Sorry, astrology haters. The rumors are true: humans are being pulled at by the gravity of all planets at all times. There is not a drop of evidence for these minuscule gravitational tugs as tools of personality formation nor as predictors of luck. Still, we are not the free-floaters we imagine ourselves to be, but perpetual victims of gravitational influences from all angles and sources.
So are asteroids. For them, however, it’s a bit more fundamental.
The classic mind’s eye image of an asteroid—in a non-sci fi context—is a banged-up rock, rambling along in empty space, maybe passing a sibling every once in a while. A rather lonely voyage around the Sun.
This is accurate in many ways. Asteroids very rarely collide with, or even closely approach, other objects. So what makes them suddenly hazardous? The culprit is the invisible but ever-present force of gravity.
If you’ve taken physics sometime in the past few hundred years, you learned that the orbits of celestial bodies—including asteroids—are driven by the gravitational force of a central body. Speeding objects are constantly being pulled into circular or elliptical paths by this centripetal gravitational force. Asteroid deflection isn’t just an Earth-protecting strategy for NASA to test; it’s also a natural universal law, as asteroids are constantly deflected from their linear paths.
While gravity drives the orbits of asteroids, it can also disrupt them. Jupiter—the “vacuum cleaner of the solar system”—has long been known to sway the orbits of comets and asteroids due to its enormous mass. Many scientists speculate that Jupiter’s gravity has historically served as a shield to protect Earth from incoming objects, allowing life to develop without too many hiccups.
But if an unlucky asteroid wobbles too far in one direction or another, Jupiter’s gravity can alter its orbit for the worse, putting it on a potentially hazardous path.
Millions of asteroids comprise the asteroid belt, located between Mars and Jupiter, so there are lots of opportunities for trouble. For asteroids in this area, Jupiter’s gravitational force may rival the Sun’s, due to its closer proximity.
Furthermore, computer simulations have shown that asteroids whose orbits are “resonant” with Jupiter’s, meaning their orbital times are a whole number multiple of one another–can become an issue. Asteroids that slip into this resonance may have their orbits turned suddenly wonky, perhaps now on the path to planetary collision.
Hazardous asteroids can also arise “from the fragments of mutual collisions in the asteroid belt,” noted Tom Gehrels, a University of Arizona astronomer. “After their break-up, some of these fragments move towards the earth under the gravitational action of Jupiter.”
Even Earth’s gravity can act against our best interests. If a near-Earth asteroid approaches close enough to pass through a gravitational keyhole, Earth’s gravitational field may pull or push it into an altered orbit. If the new orbit experiences gravitational resonance, or near resonance, with our own, it will loop back around to meet us at a later date, potentially intercepting us after all.
While scientists cannot alter planetary gravity to address our astrologically-based grievances, they can certainly mess with the gravitational forces tracking asteroids. DART is the most relevant examples of how we can attempt to right gravity’s wrongs. But we can also harness gravity for our own purposes via methods like the gravity tractor, a hypothetical craft that would fly alongside an asteroid, possibly for years, using it own gravitational force to slowly pull the asteroid away from its collision course with Earth.
Either way, as alternative rock band Yo La Tengo reminds us, gravity rules us all.