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The Rosetta spacecraft will land on a comet

Homework for next class

Rosetta's mission: land on the surface of comet 67P/Churyumov-Gerasimenko

The European Space Agency built the Rosetta spacecraft (with a little help from JPL ) and sent it on a long journey. Its primary goal is to reach comet 67P/Churyumov-Gerasimenko and place a lander on the surface.

The comet has an orbit in which it spends most of its time far from the Sun, between the orbits of Mars and Jupiter.

Rosetta was launched in March, 2004. The plan was for the spacecraft to reach the comet in 2014.

But one year after it was launched, in March, 2005, Rosetta was right back where it started --- zipping past the Earth just a few thousand kilometers above the the Pacific Ocean off Chile.

Photo: ESA

Photo: ESA

So what was the spacecraft doing near the Earth?

Using gravity assists to bounce around the solar system

The problem is that it takes a powerful rocket to reach the outer solar system directly -- and such powerful rockets are expensive. Let's do a quick and dirty calculation of the energies involved.

   Gravitational Potential Energy (GPE) of a spacecraft of
       mass m which is a distance r from a 
       planet of mass M

                          G M m 
            GPE   =   -  -------

   Increase in GPE if the spacecraft moves from r1 to
   a larger distance r2

                                   (  1       1  )
        incr in GPE  =    G M m  * ( ---  -  --- )
                                   (  r1      r2 )

Consider these three situations, and figure out how much energy you must give a spacecraft to change its position in each case:

   The Rosetta spacecraft has a mass of about   m  =  6500       kg
   The gravitational constant is roughly        G  =  6.67E-11   N*m^2/kg^2
   The mass of the Earth is                     Me =  5.98E24    kg
   The mass of the Sun is                       Ms =  1.99E30    kg

   1.  Rosetta is launched from the ground to an altitude
       of 1 Earth radius (Re = 6.37E6 km).  

   2.  Rosetta increases its distance from the Earth's surface
       from 1 Earth radius to 60 Earth radii (the Moon's distance
       from the Earth).

   3.  Rosetta moves outwards away from the Sun, from roughly
       1 AU (the Earth's distance from the Sun, 1 AU = 1.5E11 m)
       to a distance of 3.5 AU, where it will meet the comet.

You can see that the amount of energy needed to reach the outer portions of the Solar System is enormous, much larger than that needed just to reach Low Earth Orbit, or even to reach the Moon.

If one has patience, however, one can use a much less expensive rocket and a series of gravity assists to reach the desired location; it just takes time.

This Flash animation shows the trajectory of the spacecraft and its gravity assists from the Earth:

Count the flyby events:

  1. encounter with Earth (March 2005)
  2. encounter with Mars (February 2007)
  3. second encounter with Earth (November 2007)
  4. third encounter with Earth (November 2009)

Each of the encounters with Earth increases the spacecraft's orbital energy and size; the encounter with Mars, on the other hand, decreases the orbital energy just enough to cause the subsequent approaches to Earth.

The final leg of the journey, from the final Earth flyby to the meeting with Comet 67P, takes about five years. Scientists will have to wait until the year 2014 to reach the comet.

  Q:  How old will you be in 2014?
      What will you be doing then?

How do gravity assists work?

How can it help a spacecraft to fly past a planet? Sure, it will gain speed as it is pulled towards the planet by gravity ... but doesn't it lose that extra speed as it moves away from the planet?

There is a key factor involved. Let's look at a simpler situation to see it clearly. Consider Fred. He has a little rubber ball.

What will happen if he throws the ball at a car parked in the street?

  Q:  Did the ball pick up extra speed by 
      bouncing off the car?

Fred repeats the experiment, but this time, he stands out in the street and tosses his ball at a car which is speeding at 50 miles per hour.

  Q:  Did the ball pick up extra speed by 
      bouncing off the car?

      Why?  What's different in this situation?

      Which way did the ball get a boost?

      What happens to the car as a result?

In some collisions -- including the gravitational encounters between spacecraft and planets -- both momentum and kinetic energy are conserved. If you know the original velocities (magnitude AND direction) of two colliding objects, and their masses, then you can figure out their final velocities.

Fred now goes to a nearby racetrack and tosses his ball in front of a car which is going around in circles.

Even a gentle toss in front of a speeding car results in the ball shooting away from the car at high speed, in the same direction of travel.

Now, replace the racetrack with the Solar System; and replace the speeding car with a planet. You end up with a gravity assist which can greatly increase (or decrease) the speed of a spacecraft.

Figuring out the effect of a planetary encounter on a spacecraft is easiest if you transform into the center-of-mass frame; since planets are so much more massive than spacecraft, this means going into a frame in which the planet is at rest at the origin.

Homework for the next class

  1. During what month of 2014 will Rosetta begin to map the comet 67P/Churyumov-Gerasimenko?
  2. On the 15th of that month, what will the celestial coordinates (RA and Dec) of the comet be? Rough values are okay.
  3. Scientists will need to communicate with the spacecraft at this time. Where should the radio dishes be on Earth to allow the best signals to be received from the spacecraft during this crucial time? For example, would it be a good idea to put radio dishes in Rochester? (Hint: compare the Declination of the comet to the latitude of Rochester)
  4. If some astronomers do decide to place a radio telescope here in Rochester, and they try to communicate with the spacecraft, what time would be the best for them to listen?

The answers

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Last modified by MWR 3/8/2005

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.