July 6, 2015 – These are the most recent high-resolution views of Pluto sent by NASA’s New Horizons spacecraft, including one showing the four mysterious dark spots on Pluto that have captured the imagination of the world. The Long Range Reconnaissance Imager (LORRI) obtained these three images between July 1 and 3 of 2015, prior to the July 4 anomaly that sent New Horizons into safe mode.
The left image shows, on the right side of the disk, a large bright area on the hemisphere of Pluto that will be seen in close-up by New Horizons on July 14. The three images together show the full extent of a continuous swath of dark terrain that wraps around much of Pluto’s equatorial region. The western end of the swath (right image) breaks up into a series of striking dark regularly-spaced spots, each hundreds of miles in size, which were first detected in New Horizons images taken in late June.
Intriguing details are beginning to emerge in the bright material north of the dark region, in particular a series of bright and dark patches that are conspicuous just below the center of the disk in the right image. In all three black-and-white views, the apparent jagged bottom edge of Pluto is the result of image processing. The inset shows Pluto’s orientation, illustrating its north pole, equator, and central meridian running from pole to pole.
The color version of the July 3 LORRI image was created by adding color data from the Ball Aerospace-built Ralph instrument gathered earlier in the mission.
Today, NASA held a media teleconference to discuss the New Horizons spacecraft returning to normal science operations after the July 4 anomaly. Participants in the teleconference were: Jim Green, director of Planetary Science, NASA Headquarters, Washington; Alan Stern, New Horizons principal investigator, Southwest Research Institute, Boulder, Colorado; and Glen Fountain, New Horizons project manager, John Hopkins University Applied Physics Laboratory, Laurel, Maryland.
The New Horizons mission remains on track to conduct the entire close flyby sequence as planned, including the July 14 flyby of Pluto.
“We did hit a speed bump. The spacecraft had an off-nominal event over the weekend. The team was fantastic and have been working around the clock,” said Stern. “The spacecraft is ready now to execute the nine days of flyby operations.”
Fountain outlined the weekend’s events. “The 3rd of July, we really were preparing for the main event – this load that takes us through the encounter, which actually starts on the 7th. A number of things have to be done for preparation of that.”
Preparation included uploading the command sequence to both the backup and primary computers and at the same time, downloading data and compressing some of the remaining data to free up as much space as possible for the flyby. In essence,the team was doing multiple things on the spacecraft at the same time, and although they had rehearsed the sequence on the ground, the data on the spacecraft was much more complex than what had been used on the ground. Eventually, the processor said it was overloaded and the spacecraft’s autonomous system went to the backup computer and then put itself in safe mode, waiting for further instruction from the ground.
“We have a call chain on New Horizons,” said Stern. “Glen gets the call and then he calls me and I call Jim Green. All that happened very rapidly. New Horizons has been in safe mode numerous times over the nine years.”
Green added, “When these anomalies occur, we just kick on our processes. We know what to do next. The team has been methodical.”
The problem was isolated in under an hour, but because New Horizons is so far from Earth, it takes 4.5 hours to send commands and another 4.5 hours back to confirm that the command has been executed.
At that point, the New Horizons team decided to suspend science operations and focus on getting ready for the flyby, which is the primary objective. That meant that some science was lost on Saturday and all of Sunday and Monday’s science was lost as well.
“We made a decision to sacrifice observations that weren’t as useful, to let the team concentrate on tasks leading up to encounter,” said Stern.
This is the science that was lost:
16 LORRI imager observations, 3 of which were for navigation, as opposed to science
4 Ralph instrument color observations
1 Ralph composition spectroscopy
4 Alice atmospheric observations, but the Alice team does not expect to detect on the Pluto system until about July 12. These observations were being made as due diligence, in case some of the signals from the Pluto system are much brighter than expected.
Plasma Roll Observation – in which 3 instruments (SWAP, PEPPSI and REX) collaborate together to measure the interplanetary environment.
3 Days of Background Observation – SWAP, PEPPSI, and SDC lost 3 days of background monitoring on that interplanetary environment.
“We lost about 30 observations all together – that’s out of a total of about 496 observations between that date – July 4 – and the end of the close approach observations two days after flyby – about 6% of the observations by count,” said Stern. “But scientifically, we weighed the observations by how close they are to the planet, and these observations from very far away are not nearly as important as those in the Pluto system, where we will be about 100X closer than we were back this weekend, and so our assessment is that the weighted loss is far less than 1% to highest priority science.”
Since this anomaly could only happen when all of the operations were being done in tandem, the New Horizons team does not expect any problems during the Pluto encounter. Over the weekend, they focused on compressing data, uploading the command sequence, and preparing for the encounter.
“We haven’t changed anything about the encounter, and don’t need to. There is no parallel to this event,” said Stern.
Tomorrow the spacecraft will begin operating in encounter mode. That means that it will operate autonomously based on a sequence of commands that it has already been given. If there is a problem, the computer will simply re-boot itself and then return to the last point in its sequence.
“We have designed the core load so that every observation that is an important observation is redundant, so there are at least two observations of that data,” Fountain said.
Stern added, “You will be seeing daily images at closer and closer range. Satellites are small, so a few pixels when we’re very close – Pluto and Charon are larger though and we’ll be seeing different terrain each day.”
Up to five instruments will be operating at any one time and all of them will be saving the data to data storage. 99% of the data will be stored on the spacecraft during the encounter and only 1% returned to the ground. On flyby day, only engineering data will return to the ground.
“When we architected this mission, we knew it would be a long journey for a very intensive period of collection – the encounter period is about taking data,” Stern said. “After the flyby, the data coming to the ground will jump beginning on the 15th and then continuing after that.”
The team will be releasing new data sets for 16 months after the encounter. As Stern says, anyone working on a Pluto mission learns to accept delayed gratification.