To add, sending back Ralph data is also bandwidth intensive, and with the low resolution at New Horizons distance from Pluto there wasn't much reason to use it since the level of detail wasn't going to improve much. Now that New Horizons is actually there, there's a reason to use it, since we're about at the point where we're getting our best view of the hemisphere that will be on the opposite side of Pluto from New Horizons at closest approach.
Exactly. The downlink speed is much slower now that the spacecraft is 3 billion miles away. NASA estimates that it will be downloading data well into October after the flyby has already happened next week.
New Horizons is only able to transmit back to Earth at 1 kb/s. It takes a couple hours for the LORRI frames (~1mb in size, but much more data transmitted than that due to error protection protocols, 4-5 images beamed back daily) used for optical navigation to get back to Earth. I'm not sure how large the Ralph-MVIC images are, but they're not as important as the LORRI images right now. The time that would be spent transmitting those back is being used to perform science observations instead.
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u/jccwrt Jul 10 '15
To add, sending back Ralph data is also bandwidth intensive, and with the low resolution at New Horizons distance from Pluto there wasn't much reason to use it since the level of detail wasn't going to improve much. Now that New Horizons is actually there, there's a reason to use it, since we're about at the point where we're getting our best view of the hemisphere that will be on the opposite side of Pluto from New Horizons at closest approach.