Asteroids & Comets

Space stuff slams into the Earth every day .... making more Earth. Some of these objects are solid and some are made of ice. They come in all shapes and sizes. The bigger they are, the bigger the news headlines. Where does this stuff come from? Simple answer - from right here!!! They have been floating around for 4.6 billion years and finally found us. Our solar system is loaded with debris. The bigger things we call asteroids and comets.

Asteroids

As stated in the overview, most of the asteroids are irregular in shape and orbit the sun between the orbits of Mars and Jupiter. Close up images of Gaspra, Ida, and Eros show a heavily cratered surface. A few asteroids orbit farther from the sun (near Saturn and Uranus) and are known as Centaurs. Some orbit near the terrestrial planets (known as Near Earth Objects) and pose a potential threat to us if they were to hit the earth.

Dawn Spacecraft takes a picture of Ceres (notice the mysterious bright spots). Asteroid Gaspra (Credit NASA) taken by the probe Galileo

The first asteroid discovered (1801) was also the largest, Ceres, which is about 600 miles in diameter (which has since been promoted to dwarf planet status). We should have some great pictures when the Dawn spacecraft reaches Ceres in 2015. Think of Ceres during your next breakfast and have a bowl of cereal (both named for the Roman goddess of grain). Soon after the discovery of Ceres, several more large asteroids were found which measured between 250 and 325 miles in diameter. The Dawn craft recently flew by Vesta and took some great pictures. As astronomers continued to look, more and more smaller ones were found. Early astronomers initially believed that asteroids were the broken fragments of a destroyed world (have you ever heard the story of Superman and his native planet Krypton?). Modern astronomers now believe that the entire solar system looked much like the asteroid belt. However, in other regions of the solar system, these smaller objects coalesced to form planets, but the asteroid belt never did take that next step. However, it is not a mystery why. Massive Jupiter is able to exert enough of an influence on asteroids ... applying perturbing gravitational tidal forces on these smaller objects, which prevent them from grouping into a larger object. It is also these perturbing forces which make it possible for some asteroids to be ejected from the belt into Earth crossing orbits.

The year 2017 gave us a rare treat. It was the first time (that we know of) where our solar system was visited by an asteroid from interstellar space. Named Oumuamua, this asteroid entered our solar system from a strange direction (far from the plane of the ecliptic), speed (with escape velocity), and orbit (hyperbolic). It showed no signs of dust which ruled out a comet. It varied greatly in brightness leading astronomers to conclude that it was cigar shaped. It is now heading out of the solar system. Objects like this could be one possible way life (microbes) may hitchhike from one star system to another. More here. Update: It happened again in 2019 so now we know of two visitors from outside our solar system but their place of origin is still a mystery.

Asteroids vary in composition. A lot depends on where they formed and evolved since their formation. As stated before, asteroids represent a sample of the very early stages of our solar system. The asteroids closer to the sun vary from the ones further out. Generally speaking, asteroids closer to the sun contain more carbon and as you move further from the sun, the main ingredient shifts to silicates (silicon and oxygen ... the same stuff that makes up the earth's crust and mantle). There are even a few asteroids make mostly of metal (iron and nickel). These are believed to be the exposed core of body that was once much larger, differentiated, and then blown to pieces by a later collision. Click here to learn more about the composition of asteroids.

Comets

Credit NASA (Click on the image for a closer look)

Imagine a block of ice the size of a mountain. Now mix in a little dirt and put it in deep space. You just made a comet. About 1000 comets have been catalogued, and about 200 of them are comets which have short period orbits (like Halley's Comet which returns every 76 years). Each year a few new comets are discovered and each year some expected comets fail to return. Some have been observed breaking into several pieces, some crash directly into the sun and we even saw one crash into Jupiter in 1994. In 1997, we were treated to a spectacular show in the form of Hale-Bopp (I hope you had a chance to see it). On the other hand, we were all disappointed when comet ISON failed to make it around the sun in 2013.

