Jupiter Orbit Insertion Burn Status
This plot shows by how much Galileo's velocity along the line of sight between us and the spacecraft has changed, compared with what the velocity would be if there was no Jupiter Orbit Insertion (JOI) burn. Since this shows the change in velocity relative to some baseline, this is called "residual" data, because it's the velocity that is left after the baseline is subtracted out--that is, the entire velocity change along the line of sight.
The burn will last roughly 49 minutes, during which time you will see the residual velocity dropping down. The plot will start updating shortly before the start of JOI, and will cease updating shortly after the end of JOI.
What is Doppler data, and what does it have to do with velocity?
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 emitted from the spacecraft. However, since the spacecraft is 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 emitted frequency to get the Doppler shift. It's then straightforward to find the velocity that would cause the resulting Doppler shift. (Note that this only gives us the line-of-sight velocity, but the spacecraft is oriented so that the Doppler data is showing essentially all of the velocity change.).
What do the red lines represent?
Galileo's engines have some slight variability in performance. The red lines show a straight line approximation of the expected envelope of performance; as long as the data stays within that range, all is well. It's somewhat counterintuitive to have the upper line labelled underperformance, and the bottom line labelled overperformance, but that is not an error. If the engine underperforms, then it will take the spacecraft longer to slow down (the upper line). Conversely, if the engine overperforms, the spacecraft will slow down faster than expected (the lower line).
There's also some uncertainty in the "zero" point, before the engine burn starts. This is because of some uncertainty in the effects of the Io flyby on the spacecraft's trajectory. Finally, there's a line indicating the point at which Galileo's speed has dropped low enough so that we can safely say that the spacecraft has been "captured." This occurs about 45 minutes into the nominal burn duration.
Why isn't the data forming a straight line?
At the time of the Orbit Insertion Burn, the spacecraft velocity is changing radically, due to the pull of Jupiter's gravity, as well as the thrusting of the engines. As a result, the difference between the velocities for a flyby (no burn) and an Orbit Insertion burn does not change in a linear fashion (follow a straight line). Therefore, some variation will be seen in the residual (which, again, is the difference between the actual spacecraft velocity, and the velocity the spacecraft would have if no burn had occured)
Gaileo's "400 Newton" main engine produces, on average, 391 Newtons (88 lbs.) of thrust. The propulsion system was designed and built by the Federal Republic of Germany
Why is there a gap in the data?
Actually, there probably ISN'T a gap in the data. But in the event that there is, here's an explanation.
The Doppler data that you're seeing here on the WWW goes from the Deep Space Network tracking station to the Galileo navigation team at JPL where it gets further processed into the display you're seeing. If a link in that chain breaks (with the most likely problem arising in the computers doing the post-processing), the result is a seeming gap in the data. It does NOT mean that there is any problem with the spacecraft.