Comets

    OCR
    GCSE

    Comets are small Solar System bodies composed of ice, dust, and rocky particles that orbit the Sun in highly elliptical trajectories. As a comet approaches perihelion, solar radiation vaporizes volatile ices, generating a coma and a tail that invariably points away from the Sun due to solar wind and radiation pressure. The orbital mechanics of comets provide a critical context for applying the conservation of energy, specifically the interchange between gravitational potential energy and kinetic energy as orbital velocity varies with distance from the Sun.

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    Objectives
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    Exam Tips
    3
    Pitfalls
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    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating the orbit is highly elliptical (or oval) with the Sun at one focus
    • Award 1 mark for identifying that orbital speed increases as the comet approaches the Sun (perihelion)
    • Credit responses linking increased speed to stronger gravitational force at smaller orbital radii
    • Award 1 mark for explaining that total energy is conserved: GPE decreases and KE increases as the comet falls towards the Sun

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the elliptical shape, but you must place the Sun at one focus, not the center"
    • "Good description of speed change. To access higher marks, explain *why* gravity causes this acceleration"
    • "Remember that the comet tail direction is determined by the Sun's position, not the comet's direction of travel"
    • "Excellent link between distance and force. Now consider the energy transfers (GPE to KE) to fully justify the speed increase"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating the orbit is highly elliptical (or oval) with the Sun at one focus
    • Award 1 mark for identifying that orbital speed increases as the comet approaches the Sun (perihelion)
    • Credit responses linking increased speed to stronger gravitational force at smaller orbital radii
    • Award 1 mark for explaining that total energy is conserved: GPE decreases and KE increases as the comet falls towards the Sun

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When drawing the orbit, exaggerate the elongation of the ellipse to distinguish it clearly from a circular planetary orbit
    • 💡For explanation questions on speed, explicitly link 'decreased distance' to 'increased gravitational force' and 'increased kinetic energy'
    • 💡Do not confuse comets (ice/dust) with asteroids (rock/metal) when asked about composition

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Drawing the Sun at the geometric center of the elliptical orbit rather than at one focus
    • Incorrectly applying circular orbit logic to state that speed is constant; comets accelerate as they near the star
    • Confusing the direction of the tail; drawing it trailing the motion rather than pointing away from the Sun due to solar wind

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Highly elliptical orbital characteristics
    Variation in orbital speed and energy conservation
    Composition and interaction with solar radiation
    Gravitational forces in the Solar System

    Likely Command Words

    How questions on this topic are typically asked

    State
    Describe
    Explain
    Compare
    Draw

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