Comets have highly eccentric orbits and enter the inner solar system from any direction in space. As a comet enters the inner solar system, the warmth of the sun sublimates some of the frozen gasses, and the comet forms a fuzzy atmosphere called a coma. It can become as big as the earth (remember the solid nucleus is only the size of a mountain). Even closer to the sun, the comet forms a tail as the solar wind blows coma gasses away. For that reason, the tail always points away from the sun regardless which direction the comet is moving. From closer inspection, there are actually two tails. A white dust tail (most predominant) composed of small solids (size of smoke dust) that flake off and a fainter blue ion tail composed of plasma (gasses) which are ejected from the comet. After the comet reached perihelion, it moves back to cold space where the tail and coma eventually fade away. How a comet is able to make several passes around the sun was always a big question astronomers were unable to answer. That is, until spacecraft took close-up pictures of Halley's Comet in 1986. Astronomers were amazed to find a very dark surface. Apparently, when a comet makes a close approach to the sun, solids mixed within the ice remain, forming a dark insulating surface. A similar effect can be observed in snow piles (if you live in Wisconsin). As spring approaches, and much of the snow melts, the snow pile becomes very dirty as solids become exposed to the surface.

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

This is the best image (to date) of a comet. It was taken August 3, 2014 of Comet 67P/Churyumov-Gerasimenko by ESA's Rosetta spacecraft. The craft released a probe (named Philae) which soft landed on this comet in November, 2014 .... a first in astronomy. One big dirty snowball! Analysis of the data shows this comet has a layered structure with a thin rigid outer layer 10 - 50 cm thick with a less rigid interior. This is consistent with previously held thoughts.

Origin of Comets

An early astronomer, Jan Oort, wondered where comets come from. He knew they approached the inner solar system with orbits that suggest a great distance. He reasoned that some mechanism must be responsible for the new comets we see entering the inner solar system each year. Working backwards, he determined that if comets reside in a shell with a radius that approached interstellar distances, gravitational perturbations from nearby stars would be enough to knock a few into new orbits, ... producing the new comets we see. The model works well, at least enough to convince fellow astronomers that he was correct. In his honor, the Oort comet cloud is the expected origin of comets. It is unlikely that astronomers will actually be able to see such small objects at such great distances, so we will have to accept the model without observational verification. In this 2018 update, astronomers believe they may soon discover Oort clouds around other stars. If so, this would make astronomers very confident there is, indeed, an Oort cloud around our sun.

Even if the comet cloud actually exists, it is unlikely that comets formed there from the start. Computer models suggest that icy planetesimals (comets) formed in the region of the Jovian planets and were either collected together when those giant planets formed or propelled to the comet cloud as they were perturbed by the gravity of Jupiter. You can think of comets as cosmic "pin balls", which are first perturbed by Jupiter to the outer reaches of the solar system. Then nearby stars ping them back towards the sun. Finally, a few have another gravitational encounter with Jupiter, and their orbit is altered again .... giving us a short period comet. Some astronomers believe that many of the near Earth objects (see below) are corpses of burned out comets.

The earth becomes a target for these projectiles. We have been hit by comets in the past, and in the process, they brought in large quantities of water (and maybe even the chemicals necessary for life). Think about this the next time you take a shower. Speaking of showers, the next time you see a meteor shower, think of comets. The Perseid meteor shower (Aug 10-12) comes each year as the earth slams into debris left behind from comet Swift-Tuttle. Feel fortunate that we run into the material which flaked off the comet and not the comet itself. If that happens now, .... well ... good luck.

When a chunk of space rock enters our atmosphere, we call it a meteor (or meteorite once it lands). Think of these rocks as pieces of asteroids (or comets) being delivered to us. Upon examination, one thing scientists have found in these rocks are amino acids! These are the building blocks of life!!!! Could we thank comets and asteroids for seeding a very young earth with the ingredients to make life???

