key conceptsThe universe is dynamic. Everything in the universe changes. All stars, including our Sun, are born, shine for millions to billions of years, run out of fuel, and die. Galaxies can grow by cannibalizing their neighbors. Even the universe itself is evolving: it grew from a tiny, dense fireball over 13 billion years ago and continues to expand.
Collisions in space release enormous amounts of energy. Massive impacts — some as violent as the explosion of 10 thousand billion, billion, billion, billion (1040) atomic bombs — are the natural outcome of the motion of celestial objects. Collisions smash asteroids apart, created our Moon, power the stars, and build giant galaxies. For us, the most important impacts occur when particles smaller than atoms collide and rearrange matter, releasing energy as light and heat. That's what's happening by the trillions, every second, inside our Sun — and in other stars as well.
Collisions transform. Everywhere in the universe, collisions smash things apart, bring them together, and generally leave planets, stars, and galaxies very different than they were before. Earth is no exception: life has been transformed — maybe even made possible — by collisions. Notably, sixty-five million years ago an Everest-sized asteroid smashed through Earth's atmosphere, contributing to the extinction of 85% of Earth's species, including most dinosaurs. Yet this catastrophe gave rise to new ecological niches — including the one humans now occupy.
The cosmic scale of time and space is huge. There's a reason it's called space. Our Solar System is billions of miles across: to us a vast and largely empty expanse, but tiny compared to the typical distance between stars. Light from the most distant galaxies can travel for billions of light-years before it reaches us, and a light-year (the distance that light can travel in a year's time) is nearly six trillion miles! Time scales are also huge. The universe is 13.7 billion years old. Our Solar System, the Sun and the Earth, came into existence 4.5 billion years ago. The ancestors of humans may have walked on Earth 6 million years ago — an immense stretch of time, but less than 0.2% of the age of the universe. And, like the collisions that power the Sun, cosmic events can occur in the blink of an eye.
The laws of physics are the same throughout space and time. Fundamental physical forces, such as gravity and electromagnetism, operate everywhere to keep the universe in constant motion. The same laws that pull a fly ball down towards the field also pull giant galaxies towards each other. The processes we can observe from our perch on Earth — like the Aurora Borealis, planetary impacts, and galaxies colliding — are happening through-out the universe, just as they have since the Big Bang and always will. The universality of nature's laws makes it possible to under- stand things that happened far away and very long ago, and to predict how celestial bodies will move and change in the future.
The Earth system has natural defenses. Produced by the rotation of the planet's liquid outer core, the Earth's magnetic field shields us from energetic particles in the solar wind that streams off the surface of the Sun at more than a million miles an hour — and which can damage DNA in living organisms. The atmosphere also protects us: most of the tons of rock and dust from space that collide with Earth every day burn up through friction before they hit the ground.
We can use knowledge and technology to protect us. Collisions with bigger objects, like comets and asteroids, are extremely rare, but they do happen. However, unlike the dinosaurs, we know why collisions happen and can see what's coming. Scientists have been mapping the sky in ever-greater detail to track any possible threat while it's still many years away. Our knowledge — and perhaps our ability to navigate the solar system — could help us protect our planet in the future.