11  Asteroid Orbits & Planetary Defense

Will Apophis hit Earth in 2029? How do we protect our planet?

HS-ESS1-4 5–7 Days 🧠 Quiz & Evaluate ↓

☄️ Is Apophis Going to Hit Us?

12 🔥 Engage — The Apophis Scare

12.1 April 13, 2029: A 370-Meter Asteroid Passes Earth

On Friday, April 13, 2029, asteroid 99942 Apophis — named after the Egyptian god of chaos — will fly closer to Earth than many of our communications satellites. It will be visible to the naked eye from parts of Asia, Africa, and Europe.

When Apophis was first discovered in 2004, astronomers calculated a 2.7% chance of Earth impact — the highest ever assigned to a known asteroid. That’s 1 in 37. For a rock that could destroy a city the size of New York.

The good news: Further observations reduced the impact probability to zero for 2029. But the close flyby could change its orbit in unpredictable ways, and future passes remain a concern.

The bigger question: What would we actually DO if an asteroid were heading for Earth?

Plot = require("@observablehq/plot")
d3 = require("d3@7")

approachData = [
  {name: "GPS satellites", distance: 20200, type: "Satellite", color: "#74b9ff"},
  {name: "Geostationary orbit", distance: 35786, type: "Satellite", color: "#0984e3"},
  {name: "Apophis (2029)", distance: 31000, type: "Asteroid", color: "#d63031"},
  {name: "Moon", distance: 384400, type: "Moon", color: "#dfe6e9"},
  {name: "Typical near-Earth asteroid", distance: 500000, type: "Asteroid", color: "#fdcb6e"}
]

Plot.plot({
  title: "How Close Will Apophis Get? (km from Earth)",
  subtitle: "Closer than our geostationary satellites!",
  width: 700,
  height: 280,
  marginLeft: 200,
  x: {label: "Distance from Earth (km)", type: "log", grid: true},
  y: {label: null},
  marks: [
    Plot.barX(approachData, {
      x: "distance",
      y: "name",
      fill: "color",
      rx: 8,
      sort: {y: "x"}
    ,
        tip: true}),
    Plot.text(approachData, {
      x: "distance",
      y: "name",
      text: d => d.distance.toLocaleString() + " km",
      dx: 30,
      fontSize: 11,
      fontWeight: "bold"
    })
  ]
})

12.1.1 📝 Engage Questions

1. How close is Apophis’s 2029 flyby compared to the Moon’s distance?
2. If Apophis is 370 meters across and traveling at ~30 km/s, how much energy would an impact release?
3. Should we be worried about asteroids? How would we know one was coming?

13 🔍 Explore — Orbits and Gravity

13.1 What Keeps Asteroids (and Planets) in Orbit?

Everything in the solar system — planets, asteroids, comets — follows the same rules of orbital mechanics discovered by Kepler and explained by Newton.

13.2 Kepler’s Laws Applied to Asteroids

orbitData = [
  {name: "Mercury", a: 0.387, T: 0.241, type: "Planet", color: "#95a5a6"},
  {name: "Venus", a: 0.723, T: 0.615, type: "Planet", color: "#f39c12"},
  {name: "Earth", a: 1.000, T: 1.000, type: "Planet", color: "#2ecc71"},
  {name: "Mars", a: 1.524, T: 1.881, type: "Planet", color: "#e74c3c"},
  {name: "Apophis", a: 0.922, T: 0.886, type: "Asteroid", color: "#d63031"},
  {name: "Bennu", a: 1.126, T: 1.196, type: "Asteroid", color: "#6c5ce7"},
  {name: "Eros", a: 1.458, T: 1.761, type: "Asteroid", color: "#00cec9"},
  {name: "Ceres", a: 2.769, T: 4.600, type: "Dwarf planet", color: "#636e72"},
  {name: "Jupiter", a: 5.203, T: 11.86, type: "Planet", color: "#e17055"}
]

Plot.plot({
  title: "Kepler's Third Law: Orbital Period vs. Distance",
  subtitle: "T² = a³ — asteroids follow the same law as planets",
  width: 700,
  height: 400,
  grid: true,
  x: {label: "Semi-major axis (AU)", type: "log"},
  y: {label: "Orbital period (years)", type: "log"},
  color: {legend: true, domain: ["Planet", "Asteroid", "Dwarf planet"], range: ["#2ecc71", "#d63031", "#636e72"]},
  marks: [
    // Kepler's law line
    Plot.line(
      Array.from({length: 50}, (_, i) => {
        const a = 0.3 + i * 0.12;
        return {a: a, T: Math.pow(a, 1.5)};
      }),
      {x: "a", y: "T", stroke: "#00cec9", strokeWidth: 2, strokeDasharray: "8,4"}
    ),
    Plot.dot(orbitData, {
      x: "a",
      y: "T",
      fill: "type",
      r: 8,
      stroke: "white",
      strokeWidth: 1
    ,
        tip: true}),
    Plot.text(orbitData, {
      x: "a",
      y: "T",
      text: "name",
      dy: -12,
      fontSize: 10,
      fontWeight: "bold"
    })
  ]
})