Origin of our water (update 2014)

A quick analysis of the water from Comet 67P showed that it is unlike the composition of the water we find on our planet. It contains three times the deuterium (heavy water) we currently find in our oceans. This tosses a monkey wrench into earlier thought that much of our water came from comets. Researchers have concluded that much of our water was here from the very beginning ... from the meteors and asteroids that were the original building blocks of our planet. How is this possible? Rocky meteorites (of the class carbonaceous chondrite) are believed to have originally formed further from the sun (asteroid belt or beyond). Through time and nudged by gravitational interactions with Jupiter, some found their way to our planet. Analysis of these rocks currently hitting our planet (meteorites) show they contain anywhere from 3%-22% water in their chemical composition (hydrogen and oxygen tied up chemically within the rocks). All you need to do is heat it and water is released. This water has a much lower concentration of deuterium and forces astronomers to suspect that this is the major source of our oceans. This is still debated by scientists and a consensus among experts is far from complete.

Impacts that made an impact

On occasion, the Earth has a rendezvous with something from space.

Little stuff ...

The debris from comets is typically very small. When we see a typical meteor, it quickly burns up in the upper atmosphere because it is only the size of a piece of gravel .... or smaller. No big deal!

Sometimes a small rock falls to Earth and makes the news. As the rock contacts the atmosphere, we see a streak of light which lasts a second, and we call it a meteor (not a shooting star). These fragments never make it to the ground and burn into ashes about 80 miles above the surface of the earth. The original size is surprisingly small, only about the size of a pea or even smaller. If it moves across the entire sky and/or seen across several states, it is called a fireball. The original rock may fragment and may even reach the ground. If it does, it is called a meteorite. Meteorites are classified by their composition. They are Stony Meteorites (also known as Chondrites) and they make up the majority of all meteorites. Iron meteorites make up about 20% of all meteorites and even rarer (1%) are stony-iron meteorites which are a mixture of stone and iron. You can think stony meteorites as material similar to the earth's mantle and crust, iron meteorites as material similar to the earth's core, and stony-irons as material found at the core-mantle boundary.

Mrs. Hodges was injured by a meteorite in 1954.

Peekskill Meteorite: On October 9, 1992 a fireball was seen across several eastern states. It broke up into several fragments, but one fragment hit the trunk of Michelle Knapp's 1980 Chevy Malibu. Under the car she discovered a 12 kg meteorite. Police investigating the incident were unaware of the value of this rock. It was used as a door stop (at the police department), but later sold for $69,000 (and someone bought the car for $10,000). Click here to see a short video of the fireball.

Permission from Pierre Thomas

NASA had egg on their faces when the Genesis probe crashed into the desert (9/8/2004) after the parachute failed to open.

Credit: AP Photo/NASA/JPL

Chelyabinsk fireball

On February 15, 2013, the folks in Chelyabinsk Russia witnessed a rare event. That morning, an asteroid was seen streaking across the sky. It was more than just a light show. The shock wave damaged several buildings and nearly 1,500 people required medical attention (2 were serious injuries). Here are some interesting links: pictures video

Estimates of the initial mass of this asteroid were around 10,000 tons and measuring between 15-20 meters across. It broke into several fragments during its plunge into the atmosphere. The largest recovered piece (shown below) has a mass of 654 kg (1,442 pounds). It is estimated that something like this hits the earth every 100 years or so.

Credit space.com

Want to buy a rock from space? (I did) - click here. If not, just look over the prices ... some are not cheap.

Medium sized stuff ...

In 1908, in a remote area of Tunguska Siberia, an asteroid exploded in the atmosphere about 6 miles above the ground, ... knocking down trees 25 miles (from ground zero). Working backwards, scientists estimate that the initial asteroid was at least 80 meters in diameter. Luckily, this area was so remote that it is unlikely anyone was killed by this event. This was known as the Tunguska impact even though the only thing that hit the ground was a shock wave.

Image Credit: Smithsonian Institution

About 50,000 years ago, a much larger object hit the desert in Arizona. The earth ran into an iron asteroid about 50 meters in diameter. Known as Meteor Crater, this impact site measures a mile in diameter.

