This plot indicates the velocity change that the Europa gravity assist imparts to the Galileo spacecraft along the Earthline direction. This velocity change is measured as a "Doppler shift". The Doppler shift of a radio signal sent by Galileo is proportional to the line-of-sight (direction from the Earth to the spacecraft) or Earthline velocity of the spacecraft. The Doppler shift is a frequency shift measured in Hertz (Hz) and the relationship between Hertz and velocity change for the Galileo 1-way S-band radio signal is: 1 Hz = 0.131 m/sec. (For additional information on Doppler data, see the question and response below).
The gravity of Europa changes the velocity (magnitude and direction) of the spacecraft, and this change in velocity shows up as a Doppler shift of the radio signal sent from the spacecraft. The magnitude of the Doppler shift indicates the magnitude of the velocity change that the gravity assist provides. Note that this measurement is not the total spacecraft velocity change, only the Earthline component.
The plot shows that as the spacecraft approaches Europa it is accelerated along the Earthline direction (i.e., away from the Earth), and the Earthline velocity increases. It reaches its peak (Earthline) velocity change of approximately 59 m/s (450 Hz) about eight minutes before closest approach. Then as the Galileo continues past Europa, its gravity accelerates the spacecraft in the anti-Earthline direction, and the Earthline velocity decreases. The net (Earthline) velocity change of -383 m/s (-2930 Hz) is reached about three hours after the encounter.
Once the flyby is complete, Europa will have slowed the speed of the spacecraft (with respect to Jupiter) by 217 m/sec. Remember that the gravity assist changes the spacecraft velocity magnitude and direction. The total velocity (vector magnitude) change that Europa imparts to Galileo is 495 m/s.
Doppler plot is courtesy of the Galileo Navigation Team
Most people are familiar with the phenomenon of a car horn or train whistle changing its frequency as it moves towards or away from them. Electromagnetic radiation (e.g. light waves or radio signals) also experience this effect. The size of the frequency shift, or "Doppler shift," depends on how fast the light source is moving relative to the observer. Astronomers often refer to the "redshift" and "blueshift" of visible light, where the light from an object coming towards us is shifted to the blue end of the spectrum (higher frequencies), and light from an object moving away is shifted towards the red (lower frequencies).
Galileo commmunicates with controllers on the ground by radio signal. Ground controllers know the frequency of the signal that is transmitted from the spacecraft. However, since the spacecraft is always moving away from or towards us, this frequency is being Doppler shifted to a different frequency. So, engineers (or, more accurately, computers) compare the received frequency with the transmitted frequency to compute the Doppler shift. It is then straightforward to find the velocity change that would cause the resulting Doppler shift. (Note that this gives us only the line-of-sight velocity.) Again, the frequency shift is measured in Hertz (Hz), and the conversion for Galileo (1-way at S-band) is: 1 Hz = 0.131 m/sec.
What does the blue line represent?
This line is the predicted behavior of the Doppler data. The actual data will follow (and be plotted over) this line in black.
Why is there a gap in the data?
At the time of the encounter, the spacecraft will pass behind Europa, as viewed from the Earth. This is called an "occultation" (the occultation entry and exit times are marked on the plot). For about 17 minutes, the spacecraft will not be able to communicate with the Earth, since Europa will blocking the radio signal. You will therefore notice a gap in the data during the occultation.
What does the "Jupiter Closest Approach" represent?
Recall that Galileo is in an elliptical orbit about Jupiter. At Jupiter closest approach (called periapsis or perijove) the spacecraft is as its closest point to Jupiter during this orbit. Perijove occurs approximately 3 1/2 hours before the Europa encounter.
What does the "Last Updated" represent?
This plot is being updated in "real time" as the events occur. This plot will update about once every minute during the encounter activities. This label shows you how "current" the plot is. Recall that it takes about 50 minutes for a radio signal from Galileo to reach the ground at this time, so the events you see here actually took place almost an hour earlier at the spacecraft.