Beyond the orbit of Mars, but not as far as Jupiter, lurk the many hundreds of thousands of rocky bodies collectively known as the asteroid belt.
Many solar systems are thought to contain such belts, and science fiction movies and television shows often present these bands as rock-clogged expanses that would challenge any celestial navigator. It may be so, in other systems, but in our asteroid belt, the rocky bodies are actually quite far apart from each other.
Humankind will soon be getting an inside look at this often overlooked bit of celestial real estate, courtesy of NASA's Dawn mission. On Saturday, July 16, after a four-year journey, the Dawn spacecraft will reach Vesta, the second-largest body in the belt. [Photos: Asteroid Vesta and NASA's Dawn]
From there, Dawn will go on to orbit the belt's biggest object, Ceres, in 2015. Ceres accounts for nearly a third of the asteroid belt's mass, and is the largest "dwarf planet" in the solar system, outranking Pluto.
Dawn will be the first spacecraft to orbit one body, let alone two, in the asteroid belt. In so doing, Dawn will further characterize two distinct, major objects in the belt, shedding light on some of its greatest mysteries, which are:
Origin of the scattered stones
A major planet never formed where the asteroid belt lies, scientists think, because nearby of disturbances caused by Jupiter's gravitational tug. The giant planet's gravity accelerated the growing agglomerations of dust in the region of the belt, interfering with the slow, step-wise buildup to larger bodies, and booting some objects out entirely.
"The asteroid belt suffered from having this really bad neighbor next door," said Christopher Russell, a professor of geophysics and space physics at the University of California, Los Angeles, and principal investigator for the Dawn mission.
Learning more about the locations of asteroid belts in other solar systems will help confirm the theory that our belt's sparse rocks are a result of the gravitational meddling of giant planets.
Dry to wet
Although Vesta and Ceres are relatively close to each other (Vesta's orbit is about 2.4 times the Earth-Sun distance and Ceres' is 2.8 times that disance), the two objects are strikingly different. Essentially, Vesta is "dry" while Ceres is "wet."
"Vesta is very much like the Moon and Earth," Russell said. "It's a rocky body with an iron core." Ceres, for its part, "is more like rock and water," he told Life's Little Mysteries.
Scientists' best guess as to the reason behind these contrasting compositions has to do with when the bodies formed. Both Vesta and Ceres are in the ballpark of 4.6 billion years old, coming together when the rest of the solar system's major bodies took shape. "But exactly when they were made back then, if differing by a few million years, is important," said Russell.
Our solar system emerged from the collapse of a massive cloud of gas and dust. An explosion of a nearby star in a supernova seeded this cloud with heavy elements, including short-lived radioactive ones such as aluminum-26. [What If Our Solar System Formed Closer to Milky Way's Edge?]
Those bodies that accreted first contained more short-lived elements, which then decayed and heated the surrounding matter. "The body gets to the boiling point, so then water starts to boil off and that starts to dry the material," explained Russell.
The thinking is that Vesta formed just a few million years before Ceres, and as such became hot, molten and dried out. Ceres, instead, chilled out.
Not much Vesta there, but plenty here
If Vesta did indeed form before Ceres, that might also explain the mystery of why there are so few "V-type," or Vesta-like asteroids observed in the belt. Most of those known appear to have come from Vesta itself, having been blasted out by a collision long ago.
That blast apparently sent some Vesta fragments Earth's way, too. About one out of 20 meteorites — space rocks that survive passage through Earth's atmosphere all the way to the ground — appear to have come from Vesta, Russell said.
More puzzingly, none of the meteorites that have ever been recovered appear to have originated from Ceres. Russell said this is probably because the icy chunks that have been knocked off of Ceres sublimate — that is, turn to gas — when subjected to sunlight or the heat of entry into Earth's atmosphere, and so they never reach terra firma.
The Dawn probe will study Ceres' surface to gauge this hypothesis. Alternatively, Jupiter's gravity might again play a role, pumping much more of Vesta's shrapnel our way compared to Ceres'.
Bonus boggler: Bringers of life and death?
While planning the Dawn mission, some scientists voiced concerns about sending the probe to Ceres. "They said Ceres is an object of astrobiology interest," Russell said. "If it's got water and a good temperature out there under its surface, we don’t want [the Dawn mission] contaminating it."
Russell said his team will certainly aim to prevent Dawn from accidentally crashing into Ceres. A future mission could someday assess the dwarf planet's habitability.
That Ceres or other objects in the asteroid belt might harbor life, or its ingredients, speaks to the "panspermia" theory of life's origins here on Earth. The panspermia theory suggest that life did not begin here, but rather that biological entities developed elsewhere, and then a meteorite delivered them to Earth. Perhaps that rock chunk chipped off Ceres, or another icy asteroid, and somehow made it to Earth.
Overall, asteroids certainly appear to have had quite an impact, literally and figuratively, on life on Earth. A minimum six-mile-wide asteroid helped doom the dinosaurs when it crashed here 65 million years ago.
Yet bombardment from icy asteroids early in Earth's history possibly brought huge amounts of water and carbon-containing compounds to the planet, both of which are critical for creating and supporting life.
"You're looking at two scenarios, where alternatively life was negatively affected by asteroids and other times positively affected by asteroids," Russell said. "Asteroids are neither bad nor good."