13.3 Newton’s Law of Universal Gravitation

The force of gravity between any two objects:

\[F = G \frac{m_1 \cdot m_2}{r^2}\]

where \(G = 6.674 \times 10^{-11}\) N⋅m²/kg², \(m_1\) and \(m_2\) are the masses, and \(r\) is the distance between them.

viewof asteroidMass = Inputs.range([1e8, 1e13], {
  label: "Asteroid mass (kg):",
  step: 1e8,
  value: 2.7e10,
  format: d => d.toExponential(1)
})

viewof asteroidDistance = Inputs.range([6371, 400000], {
  label: "Distance from Earth center (km):",
  step: 1000,
  value: 31000
})

{
  const G = 6.674e-11;
  const earthMass = 5.972e24;
  const r = asteroidDistance * 1000; // convert km to m
  const force = G * earthMass * asteroidMass / (r * r);
  const acceleration = force / asteroidMass;
  
  const div = d3.create("div")
    .style("padding", "20px")
    .style("background", "linear-gradient(135deg, #ffecd2, #fcb69f)")
    .style("border-radius", "12px")
    .style("border-left", "6px solid #e17055");

  div.append("h3").style("margin-top", "0").text("⚡ Gravitational Interaction");
  div.append("p").html(`<strong>Asteroid mass:</strong> ${asteroidMass.toExponential(2)} kg`);
  div.append("p").html(`<strong>Distance:</strong> ${asteroidDistance.toLocaleString()} km from Earth's center`);
  div.append("p").html(`<strong>Gravitational force:</strong> ${force.toExponential(3)} N`);
  div.append("p").html(`<strong>Acceleration toward Earth:</strong> ${acceleration.toFixed(6)} m/s²`);
  
  if (asteroidDistance < 35786) {
    div.append("p").style("color", "#d63031").style("font-weight", "bold")
      .text("⚠️ This is closer than geostationary orbit!");
  }
  if (asteroidDistance < 6500) {
    div.append("p").style("color", "#d63031").style("font-weight", "bold")
      .text("💥 IMPACT — this is at or below Earth's surface!");
  }
  
  return div.node();
}

13.3.1 💡 Why Orbits Change

An asteroid’s orbit is stable as long as no outside forces act on it significantly. But orbits can change due to:

• Gravitational perturbations — close passes by planets bend the orbit
• Yarkovsky effect — sunlight heats one side, creating a tiny thrust
• Collisions — impacts with other asteroids can redirect them
• YORP effect — sunlight changes rotation rate, which changes orbit over time

This is why Apophis’s 2029 flyby matters — Earth’s gravity will bend its orbit, making future predictions harder.

14 💡 Explain — Orbital Mechanics

14.1 Why Some Asteroids Are Dangerous

Not all asteroids pose a threat. The dangerous ones are Near-Earth Objects (NEOs) — asteroids whose orbits bring them within 1.3 AU of the Sun, crossing or approaching Earth’s orbit.

neoData = [
  {size: "< 1 m", count: 1000000000, damage: "Burns up in atmosphere", risk: "None", color: "#2ecc71"},
  {size: "1–10 m", count: 500000000, damage: "Fireball, possible meteorites", risk: "Very Low", color: "#74b9ff"},
  {size: "10–50 m", count: 10000000, damage: "Chelyabinsk-type airburst", risk: "Low", color: "#fdcb6e"},
  {size: "50–140 m", count: 25000, damage: "City destroyer", risk: "Medium", color: "#f39c12"},
  {size: "140 m–1 km", count: 8000, damage: "Regional devastation", risk: "High", color: "#e17055"},
  {size: "> 1 km", count: 900, damage: "Global catastrophe", risk: "Very High", color: "#d63031"}
]

Plot.plot({
  title: "Near-Earth Objects by Size Category",
  subtitle: "Estimated total population — most small ones are still undiscovered",
  width: 700,
  height: 300,
  marginLeft: 100,
  x: {label: "Estimated number (log scale)", type: "log", grid: true},
  y: {label: null},
  marks: [
    Plot.barX(neoData, {
      x: "count",
      y: "size",
      fill: "color",
      rx: 8
    ,
        tip: true}),
    Plot.text(neoData, {
      x: "count",
      y: "size",
      text: d => d.damage,
      dx: 10,
      textAnchor: "start",
      fontSize: 10,
      fontWeight: "bold"
    })
  ]
})

🧠 We’ve found about 95% of the “planet killer” asteroids (>1 km) and NONE are on collision course with Earth. But we’ve found less than 40% of the “city killers” (>140 m). Apophis is 370 m — big enough to devastate a continent.

15 🔬 Elaborate — Planetary Defense

15.1 The DART Mission: Punching an Asteroid

In September 2022, NASA’s DART (Double Asteroid Redirection Test) spacecraft deliberately crashed into the asteroid moonlet Dimorphos at 22,530 km/h. The goal: prove we can change an asteroid’s orbit.

