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Emily LakdawallaJune 24, 2015

What to expect when you're expecting a flyby: Planning your July around New Horizons' Pluto Pictures (version 2)

New Horizons is getting close to Pluto. Pluto and Charon have enlarged from featureless dots into worlds. Pluto's surface clearly bears streaks and splotches, while Charon is beginning to show the first hints of discernible features. Excitement is building for flyby day, July 14!

Key places to watch for New Horizons information:

Three months ago, I posted an article explaining what to expect during the flyby. The following is a revised version of the same post, with some errors corrected, the expected sizes of Nix and Hydra updated, and times of press briefings added.

UPDATE July 8: Added newly scheduled July 15 press briefing.
UPDATE July 12: Changed schedule of July 13 press briefing.
UPDATE July 14: Changed schedule of July 14 evening press briefings.


As New Horizons approaches, every image of Pluto and Charon that each instrument returns will be the best it has ever taken. All of these images will be thrilling to see. But until mid-July, all the images will still be pretty small. When will we get pictures that really answer questions about Pluto? In this blog entry I'll try to explain when we'll get the images that you're hoping for. First, the executive summary:

It's hard to get data from Pluto

Data will arrive on Earth in a series of downlinks. Downlink sessions can last as long as about 8 hours, but are usually somewhat shorter. Whenever New Horizons is downlinking data, it can't take new photos, so the downlinks get shorter and less frequent as the spacecraft gets close to the time of the flyby, when it concentrates on collecting as much data as possible. Because data downlinks are slow, there will be much less data downlinked than New Horizons has stored on board. After data is downlinked, it must be processed before posting online. How long that will take is not yet known.

On Sunday, July 12, New Horizons will transmit the last of its optical navigation data. Then, on Sunday and Monday, July 12 and 13, there will be a series of four "Fail Safe" downlinks. These are designed to return a minimum set of data from all instruments, just in case New Horizons does not survive the flyby. A last downlink ending overnight Monday July 13, called "E-Health 1," will include one last pre-closest approach photo of Pluto.

Then there is a nail-biting 24-hour period of waiting while New Horizons concentrates on flyby science and does not communicate with Earth, followed by the much-anticipated beep of the "Phone Home" downlink on Tuesday night, July 14. Following closest approach, on Wednesday and Thursday, July 15 and 16, there will be a series of "First Look" downlinks containing a sampling of key science data. Another batch of data will arrive in the "Early High Priority" downlinks over the subsequent weekend, July 17-20. Then there will be a hiatus of 8 weeks before New Horizons turns to systematically downlinking all its data. Almost all image data returned during the week around closest approach will be lossily compressed -- they will show JPEG compression artifacts. Only the optical navigation images are losslessly compressed.

The transmission of the High Priority data set will be complete on July 20, and then image transmission will pause. For nearly two months, until September 14, New Horizons will switch to near-real-time downlinking of data from other, so-called "low-speed" instruments while it transmits just housekeeping information for all of the rest of the data. No new images will arrive on the ground during this time. I asked Kim Ennico for a little more explanation of the "low-speed" distinction, and she said:

SWAP, PEPSSI and SDC are "low-speed" compared to LORRI, Ralph, Alice and REX who are called "high-speed." They are separated in type by the spacecraft bus to which they write their data, the "low speed bus" or the "high speed bus." As an added complication, there is a science mode in Alice which writes to the "low speed bus." We've learned that when we compress and packetize the data for downlink, we have to treat the "high-speed" differently than the "low-speed." We've found you can be most efficient at downloading the "low-speed" data if you do it as a series of playbacks of data from specified mission elapsed time ranges with playback pauses. It made sense to dedicate a period of time to return all the "low-speed data." Plus during the same Juy 20 to September 14 period we can get all the spacecraft data, so if we need to reconstruct the actual pointing of the handful of near-encounter images that came down, we can do so.

On September 14, New Horizons will begin downlinking a "browse" version of the entire Pluto data set, in which all images will be lossily compressed. It will take about 10 weeks to get that data set to the ground. There will be compression artifacts, but we'll see the entire data set. Then, around November 16, New Horizons will begin to downlink the entire science data set losslessly compressed. It will take a year to complete that process.

