Astrophysics
5 marks · NormalA space probe is launched from Earth to study a distant comet. The probe travels in orbit and astronomers collect data about its motion and the comet's spectrum.
Describe two ways in which the orbit of a comet usually differs from that of a planet in our solar system.
State the reason why an object stays in orbit around another object, such as a probe orbiting a comet.
Scientists observe that the absorption lines in the spectrum of the comet have shifted towards longer wavelengths compared to those from the Sun. What does this tell us about the motion of the comet relative to Earth?
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(a) Comet orbits are much more elongated (highly elliptical) and not close to circular; comets have much longer orbital periods and travel far beyond the planets; comets move fastest near the Sun and slowest when far away—planets have nearly circular, shorter orbits.
(b) Because of gravitational attraction between the two objects.
(c) The shift to longer wavelengths (red-shift) indicates the comet is moving away from Earth.
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Marking points
B1 (a) · 1 mark · Identifies one difference: comet orbits are much more elliptical/elongated than planets'
B2 (a) · 1 mark · Identifies another difference: longer orbital periods for comets, or comets travel further from the Sun, or orbital speed varies greatly
B3 (b) · 1 mark · States objects stay in orbit due to gravitational attraction
B4 (c) · 1 mark · States that longer wavelengths = red-shift
E1 (c) · 1 mark · Explains that this shows the comet is moving away from Earth
Worked solution
a) Comets have highly elliptical orbits, unlike planets' nearly circular ones. Comets' orbits take them much farther from the Sun and their orbits have much longer periods.
b) Objects stay in orbit because of the gravitational force between them.
c) The absorption lines have shifted to longer wavelengths (red-shift). This shows the comet is moving away from Earth.