Though too small to earn the distinction of "planet", asteroids and comets loom large in literature and folklore. The reason is clear: one of the chunky rocks or icy mud balls will eventually slam into Earth and change the planet irreversibly. Such an impact 65 million years ago is widely believed to have killed off the dinosaurs.
Asteroids and comets are considered remnants from the giant cloud of gas and dust that condensed to create the sun, planets, and moons some 4.5 billion years ago. Today, most asteroids orbit the sun in a tightly packed belt located between Mars and Jupiter. Comets are relegated to either a cloud or belt on the solar system fringe. Gravitational tugs, orbital collisions, and interstellar jostles occasionally perturb an asteroid or comet onto a wayward path.
The distinction between asteroids and comets is fuzzy—comets tend to have more chemical compounds that vaporize when heated, such as water, and more elliptical (egg-shaped) orbits than asteroids do. And when observed through a telescope, comets appear fuzzier.
Asteroids are essentially chunks of rock that measure in size from a few feet to several miles in diameter. (Small asteroids are called meteoroids.) The largest asteroid, Ceres, is about 590 miles (950 kilometers) wide. Like most asteroids, it lies in the asteroid belt between Mars and Jupiter. Many astronomers believe the belt is primordial material that never glommed into a planet because of Jupiter's gravitational pull. Other astronomers say the belt is a planet that was broken apart during a collision.
Comets are balls of rock and ice that grow tails as they approach the sun in the course of their highly elliptical orbits. As comets heat up, gas and dust are expelled and trail behind them. The sun illuminates this trail, causing it to glow. The glowing trails are visible in the night sky.
While there are perhaps trillions of comets ringing the outer fringes of the solar system, bright comets appear in Earth's visible night sky about once per decade. Short-period comets such as Halley's are perturbed from the so-called Kuiper belt out beyond the orbit of Neptune and pass through the inner solar system once or twice in a human lifetime. Long-period comets come from the Oort Cloud, which rings the outer reaches of the solar system, and pass near the sun once every hundreds or thousands of years.
Occasional collisions and gravitational tugs send asteroids and comets careering toward the sun on highly elliptical orbits, some close enough to Earth to pose a risk of impact. Astronomers are constantly on the lookout for bodies on such a catastrophic trajectory. Most asteroids, fortunately, are too small to cause any damage. Instead, they burn up in the atmosphere and appear to us as a shooting star.
Water Discovered on Second Asteroid, May Be Even More Common Than Expected
Water ice on asteroids may be more common than expected, according to a new study.
A UCF researcher who made national headlines for showing the first evidence of water ice and organic molecules on an asteroid has discovered another one. Image Credit: Gabriel Pérez, Instituto de Astrofisica de Canarias, Spain
Two teams of researchers who made national headlines in April for showing the first evidence of water ice and organic molecules on an asteroid have now discovered that asteroid 65 Cybele contains the same material.
“This discovery suggests that this region of our solar system contains more water ice than anticipated,” said University of Central Florida Professor Humberto Campins. “And it supports the theory that asteroids may have hit Earth and brought our planet its water and the building blocks for life to form and evolve here.”
Campins presented the teams’ findings during the 42nd annual meeting of the American Astronomical Society’s Division for Planetary Sciences (DPS) in Pasadena, Calif.
Asteroid 65 Cybele is somewhat larger than asteroid 24 Themis — the subject of the teams’ first paper. Cybele has a diameter of 290 km (180 miles). Themis has a diameter of 200 km (124 miles). Both are in the same region of the asteroid belt between Mars and Jupiter.
The academic article reporting this new finding has been accepted for publication in the European Journal “Astronomy and Astrophysics.”
New Clue to Whether Phaeton is an Asteroid or a Comet
In another talk at the DPS meeting, Campins presented new findings about the asteroid 3200 Phaethon.
There are hundreds of asteroids in space, like this one named Ida. It also is found in the asteroid belt between Mars and Jupiter. Credit: NASA JPL/California Institute of Technology
This asteroid sometimes acts like a comet, lighting up the Earth’s night sky with meteor showers. Sometimes it’s like an asteroid, a hunk of rock floating in space.
Phaethon has baffled scientists for years, because it doesn’t behave the way they expect. But Campins and his team may have uncovered a major clue — Phaethon’s daddy.
Using telescopes and mathematical modeling, researchers looked at the chemical composition – the DNA fingerprint – of Phaethon and compared it to the composition of the second-largest asteroid found in the main asteroid belt located between Mars and Jupiter. They found significant similarities between Phaethon and 2 Pallas.
“Pallas and Phaethon appear to be father and son,” Campins said. “But the son doesn’t act anything like dad.”
Based on the results, the team, which includes scientists from Spain, Greece and France, believes that the size of asteroids and their orbits play a role in how they act.
Phaethon’s orbit crosses the Earth’s orbit, and debris left by Phaethon in its orbit produces the Geminid meteor shower every December. In addition, Phaethon probably contains organic material that may have been part Earth before life, as we know it, developed here.
So what should we call Phaethon? Campins smiles.
“It just shows you that Mother Nature doesn’t care what we call it,” Campins said. “She will continue to amaze us, regardless of what we call her wonders.”