In 1801, while making a star map, Italian priest and astronomer Giuseppe Piazzi accidentally discovered the first and largest asteroid, Ceres, orbiting between Mars and Jupiter. Although Ceres is classified today as a dwarf planet, it accounts for a quarter of all the mass of all the thousands of known asteroids in or near the main asteroid belt.


According to the National Aeronautics and Space Administration (NASA), asteroids can be divided into the following three categories:

  • Main Asteroid Belt: The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today;
  • Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points). There, the gravitational pull from the sun and the planet are balanced by a Trojan’s tendency to otherwise fly out of the orbit. The Jupiter Trojans form the most significant population of Trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune Trojans, and NASA announced the discovery of an Earth Trojan in 2011; and
  • Near-Earth Asteroids: These objects have orbits that pass close by that of Earth. Asteroids that actually cross Earth’s orbital path are known as Earth-crossers. As of June 19, 2013, 10,003 near-Earth asteroids are known and the number over 1 kilometer in diameter is thought to be 861, with 1,409 classified as potentially hazardous asteroids – those that could pose a threat to Earth.

Here is some background information:

  • The first spacecraft to take close-up images of asteroids was NASA’s Galileo in 1991, which also discovered the first moon to orbit an asteroid in 1994;
  • In 2001, after NASA’s NEAR spacecraft intensely studied the near-earth asteroid Eros for more than a year from orbit, mission controllers decided to try and land the spacecraft. Although it wasn’t designed for landing, NEAR successfully touched down, setting the record as the first to successfully land on an asteroid;
  • In 2006, Japan’s Hayabusa became the first spacecraft to land on and take off from an asteroid. It returned to Earth in June 2010, and the samples it recovered are currently under study;
  • NASA’s Dawn mission, launched in 2007, began exploring Vesta in 2011. After a year, it left the asteroid for a trip to Ceres, with a planned arrival time of 2015. Dawn was the first spacecraft to visit Vesta, and will also be the first to explore Ceres; and
  • In 2012, a company called Planetary Resources, Inc. announced plans to eventually send a mission to a space rock to extract water and mine the asteroid for precious metals. Since then, NASA has begun to work on plans for its own asteroid-capture mission.


Asteroids are small, airless rocky worlds revolving around the sun that are too small to be called planets. They are also known as planetoids or minor planets. In total, the mass of all the asteroids is less than that of Earth’s moon. But despite their size, asteroids can be dangerous. Many have hit Earth in the past, and more will crash into our planet in the future. That’s one reason scientists study asteroids and are eager to learn more about their numbers, orbits and physical characteristics. If an asteroid is headed our way, we want to know that.

Most asteroids lie in a vast ring between the orbits of Mars and Jupiter. This main asteroid belt holds more than 200 asteroids larger than 60 miles (100 kilometers) in diameter. Scientists estimate the asteroid belt also contains more than 750,000 asteroids larger than three-fifths of a mile (1 km) in diameter and millions of smaller ones. Not everything in the main belt is an asteroid — for instance, comets have recently been discovered there, and Ceres, once thought of only as an asteroid, is now also considered a dwarf planet.

Here is how the NASA summarized the attributes of an asteroid under their Solar System Exploration program:

  • If all of the asteroids were combined into a ball, they would still be much smaller than Earth’s moon. If the sun were as tall as a typical front door, Earth would be the size of a nickel, the moon would be about as big as a green pea and Ceres (the largest object in the main asteroid belt) would be as small as a sesame seed;
  • Most asteroids orbit our sun, a star, in a region of space between the orbits of Mars and Jupiter known as the Asteroid Belt;
  • Days and years vary by asteroid. A day on asteroid Ida, for example, takes only 4.6 hours (the time it takes to rotate or spin once). Ida makes a complete orbit around the sun (a year in this asteroid’s time) in 4.8 Earth years;
  • Asteroids are solid, rocky and irregular bodies;
  • Asteroids do not have atmospheres.
  • More than 150 asteroids are known to have a small companion moon (some have two moons). The first discovery of an asteroid-moon system was of asteroid Ida and its moon Dactyl in 1993;
  • One asteroid, named Chariklo, is known to have two dense and narrow rings;
  • More than 10 spacecraft have explored asteroids. NEAR Shoemaker even landed on an asteroid (Eros). The Dawn mission is the first mission to orbit (2011) a main belt asteroid (Vesta);
  • Asteroids cannot support life as we know it; and
  • Ceres, the first and largest asteroid to be discovered (1801 by Giuseppe Piazzi) and the closest dwarf planet to the sun, encompasses over one-third of the estimated total mass of all the asteroids in the asteroid belt.


The three broad composition classes of asteroids are:

  • The C-type (Chondrite) Asteroids are most common, probably consist of clay and silicate rocks, and are dark in appearance. They are among the most ancient objects in the solar system;
  • The S-types (“Stony”) Asteroids are made up of silicate materials and nickel-iron; and
  • The M-types are metallic (Nickel-Iron).

