What is space weather?
Weather on Earth is rain, wind and heat. Space weather is the changing condition of space near Earth, and it is driven by the Sun. The Sun has a strong, twisting magnetic field. From it come:
- Sunspots: cooler dark patches.
- Flares: sudden flashes of light and X-rays. Light takes about 8 minutes to reach us.
- Coronal mass ejections (CMEs): huge clouds of charged particles. They are slower than light.
The Sun also blows a steady solar wind, about 400 km/s. Activity goes up and down in an 11-year cycle.
How storms reach Earth and what they do
Distance from the Sun is about 1.5 × 10⁸ km. Travel time = distance ÷ speed. A fast CME at 1000 km/s takes about 42 hours. A slow solar wind at 400 km/s takes about 4.3 days.
Earth's magnetic field forms a shield called the magnetosphere. Most particles are bent around the Earth. Some are guided down towards the poles, where they hit the air and make glowing green and red aurora.
A strong geomagnetic storm can:
- damage or confuse satellites and GPS,
- black out some radio signals,
- push extra current into long power lines and trip transformers,
- raise radiation for astronauts and high-flying aircraft.
A spacecraft sitting 1.5 million km sunward of Earth sees the wind early and gives about 30 to 60 minutes of warning.
Near-Earth objects: asteroids
An asteroid is a rocky body that goes round the Sun, mostly in a belt between Mars and Jupiter. A near-Earth object (NEO) is one whose orbit brings it within about 1.3 AU of the Sun, so its path can pass close to Earth's.
Astronomers photograph the sky again and again, measure each rock's position, and work out its orbit many years ahead. Because asteroids move on smooth, predictable paths, we know where they will be. Small pieces hit the air every day and burn up as shooting stars. Big, dangerous ones are rare, and all large known ones are tracked.
An asteroid orbit is an ellipse. A close approach is when the rock's path takes it near Earth; the number to watch is how many km it misses by.
Try it: warning time
- In the 3D, set the storm to 1. Read the arrival time. Then set it to 9.
- Predict first: will a storm that is 2 times faster take half the time? Check with the numbers.
- At home: look up tonight's aurora forecast online or note how a weather app gives a warning. Which warning gives more time, a storm from the Sun or an asteroid?
Key formulas and definitions
- travel time = distance ÷ speed
- Sun to Earth ≈ 1.5 × 10⁸ km = 1 AU
- 1 day = 86 400 s
- NEO: orbit comes within about 1.3 AU of the Sun
Worked examples
1. A CME travels at 1000 km/s. How long does it take to reach Earth (1.5 × 10⁸ km)?
t = 1.5 × 10⁸ / 1000 = 1.5 × 10⁵ s. In hours: 1.5 × 10⁵ / 3600 ≈ 41.7 h.
2. The solar wind moves at 400 km/s. How many days does it take to reach Earth?
t = 1.5 × 10⁸ / 400 = 3.75 × 10⁵ s. In days: 3.75 × 10⁵ / 86 400 ≈ 4.3 days.
3. A spacecraft 1.5 × 10⁶ km towards the Sun sees wind at 500 km/s. How much warning does it give?
t = 1.5 × 10⁶ / 500 = 3000 s = 50 minutes.
4. What speed brings a CME to Earth in exactly 1 day?
v = distance / time = 1.5 × 10⁸ / 86 400 ≈ 1736 km/s.
5. An asteroid passes at 0.01 AU from Earth. How many km is that?
0.01 × 1.5 × 10⁸ = 1.5 × 10⁶ km, about 4 times the Earth–Moon distance.
Common mistakes
- Thinking a flare and a CME arrive together. Light from a flare comes in 8 minutes; the CME takes 1 to 4 days.
- Thinking an aurora is a fire or only happens at one pole. It happens in a ring around both poles.
- Mixing km/s with km/h when finding travel time.
- Believing every asteroid is a danger. Most near-Earth asteroids will never hit Earth, and orbits are tracked for years.