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Watching the New Horizons flyby, I notice that the probe stops probing and points back at Earth for about an hour at a time, right around the closest approach.

This happens on one occasion 6 hours before closest approach, lasting for 30 minutes, and again just 24 minutes after closest approach, lasting for about 2.5 hours.

What is the reason for this? Does it entail reduced scientific results?

It seems impossible that additional commands could make the round trip in time. And the data returned in just a half hour can't put much of a dent in its storage.

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This last downlink transmission, ending ~24 hours before the flyby is a contingency plan. If New Horizons has a catastrophic collision as it passes through the Pluto system, we'll at least have some data. This downlink is called "Fail Safe D".

NH Closest approach: July 14, 2015 11:49:57 UTC

From Emily Lakdawalla's list, all times are Earth time (i.e. the time signals are received here, 4.5 hours after they were sent):

  • Monday, July 13 at 16:24 UT / 12:24 ET / 09:24 ET: 3.5hr downlink: Fail Safe D

  • Tuesday, July 14 at 03:15 UT / Monday, July 13 at 23:15 ET / 20:15 PT: 0.9hr downlink: E-Health 1

  • Flyby occurs at 13:49:57 UT

  • Wednesday, July 15 at 01:09 UT / Tuesday, July 14 at 21:09 ET / 18:09 PT: 0.3hr downlink: Phone home. No science data, just a brief burst of telemetry confirming that the spacecraft survived the flyby.

Your "6 hours before closest approach" would be July 14 around 10:20 which doesn't line up with any of the downlinks in this schedule, so somebody's off. I haven't found an original NASA downlink schedule yet.

Live view of events

Full list of probe activities

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    $\begingroup$ I was going by "6 hours before closest approach", that has to be Failsafe D. $\endgroup$
    – Hobbes
    Jul 14, 2015 at 12:42
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    $\begingroup$ I think those times are on Earth. You might be using a closest approach time form NH's perspective. $\endgroup$
    – Potatoswatter
    Jul 14, 2015 at 12:45
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    $\begingroup$ Mainly going by what's logical. "Fail safe" has to be before NH crosses Pluto's orbital plane (where a collision is most likely). $\endgroup$
    – Hobbes
    Jul 14, 2015 at 13:08
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    $\begingroup$ Where did you get your flyby time? Again, it sounds like you're mixing up the spacecraft and ground timelines. The flyby occurs between the ground reception of E-Health and Phone Home, but several hours after Phone Home from the spacecraft perspective — according to the simulation. $\endgroup$
    – Potatoswatter
    Jul 14, 2015 at 14:47
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    $\begingroup$ I'm too exhausted to get to the bottom of this today. IMO, NH will not communicate for 12 hours on either side of closest approach, see the last link "Full list of probe activities" $\endgroup$
    – Hobbes
    Jul 14, 2015 at 18:20
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Let's take a look the timeline when the signal was sent.

  • 15:52:57 New Horizons is taking radio uplink data with REX. New Horizons is 31.9 AU from Earth.
  • 15:53:12 New Horizons is taking radio uplink data with REX. New Horizons is 31.9 AU from Earth.
  • 16:27:00 Signal Has Left New Horizons that it crossed through the Pluto System
  • 17:02:32 New Horizons is taking 3 images of Pluto with LORRI from 456694.82 km away at est. resolution 2.3 km/pix.

I believe that during this time Pluto and Charon are essentially in complete eclipse, making for limited observation capabilities during this time. The antenna also happens to be pointed at Earth, to do the data collection with REX. Also, the team would really like to know the status of things so they can have a party, before they have to start working analyzing the data. Bottom line, it was a lucky happenstance that they are taking advantage of.

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  • $\begingroup$ July 14 16:27 EDT is 20:27 UTC, 9 hours after the simulator depicts the closest approach. That timeline agrees and mentions closest approach at 07:50 EDT. The radio event 6 hours before closest approach is explained as "New Horizons is taking radio uplink data with REX." $\endgroup$
    – Potatoswatter
    Jul 15, 2015 at 3:29
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    $\begingroup$ It looks like the question is a bit confused and ambiguous, since I mis-identified the signal. The vantage point is much worse 9 hours past Pluto on the dark side, so there's not as much of a question about why they stop the observations at that point. $\endgroup$
    – Potatoswatter
    Jul 15, 2015 at 3:34
  • $\begingroup$ I've rolled back the question to remove the speculation, and added a self answer. $\endgroup$
    – Potatoswatter
    Jul 15, 2015 at 4:23
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According to the full activity timeline, the two communication sessions before and after the closest approach are uplinks, not downlinks. Presumably these are used to provide refined targeting based on the preview observations.

There are also DSN tracking sessions after the uplinks, which don't correspond to any returned results. I guess that either

  • the REX transmitter can transmit a carrier wave during an uplink, but can't encode data on it, or
  • the DSN antennas are "illuminating" the spacecraft with sideband signals during the uplink, and then looking for a passive reflection after the round trip.

The first session:

July 13 21:00:00 EDT New Horizons soon to be communicating with Madrid 70 m (DSS-63)

July 13 23:30:00 EDT New Horizons' track ending with Madrid 70 m (DSS-63)

July 14 01:57:59 EDT New Horizons is taking radio uplink data with REX. New Horizons is 31.8 AU from Earth.

July 14 02:05:00 EDT New Horizons soon to be communicating with Goldstone 70 m (DSS-14)

July 14 06:40:00 EDT New Horizons' track ending with Goldstone 70 m (DSS-14)

The second session, overlapping the first:

July 14 03:15:00 EDT New Horizons soon to be communicating with Canberra 70 m (DSS-43)

July 14 06:40:00 EDT New Horizons' track ending with Canberra 70 m (DSS-43)

July 14 09:28:02 EDT New Horizons is taking radio uplink data with REX. New Horizons is 31.9 AU from Earth.

July 14 10:55:00 EDT New Horizons soon to be communicating with Canberra 70 m (DSS-43)

July 14 12:00:00 EDT New Horizons' track ending with Canberra 70 m (DSS-43)

I don't see any REX activity corresponding to the last Canberra tracking, so I guess that indicates they're looking either for reflection of signals from the earlier part of the preceding track, 9 hours earlier, or simply looking for it without any illumination.

Also, the simulator's depiction of a 2.5 hour radio session immediately following closest approach disagrees with that timeline. Perhaps such a session was scheduled to conservatively return results ASAP, before passing through the entire system, but then they decided to perform more observations instead and delay the First Look returns.

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