By Jonny Lupsha, Wondrium Staff Writer
A major asteroid has a one in 1,700 chance of striking Earth before the year 2300. NASA says Bennu, which is coincidentally about 1,700 feet wide, has a 99.94% chance of missing Earth whenever it passes. Most asteroids come from the asteroid belt.
NASA scientists have calculated that Bennu, an asteroid with a diameter of 1,722 feet, has just a 0.06% chance of hitting Earth between now and the year 2300. Nearly all other asteroids in the solar system have an even slimmer chance of ever impacting our planet.
That means that even at such a low likelihood of impact, Bennu is technically one of the most hazardous asteroids in our entire solar system. While an interesting fact, NASA has clarified that there is no reason for concern. In her video series A Field Guide to the Planets, Dr. Sabine Stanley, a Bloomberg Distinguished Professor in the Morton K. Blaustein Department of Earth and Planetary Sciences at Johns Hopkins University, said that most asteroids do indeed come from the asteroid belt.
Loosening the Belt
According to Dr. Stanley, the asteroid belt, which lies between Mars and Jupiter, is home to millions of asteroids. However, the entire asteroid belt only makes up about 4% of the mass of Earth’s Moon. Half of the asteroid’s belt comes from just four asteroids: Ceres, Vesta, Pallas, and Hygiea.
“We now know Jupiter’s gravity makes it nearly impossible to form a planet in the asteroid belt,” she said. “This is because Jupiter’s gravity strongly perturbs objects in this region of space, giving too much energy to these small objects, which prevents them from clinging together in large enough quantities to produce a planet.”
These perturbations keep asteroids’ orbits too eccentric and inclined. Pallas, the third-largest asteroid in the belt, orbits anywhere from 2.1 astronomical units (AU) to 3.4 AU, at an inclination of 35 degrees.
“As the orbits get more eccentric, there is a larger chance of collision between the asteroids,” Dr. Stanley said. “These collisions have resulted in the breakup of many objects in the asteroid belt. Some asteroids become moons of other asteroids; other fragments are rejected from the asteroid belt entirely, some ending up as near-Earth asteroids (NEAs).”
Making Notches in the Belt
Dr. Stanley said that Jupiter has influenced the shape and composition of the asteroid belt in other ways, as well. Not only does its gravity keep asteroid orbits far from being circular, but it’s also responsible for gaps or holes in the asteroid belt.
“If we plot the number of asteroids as a function of orbital distance, we notice certain locations are lacking asteroids,” she said. “These gaps happen to occur where the missing asteroids would have orbital periods that are integer multiples of Jupiter’s orbital period.
“So, for example, the gap near 2.5 AU turns out to be a 3:1 orbital resonance, where an asteroid would orbit three times the Sun every one orbit of Jupiter.”
In simpler terms, the holes in the asteroid belt happen in places where the gravitational perturbations of Jupiter are always in the same direction and become amplified, which causes larger and larger disturbances to an asteroid’s orbit until it gets flung out of that orbital location.
These gaps are known as Kirkwood Gaps, named after Daniel Kirkwood, who noticed them in 1860 and explained their origin.
In the 0.06% chance that Bennu impacts Earth in the next 280 years, at least we’ll have a good idea of where it came from.