What to expect, when

Here is a graphical summary of all of the LORRI data that New Horizons is expected to downlink in the two weeks surrounding closest approach. I used Voyager images of Jupiter and Saturn moons to stand in for Pluto, Charon, Nix, and Hydra: Ganymede for Pluto, Tethys and Rhea for Charon, and Janus and Hyperion for Nix and Hydra. Don't take the comparisons between the moons and Kuiper belt objects too literally -- this comparison is just meant to give you a sense of the scope of the near-encounter data set at a glance. Since the first time I posted this article, I've had to reduce the apparent sizes of Nix and Hydra by about half to match the new Hubble estimates of their diameters.

Simulation of the New Horizons Pluto flyby LORRI data set

Voyager images: NASA/JPL. Chart by Emily Lakdawalla.

Simulation of the New Horizons Pluto flyby LORRI data set
In the two weeks surrounding New Horizons' flyby of Pluto, only 1% of the science data that it acquires will be downlinked to Earth. This chart uses Voyager images of Jupiter's and Saturn's moons to stand in for the images that New Horizons' highest-resolution camera, LORRI, is expected to downlink in the summer of 2015. Visit planetary.org/plutodata for futher explanation.

Following is a complete list of all the planned downlinks of image data during the highest-intensity period around closest approach. The times given are planned downlink end times. Downlinks containing LORRI images, which will be released on the New Horizons raw image website automatically, are in bold. Automatic release is promised within 48 hours of receipt; lately, the images have been released faster than that. The times and ranges of the images may change by as many as 7 minutes and 6000 kilometers as navigators update their knowledge of Pluto's position; I will update this post as necessary. Therefore, I do not recommend the copying and reposting of this text, because it will become out of date!

Sunday, July 12 19:39 UT / 15:39 ET / 12:39 PT: 7.5hr downlink: Final optical navigation images

Monday, July 13 02:23 UT / Sunday, July 12 22:23 ET / 19:23 PT: 4.2hr downlink: Fail Safe A

Monday, July 13 06:14 UT / 02:14 ET / Sunday, July 12 23:14 PT: 2.5hr downlink: Fail Safe B

Monday, July 13 at 10:39 UT / 06:39 ET / 03:39 PT: 1.4hr downlink: Fail Safe C

Monday, July 13 14:30 UT / 10:30 ET / 07:30 PT: NASA TV briefing: Mission status and what to expect

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

Tuesday, July 14 11:30 UT / 07:30 ET / 04:30 PT: NASA TV briefing: Arrival at Pluto, Inside the Pluto System and New Horizons’ Perilous Path

Wednesday, July 15 00:30 UT / Tuesday, July 14 20:30 ET / 17:30 PT: LIVE NASA TV Coverage of “Phone-home signal”

Wednesday, July 15 at 01:09 UT / Tuesday, July 14 at 21:09 ET / 18:09 PT: 0.3hr downlink: Phone home

Wednesday, July 15 01:30 UT / Tuesday, July 14 21:30 ET / 18:30 PT: LIVE TV Press Briefing: Spacecraft Status Report

Wednesday, July 15 01:15 UT / Tuesday, July 14 21:15 ET / 18:15 PT: NASA TV briefing: New Horizons health and mission status

Wednesday, July 15 at 10:59 UT / 06:59 ET / 03:59 PT: 1.5hr downlink: First Look A

Wednesday, July 15 19:00 UT / 15:00 ET / 12:00 PT: NASA TV briefing: release of close-up Pluto images

Wednesday, July 15 at 19:25 UT / 15:25 ET / 12:25 PT: 6.9hr downlink: First Look B

Thursday, July 16 at 04:23 UT / 00:23 ET / Wednesday, July 15 at 21:23 PT: 1.9hr downlink: First Look C

Thursday, July 16 at 07:23 UT / 03:23 ET / 00:23 PT: 1.9hr  downlink: First Look D

Thursday, July 16 at 13:22 UT / 09:22 ET / 06:22 PT: 4.3hr downlink: First Look E

Friday, July 17 at 16:32 UT / 12:32 ET / 09:32 UT: 3.3hr downlink: High Priority A

Saturday, July 18 at 10:29 UT / 06:29 ET / 03:29 PT: 4.6hr downlink: High Priority B

For the rest of Saturday and Sunday, downlinks include REX and LEISA data, with no LORRI or MVIC data.

Monday, July 20 at 16:20 UT / 12:20 ET / 09:20 PT: 3.3hr downlink: High Priority G

 Following that, no images will be returned until September 14, at which point we will gradually get the entire image data set.

Read more: New Horizons, Pluto, Charon, mission status, Pluto's small moons

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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