# Practical Exercise 4:

The closest star situated outside our Solar System is Proxima Centauri and its distance is approx. 4.2 ly. How long would take the journey there:

a) By a train set Pendolino at a speed of 250 km/h;

b) By an aeroplane Airbus at a travelling speed of 800 km/h;

c) By a Voyager spacecraft at a speed of 17 km/s?

Answer: We convert the distance to kilometres: d = 4,2 ly = 4,2 · 9 500 000 000 000 km ≐ 40 000 000 000 000 km.

For the conversion to years, we use the given value: 1 year ≈ 8760 h. For the particular times we get:

ta = 40 000 000 000 000 km/250 km/h ≐ 160 000 000 000 h ≐ 18 000 000 years,

tb = 40 000 000 000 000 km/800 km/h = 50 000 000 000 h ≐ 5 700 000 years,

tc = 40 000 000 000 000 km/17 km/s ≐ 2 400 000 000 000 s ≐ ≐ 650 000 000 h ≐ 75 000 years.

For the Voyager spacecraft, we can calculate the speed in km/h v = 17x3600 km/h = 61 200 km/h. Then we can get similarly to a) and b):

tc = 40 000 000 000 000 km/61 200 km/h ≐ 650 000 000 h ≐ 75 000 years

Objective of practical exercise is to get a visualisation of big distance based on a travelling experience by a common means of transport (train, aeroplane) and, up to this date, the most remote man-made object of our Solar System, Voyager 1 spacecraft. It was launched on 5th September 1977 and for 40 years, it has reached the distance of „only“ 140 au (i.e. approximately 19 ly; according to the date for June 2018, currently it is possible to update on internet. Even though we will lose the contact with the spacecraft in the following years, it is supposed that after the year 2025 the spacecraft will not have energy for communication, acquisition or sending of any data).

First two journey times are enormous – 5.7 million or 18 million years ago, there were neither humans, nor Australopithecus hominids on Earth yet. And besides, we do not take into account the huge volume of fuel and energy necessary for the train or aeroplane operation for such a long time. And we consider only the closest star just beyond the gates of the Solar System!