Credit NASA

Meteor Crater, AZ

About 120 impact sites have been identified on the surface of the earth. This small number reminds us that the actions of erosion and plate tectonics quickly hides evidence of earlier impacts. It also made us unaware of the potential damage an impacting rock can produce. That is, until a geologist named Gene Shoemaker came along. Gene Shoemaker wanted to become an Apollo astronaut (who would eventually land on the moon), ... but health problems eliminated him from the pool of candidates. He turned his attention to the geology of impacts. It was Shoemaker who identified the crater in Arizona as an impact site. Previously, it was believed that most craters of this type were produced by volcanic activity. By studying the results of underground nuclear tests, Shoemaker was able to convince most geologists that impacts have happened frequently in the past and will occur again in the future. He supported his claims by identifying several Earth approaching asteroids which represent potential threats to the earth. He tragically died in a car accident and as a tribute, his ashes crashed to the lunar surface in a probe sent by NASA to study the moon.

Gene Shoemaker (1928-1997)

One piece of Shoemaker - Levy 9 smashing into Jupiter

Credit NASA

The point was made in dramatic style in 1994 when the earth witnessed (for the first time), a comet smash into Jupiter. The comet, Shoemaker - Levy 9, was named for Caroline Shoemaker (Gene's wife) and David Levy. Soon afterwards, 21 fragments plunged into the giant planet, creating plumes the size of the earth. Lucky for us it hit Jupiter. This event inspired Hollywood to make Armageddon (which I hated) and Deep Impact (which I thought was OK).

Really big stuff ...

When chicken little cried "the sky is falling" ... she could be very correct! Most geologist now accept the idea that an impact with the earth 65 million years ago, known as the K/T event or the Cretaceous–Paleogene (K–Pg) extinction event, lead to the mass extinction of almost ¾ of all species (including the dinosaurs). Another smaller impact about 800,000 years ago likely changed the human population on the earth. This is based on the age of Australasian tektites. It's not a matter of if it could happen again ... only when! If interested, click here for a detailed account.

Mass extinctions

The geologic record shows that the earth periodically goes through mass extinction events which changes the course of life on the planet. About 65 million years ago, about ¾ of all life forms went extinct, ... including the dinosaurs. Geologists initially laughed off the claim (in 1980) by a father-son team of geologists (Luis & Walter Alvarez) that this event was caused by an impact from space, ... known as the K/T event. Their claim came from examining a thin layer of clay found around the earth dating to that time period. In this layer, they found high concentrations of the element iridium, which is rare in surface rocks but found in higher concentrations in meteorites. The impact, they claimed, would blow a blanket of dust into the atmosphere (which could last for years) in a scenario known as "nuclear winter". The collapse of the food chain would bring on the mass extinction. However, they were unable to provide an impact site. Later, studies from space showed that the earth was hit by a huge (10 km diameter) rock or comet, ... leaving a 180 km crater in the area of Chicxulub (in the Yucatan Peninsula). The date of the impact dates to 65 million years. Apparently, the smoking gun has been found. Geologists are still debating this idea. They point to the evidence that the earth also experienced volcanism on a mammoth scale about the same time. If so, the two events may not be unrelated. Did one event trigger the other (leaving Dino in the dust)?

Spot of the K/T impact

Since Gene Shoemaker, scientists have begun to study Near Earth Objects (NEO's) in detail. The first step is to be aware of the potential threat, and then what action (if any) can be taken to alter the path of the asteroid or comet or destroy it before hitting the earth. For example, astronomers have long known an Earth approaching asteroid named 1036 Ganymed is a potential extinction event waiting to happen. It is about 4 times the size of the K/T object! Don't lose sleep .. it is not projected to hit anything soon.

Here is a list of other NEO's. Every once in a while we receive news of an object that could potentially hit the earth. Can we currently do anything if we find out a large object is about to slam into the earth? Not yet! More bad news ... after every extinction event in the earth's history, the top life forms of the food chain were taken off the map. Guess where we are? Have a nice day. ☺

Near Earth Objects by NASA

So here is the deal. On one hand comets and asteroids may have seeded the early earth with the building blocks of life (amino acids) and some water, but a future collision with a comet or asteroid could also take us out of the picture.