Studyledge

Space Physics: Question 3

Syllabus 6.1.2

Structured Core 7 marks

Mission control for a space agency operates a probe named Meridian-6, which is currently orbiting Neptune, one of the giant planets that lie far beyond the asteroid belt.

(a) State the force that keeps Meridian-6 in orbit around Neptune, and state why Neptune itself orbits the Sun rather than the Sun orbiting Neptune. [2]

(b) State how the strength of Neptune's gravitational field changes as Meridian-6 moves from an orbit close to Neptune's cloud tops to a much wider orbit further away, and state the one property of Neptune that determines the strength of its gravitational field at any fixed distance from its centre. [2]

(c) Mission control sends a radio signal to Meridian-6. At the moment the signal is sent, Meridian-6 is 1.20×1012 m1.20\times10^{12}\text{ m} from Earth. Radio signals, like all electromagnetic waves, travel at the speed of light, 3.00×108 m/s3.00\times10^8\text{ m/s}. Calculate the time taken for the signal to reach Meridian-6, giving your answer in minutes. [3]

Show worked solution Hide worked solution

Worked solution

Part (a): What keeps Meridian-6 in orbit

The force that keeps Meridian-6 in its orbit around Neptune is the gravitational attraction between the probe and Neptune.

By the same reasoning, Neptune orbits the Sun rather than the Sun orbiting Neptune because the Sun contains almost all of the mass of the Solar System. This huge mass means the Sun’s gravity dominates, so it is the planets (including Neptune) that orbit the much more massive Sun.

Part (b): Gravitational field strength with distance

As Meridian-6 moves from an orbit close to Neptune out to a much wider orbit, the strength of Neptune’s gravitational field decreases. The further Meridian-6 is from Neptune, the weaker the pull of gravity it experiences.

At any one fixed distance, the strength of a planet’s gravitational field depends on the mass of the planet. A more massive planet produces a stronger gravitational field at the same distance.

Part (c): Time for the radio signal to travel

Radio signals are electromagnetic waves, so they travel at the speed of light, c=3.00×108 m/sc = 3.00\times10^8\text{ m/s}.

Rearranging speed =distancetime=\dfrac{\text{distance}}{\text{time}} for time:

time=distancespeed\text{time} = \frac{\text{distance}}{\text{speed}}

time=1.20×1012 m3.00×108 m/s\text{time} = \frac{1.20\times10^{12}\text{ m}}{3.00\times10^8\text{ m/s}}

time=4.00×103 s=4000 s\text{time} = 4.00\times10^3\text{ s} = 4000\text{ s}

Converting to minutes:

time=400060 min=66.7 minutes (3 s.f.)\text{time} = \frac{4000}{60}\text{ min} = \boxed{66.7\text{ minutes (3 s.f.)}}

Final answers

  • (a) Gravitational attraction keeps Meridian-6 in orbit around Neptune; Neptune orbits the Sun because the Sun holds almost all the Solar System’s mass
  • (b) The field decreases in strength with distance from Neptune; its strength at a given distance depends on Neptune’s mass
  • (c) Signal travel time == 66.766.7 minutes (to 3 s.f.)