On a cold morning in February 2013, the skies above Chelyabinsk, Russia, were torn open by a flash of light brighter than the sun. An asteroid, no bigger than a house, had entered the atmosphere at over 40,000 miles per hour. It exploded with the force of 30 Hiroshima bombs, creating a shockwave that shattered windows and injured over 1,500 people. For a few terrifying moments, the cosmos ceased to be a distant, silent backdrop and became an immediate, violent presence. As James Burke taught us, a single event can change our perspective entirely. The Chelyabinsk event was a stark reminder that Earth is not isolated; we are a target in a cosmic shooting gallery. To understand this risk, we must expand our study of forces from the Earth’s crust and our highways to the vast emptiness of space.

What holds the solar system together? What keeps the Moon from flying away from the Earth, or the Earth from careening away from the Sun? The answer is a force that acts across unimaginable distances, a silent, persistent pull called gravity. Isaac Newton revealed its secret: every object with mass in the universe pulls on every other object with mass. This pull, described by his Law of Universal Gravitation, depends on just two things: the mass of the objects and the distance between them. This is not just an earthly force; it is a cosmic one. The same force that makes an apple fall from a tree is the one that orchestrates the grand waltz of the planets.

An object in orbit, like the International Space Station, is in a constant state of falling. It is continuously being pulled toward the Earth by gravity, but it is also moving sideways so fast that it constantly ‘misses’ the ground. This delicate balance between forward velocity and gravitational pull is what defines an orbit. If an object moves too slowly, it will spiral down to the surface. If it moves too fast, it will escape Earth’s gravity altogether. For the thousands of satellites, and the millions of asteroids, staying in a stable orbit is a perpetual high-wire act governed by the precise laws of physics.

An orbiting object possesses two kinds of energy. It has kinetic energy due to its motion, and it has gravitational potential energy due to its position within a gravitational field. The sum of these two is its total orbital energy. In a perfectly circular orbit, both the speed and distance are constant, so the energy is constant. But most orbits, like those of asteroids, are elliptical. As an asteroid swings closer to the Sun, it speeds up, converting potential energy into kinetic energy. As it moves away, it slows down, converting kinetic energy back into potential. This constant exchange is the heartbeat of an orbit. A small, unwelcome nudge from another object can alter this energy balance, changing an asteroid’s path from a safe, predictable orbit to a collision course with Earth.

Nowhere is the Scale, Proportion, and Quantity thinking lens more critical than in astronomy. The force of gravity between you and your desk is technically not zero, but it is so infinitesimally small that we ignore it. The force between the Earth and the Sun, however, is immense enough to hold a planet in its path. The Chelyabinsk asteroid was tiny on a cosmic scale, yet its energy was vast on a human scale. Understanding the universe requires a constant shifting of perspective, an appreciation for the numbers that define our cosmos—from the masses of planets to the velocities of asteroids and the immense distances between them.