Rosetta: Rendez-vous with a comet

Rosetta is a robotic space probe built and launched by the European Space Agency. Along with Philae, its lander module, Rosetta is performing a detailed study of comet 67P/Churyumov–Gerasimenko.

Rosetta

Philae

Comet 67P
Rosetta: © (CC) ESA/ATG medialab – https://www.flickr.com/photos/europeanspaceagency/11206647984/
Philae: © (CC) ESA/ATG medialab – https://www.flickr.com/photos/europeanspaceagency/10796307373/
Comet 67P: © (CC) SA/Rosetta/NAVCAM – http://blogs.esa.int/rosetta/2014/09/19/cometwatch-19-september/

Your Challenge

At the time this blog post was created, the European Space Agency just received new photographs of comet 67P taken by Rosetta. The spaceship was 500 million km away from Earth.

Write a Python script to answer the following question:

Considering that electro-magnetic signals used to communicate with Rosetta travel at the speed of light (300,000 km/s), how long did these pictures take to travel from Rosetta to planet Earth?

To help you solve this problem you may need to investigate the following formulas:

You will need to make sure your script gives the answer using in the correct format: hh:mm:ss.

Challenge #2: Time travelling?

Did you know that when you look at the stars in the night sky you are time travelling. You are in fact looking at what the stars did look like years and years ago! This is because the stars are extremely far away from planet Earth, and the light they create takes several years to reach us.

When astronomers measure the distance of stars (from planet Earth) they do not use the kilometer (km) unit but instead they use the light-year unit. One light-year represents the distance that light travels in one year, at the speed of light (300,000 km/s).

Your challenge consists of writing a Python script to calculate the distance in km of one light-year.

Then you will use your script to calculate the distance in km for the following stars:

Star	Distance from Earth (in light-years)
Alpha Centauri	4.24 light-years
Barnard’s Star	5.96 light-years
Luhman 16	6.59 light-years
WISE 0855-0714	7.2 light-years
Wolf 359	7.78 light-years

Source:
http://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs

Learning Objectives

In this challenge we are going to focus on accessing a text file in Python to read the content of the file line by line.

Challenge

Your first challenge consists of writing a Python script that will read the following text file, one line at a time and display the content of each line on screen.

My Playlist.txt

To read the content of a text file line by line we are going to use a for loop that will loop through and extract each line of the text file one at a time. It will stop looping only once the it will have reached the last line.

Check this animation that explains this process:

When the text file uses a CSV format (Comma Separtred Values), each line contains several values separated using a special character, often a comma or a semi-colon. In this case we can use the split() method to extract each value of each line and store them in a list.

Check this animation that explains this process further:

Solution

The solutions below contains 5 main steps:

1. Step 1: Open the text file using the open() function. This function takes two parameters: The name of the file (e.g. “playlist.txt”) and the access mode. In our case we are opening the file in read-only mode: “r”.
2. Read through the file one line at a time using a for loop.
3. Split the line into an array. This is because in this file each value is separated with a semi-column. By splitting the line we can then easily access each value (field) individually.
4. Output the content of each field using the print method.
5. Once the for loop is completed, close the file using the close() method.

 

Main Access Modes when opening a file with the open() function.

Mode	Description
r	Opens a file in read only mode. This is the default mode.
r+	Opens a file for both reading and writing.
w	Opens a file for writing only. Overwrites the file if the file exists. If the file does not exist yet, it will create the new file for writing.
w+	Opens a file for both writing and reading. Overwrites the file if the file exists. If the file does not exist yet, it will create the new file for writing.
a	Opens a file for appending. The file pointer is at the end of the file. So new data will be added at the end of the file. If the file does not exist, it creates a new file for writing.
a+	Opens a file for both appending and reading. The file pointer is at the end of the file. The file opens in the append mode. If the file does not exist, it creates a new file for reading and writing.

Your Challenge

Use what you have learnt in this blogbpost to complete these challenges:
My mp3 playlist(Reading a text file using Python) US Population(Reading a text file using Python)

In most of your Python programs you will want to interact with the end-user by asking questions and retrieving user inputs.

To do so you can use the input() function: e.g.

username=input("What is your username?")

Sometimes you will need to retrieve numbers. Whole numbers (numbers with no decimal place) are called integers. To use them as integers you will need to convert the user input into an integer using the int() function. e.g.

age=int(input("What is your age?"))

This line of code would work fine as long as the user enters an integer. If, by mistake, they enter letters or punctuation signs, the conversion into an integer will fail and generate an exception (error) and the program would stop running.

To validate the user entry and ensure that is a number it is possible to catch this exception when it occurs using the try…except….else block as follows:

try:
    value=int(input("Type a number:"))
except ValueError:
    print("This is not a whole number.")

See how we can use this approach to define our own function (called inputNumber()) to ask for a number. This new function can then be used instead of an input() function whenever we expect the user to enter a whole number. This function uses the try…except….else block within a while loop so that it keeps asking the user to enter a number till the user gets it right.

Final Code:

def inputNumber(message):
  while True:
    try:
       userInput = int(input(message))       
    except ValueError:
       print("Not an integer! Try again.")
       continue
    else:
       return userInput 
       break 
     

#MAIN PROGRAM STARTS HERE:
age = inputNumber("How old are you?")