observatory has a project to study Kuiper Belt objects, which may discover new Oort Cloud objects as well.
While NASA has a space watch program to find and catalog every object that poses a risk to Earth, there is a long way to go. They are aided by amateur astronomers watching the skies today and analysts comparing slides taken by astronomers over decades to look for very slight changes that reveal Oort Cloud objects like Sedna.
Why the Oort Cloud Matters
According to Bode's Law, each planet is about twice as far from the Sun as its neighbor. This law holds relatively constant until one reaches Neptune and the Pluto-Charon system. One possible answer for this deviation is that Neptune and Pluto and its moons have had their orbits altered over time. This led to a theoretical planet in the Oort Cloud, though nothing much larger than Pluto has been discovered to date in the Oort Cloud.
Another theory is that stars have passed close enough to the Solar System to alter the orbits of at least Neptune and Pluto; if true, this means that the Sun has passed close enough to other stars to alter the bodies of much smaller objects in the Oort Cloud as well.
Stars passing near our Sun are thought to disturb the orbits of bodies in the Oort Cloud, causing some of them to fall toward the inner solar system. Comets with periods of several million years probably started out in the Oort Cloud. And some of these objects could hit the Earth and cause massive extinctions such as the asteroid that hit the Yucatan Peninsula and wiped out the dinosaurs.
According to "Global Collective Action" by Todd Sandler, a comet found in 1996 has a 50% chance of hitting the Earth over the next 100,000 years. There are an estimated 35,000 objects in the Kuiper Belt over 100 kilometers across. An unknown number of large objects reside in the Oort Cloud, and any of them could be on a collision course with Earth right now. Thus the most urgent reason to study the Oort Cloud is to discover possible threats to life on Earth.
Another reason to study the Oort Cloud is to gain deeper understand the history of our solar history. Exploring the Oort Cloud could help us understand the material that eventually coalesced to become the inner planets. Studying objects in the Oort Cloud has the potential to reveal the history of objects that have passed close enough to our solar system to affect the orbits of objects.
Theoretically, the frozen comets of the Oort Cloud could provide water, hydrocarbons and oxygen for interstellar space travelers.
1. "The Space Environment and its Effects on Space Systems" by Vincent L. Pisacane
2. "Global Collective Action" by Todd Sandler
3. "Fundamentals of Geophysics, Second Edition" by William Lowrie
4. "Frontiers of Propulsion Science" by Marc G. Millis and Eric W. Davis