The asteroids’ compositional differences are related to how far from the sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. Only one such asteroid, Vesta, survives to this day.

Jupiter’s massive gravity and occasional close encounters with Mars or another object change the asteroids’ orbits, knocking them out of the main belt and hurling them into space in all directions across the orbits of the other planets. Stray asteroids and asteroid fragments slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth.

Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth’s orbit, and near-Earth asteroids that approach Earth’s orbital distance to within about 45 million kilometers (28 million miles) and may pose an impact danger. Radar is a valuable tool in detecting and monitoring potential impact hazards. By reflecting transmitted signals off objects, images and other information can be derived from the echoes. Scientists can learn a great deal about an asteroid’s orbit, rotation, size, shape, and metal concentration.

According to the Huffington Post, researchers at St. Petersburg State University have marked April 13, 2036, as the date that the asteroid, named Apophis, might strike the planet. Professor Leonid Sokolov said that while the chances of impact were slim, all possible outcomes should be planned for. He also said that Apophis could break up into smaller pieces that could smack into the Earth in years to come.


NASA scientists have known in 2013 for nearly a year that a small asteroid called 2012 DA14, about 150 ft. across, would whiz past Earth at the nail-biting distance of 17,000 miles or so — significantly closer than the 22,500-mile altitude occupied by geosynchronous satellites. It happened right on schedule on Friday, February 15, 2013, with no consequences at all for Earth.

But nobody had a clue that a space rock about a third as large would scream into the atmosphere in the early hours of Friday morning, blaze across the Siberian sky at a blistering 40,000 mph., and explode, triggering a shock wave that broke windows and toppled walls in and around the city of Chelyabinsk, injuring more than 1,000 people, mostly with flying glass. It was, said Paul Chodas, of NASA’s Near Earth Object Program, at a press conference, “an incredible coincidence.”


Taken together, the two events were a grim reminder that the Earth faces dangers from outer space that has nothing to do with alien invaders. Back in 1908, a meteor exploded over the Tunguska region of Siberia with estimated force of 80 megatons, flattening some 80 million trees in a largely uninhabited area. 50 million years ago an asteroid fragment slammed into the Arizona desert, excavating a crater a mile across and 550 feet deep.

Either of those historic strikes could have killed hundreds of thousands of people if they’d happened in or over a major metropolitan area — or if they’d plunged into the ocean, triggering a massive tsunami.

But devastating as that would be, it pales next to the strike that happened 65 million years ago, when a much bigger chunk of space rock, a few miles across, slammed into the sea right off the Yucatan Peninsula in what is now Mexico. The debris thrown into the atmosphere in the aftermath of that gigantic impact is thought by many to have caused one of the greatest mass extinctions of species in the planet’s history, by blocking off enough sunlight to chill the planet dramatically.

About 70 percent of all living species disappeared during that episode of abrupt climate change. Now many scientists believe another mass extinction is under way – this one entirely of our own making. A combination of pollution, habitat destruction and the global warming from greenhouse-gas emissions has already driven the species-extinction rate well above normal, and there’s every reason to believe it will continue to skyrocket as the warming starts to overwhelm these other effects during the coming century and beyond.


It should be kept in mind that asteroid impacts do two things that may affect climate:

  • Release heat upon impact (several hundreds to thousands worth of atomic bombs, depending on its size); and
  • Release a dust cloud so large that may potentially block out the sun, cover flora and fauna in debris, and choke them as well from nutrient gasses in the atmosphere.

University of Michigan space scientist is worried that global warming is increasing the risk that an asteroid could, one day, collide with the Earth. The seemingly impossible connection is due to a basic law of physics: when a gas is heated, it expands. “Some large meteoroids have skimmed the outer reaches of Earth’s atmosphere, skipping off back into outer space”, said Professor Charles Boyle, chairman of University of Michigan’s Near-Earth Asteroid Team (NEAT). “As the atmosphere warms, it expands outward, potentially capturing large asteroids that would have otherwise been a near-miss. It seems that the dangers from global warming just keep mounting up…like the national debt.” Now that all reputable scientists agree that global warming is the greatest threat to face mankind since the secretly-thwarted alien invasion of 1955, it seemed that just the warming alone was enough to strike terror in the hearts of Earth’s imperiled inhabitants.

But this new connection between global warming and the asteroid threat makes immediate action to avoid planetary collision just as important as avoiding planetary meltdown. “Our NEAT researchers have been feverishly calculating the risk enhancement factors associated with the thermal expansion of the atmosphere”, said Prof. Boyle, as several scientists nearby made animated gestures as one of them guided two billiard balls past each other. “We hope to have risk assessments in place, ready to assist future defensive measures that might be taken to divert potentially dangerous asteroids from colliding with the Earth.”

28 August 2015 – Ottawa, Ontario, Canada. 


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