Comets

Comet anatomy

JavaScript — comet calculations

const G = 6.674e-11;
const M_SUN = 1.989e30;

// Halley's comet orbit
const a_HALLEY = 17.83;  // AU
const e_HALLEY = 0.967;
const T_HALLEY = Math.pow(a_HALLEY, 1.5);
console.log(`Halley period: ${T_HALLEY.toFixed(1)} years`);
const peri_HALLEY = a_HALLEY * (1 - e_HALLEY);
const aph_HALLEY = a_HALLEY * (1 + e_HALLEY);
console.log(`Halley peri: ${peri_HALLEY.toFixed(2)} AU; aph: ${aph_HALLEY.toFixed(2)} AU`);
// 0.59 AU (inside Earth) to 35.1 AU (past Neptune)

// Speed at perihelion
function visViva(distance_AU, semi_major_AU) {
  // v = sqrt(GM(2/r - 1/a))
  const GM = G * M_SUN;
  const r = distance_AU * 1.496e11;
  const a = semi_major_AU * 1.496e11;
  return Math.sqrt(GM * (2/r - 1/a));
}

console.log(`Halley v at peri: ${(visViva(0.586, a_HALLEY) / 1000).toFixed(2)} km/s`);
console.log(`Halley v at aph: ${(visViva(35.08, a_HALLEY) / 1000).toFixed(3)} km/s`);

// Comet sublimation rate (rough)
function sublimation(distance_AU, latent_heat = 2.83e6, ice_density = 920) {
  const flux = 1361 / (distance_AU * distance_AU);
  // Assuming all flux goes to sublimation
  return flux / latent_heat * ice_density;  // kg/(m²·s)
}

console.log(`At 1 AU: ${sublimation(1).toExponential(2)} kg/m²/s`);
console.log(`At 5 AU: ${sublimation(5).toExponential(2)} kg/m²/s`);

// Long-period comet: from Oort cloud
const oort_distance = 50000;  // AU
const oort_speed_at_peri = visViva(0.5, oort_distance / 2);
console.log(`Long-period at 0.5 AU: ${(oort_speed_at_peri / 1000).toFixed(0)} km/s`);

Why comets matter

• Solar system origin. Pristine 4.5 Gyr old material.
• Water and organic delivery. Comets may have brought water and life ingredients to early Earth.
• Astrobiology. Organics on comets — building blocks of life.
• Space hazards. Some long-period comets pose impact risks.
• Public engagement. Bright comets — visible naked-eye spectacles.
• Mission targets. Rosetta, Deep Impact, Stardust returned data and samples.
• Interstellar science. Oumuamua, Borisov — visitors from other star systems.

Common misconceptions

• Tails point in direction of motion. Always away from Sun.
• Comets are fast and bright always. Most appear quiet; only bright when close to Sun.
• Comets are bad omens. Historical superstition; no scientific basis.
• All comets are alike. Varies in composition, orbit, dust/ion ratio, age.
• Comets melt and disappear. Sublimation, not melting (no liquid phase). Material lost over many passes; eventually disintegrate.
• Comets are mainly water. ~50% water ice; rest is dust, organics, other ices, rock.