Result: It worked. DART changed Dimorphos’s orbital period by 33 minutes — far more than the minimum 73 seconds needed for success.

15.2 Deflection Strategies

viewof warningYears = Inputs.range([0.1, 30], {
  label: "Years of warning before impact:",
  step: 0.1,
  value: 10
})

{
  const strategies = [
    {name: "Kinetic Impactor (DART)", minYears: 5, maxYears: 30, effectiveness: "Proven", cost: "Medium", desc: "Crash a spacecraft into the asteroid at high speed. Proven by DART in 2022.", color: "#2ecc71"},
    {name: "Gravity Tractor", minYears: 10, maxYears: 30, effectiveness: "Theoretical", cost: "High", desc: "Park a massive spacecraft near the asteroid. Its gravity slowly tugs the asteroid off course.", color: "#0984e3"},
    {name: "Ion Beam Deflection", minYears: 5, maxYears: 20, effectiveness: "Theoretical", cost: "Medium", desc: "Fire ion thrusters at the asteroid surface, pushing it slowly.", color: "#6c5ce7"},
    {name: "Nuclear Standoff", minYears: 0.5, maxYears: 5, effectiveness: "Last resort", cost: "Very High", desc: "Detonate a nuclear device near (not on!) the asteroid. Radiation vaporizes the surface, creating thrust.", color: "#d63031"},
    {name: "Paint it white", minYears: 15, maxYears: 30, effectiveness: "Theoretical", cost: "Low", desc: "Change the asteroid's reflectivity to use sunlight pressure (Yarkovsky effect) to nudge its orbit.", color: "#f39c12"}
  ];

  const viable = strategies.filter(s => warningYears >= s.minYears && warningYears <= s.maxYears);
  const notViable = strategies.filter(s => warningYears < s.minYears || warningYears > s.maxYears);

  const div = d3.create("div");
  div.append("h3").style("font-family", "Space Grotesk, sans-serif").style("color", "#00cec9")
    .text(`With ${warningYears.toFixed(1)} years of warning:`);

  if (viable.length > 0) {
    div.append("h4").style("color", "#2ecc71").text("✅ Viable Options:");
    viable.forEach(s => {
      const card = div.append("div").style("padding", "12px").style("margin", "8px 0")
        .style("border-radius", "10px").style("border-left", `5px solid ${s.color}`)
        .style("background", `${s.color}15`);
      card.append("strong").text(s.name);
      card.append("span").style("margin-left", "10px").style("font-size", "0.85em")
        .style("color", "#636e72").text(`[${s.effectiveness}]`);
      card.append("p").style("margin", "5px 0 0 0").style("font-size", "0.95em").text(s.desc);
    });
  }

  if (notViable.length > 0) {
    div.append("h4").style("color", "#d63031").text("❌ Not enough time for:");
    notViable.forEach(s => {
      div.append("div").style("padding", "5px 12px").style("margin", "4px 0")
        .style("color", "#636e72").style("text-decoration", "line-through")
        .text(`${s.name} (needs ${s.minYears}–${s.maxYears} years)`);
    });
  }

  if (warningYears < 0.5) {
    div.append("div").style("padding", "15px").style("margin-top", "10px")
      .style("background", "#d63031").style("color", "white").style("border-radius", "10px")
      .style("font-weight", "bold").style("text-align", "center")
      .text("⚠️ With less than 6 months warning, there may be no viable deflection option. Evacuation only.");
  }

  return div.node();
}

15.2.1 💡 The Key Lesson

Planetary defense isn’t science fiction — it’s active science:

• Detection is the most important step. We need to find asteroids years before they arrive.
• Small nudges early are better than big pushes late. A 1 cm/s change 10 years out can miss Earth by thousands of kilometers.
• DART proved kinetic impact works. The next step is ESA’s Hera mission to study the impact site.
• The biggest risk isn’t a known asteroid — it’s the ~60% of city-killers we haven’t found yet.

16 ✅ Evaluate

16.1 Applying Orbital Mechanics to Real Threats

16.1.1 🧪 Evaluate Questions

1. Apply Kepler’s Third Law: If an asteroid has a semi-major axis of 1.5 AU, what is its orbital period? How does this compare to Mars?

2. Use Newton’s gravity law: Why does Apophis’s orbit change when it passes close to Earth? What determines how much it changes?

3. Evaluate the DART mission. Why was it important to test kinetic impact on a real asteroid rather than just simulating it?

4. Argue which planetary defense strategy is best for:

   • A 100-meter asteroid detected 20 years before impact
   • A 500-meter asteroid detected 2 years before impact
   • A 1-km asteroid detected 6 months before impact

5. Connect to the unit: How do asteroid impacts connect to the probability of life existing in the universe? Consider both destructive AND constructive effects.

16.1.2 📝 Model Update

Update your thinking about the probability of life:

• Asteroid impacts are both threats and opportunities for life
• The ability of a civilization to defend itself from asteroids may be a factor in whether intelligent life survives long enough to be detected
• How does this connect to the Fermi Paradox?