Mission Elapsed Time:
Beginning 1/19/06 19:00:00 UTC
2677 Days (7.33 yrs.) 07 Hours 44 Minutes
Pluto Closest Encounter Operations Begin:
4/12/15 00:00:00 UTC
628 Days (1.72 Yrs.) 21 Hours 27 Minutes
Pluto Closest Approach:
7/14/15 11:49:59 UTC
785 Days (2.15 yrs.) 09 Hours 17 Minutes
Waking Up New Horizons for Summer 2013:
Currently, New Horizons itself is about 2.6 billion miles from the Sun, and only about 600 million miles from Pluto. Arrival at Pluto is just under 700 days away – still a long time, but much less than the nearly 2,700 days New Horizons has been traveling since launch. New Horizons is healthy and on course, with all systems and science sensors working. On May 21, the team wake the spacecraft from its most recent, 100-plus day hibernation to begin a busy annual checkout, which will include thorough checks of all backup systems, instrument payload calibrations, and an update of the fault protection software with the next-to-last planned set of enhancements before New Horizons starts the Pluto encounter in January 2015 – just over 19 months from now.
New Horizons Present Position in Three Dimensions:
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This summer’s wakeup will also include the most comprehensive on-the-spacecraft close-encounter rehearsal. For nine days, beginning July 5, New Horizons will execute all the activities of its final week on approach to Pluto, closest approach day, and then some of the post-encounter timeline as well. After the nine-day rehearsal, New Horizons will be ordered to downlink a large amount of data through NASA’s Deep Space Network to evaluate how the rehearsal went, collect more cruise science data, conduct more spacecraft navigation tracking, and then put New Horizons back into hibernation on August 21 for another 4½ months, Also this summer, New Horizons will be close enough to resolve Pluto from its large moon Charon using the long-focal length telescopic imager called LORRI. The first week of July is also the 35th anniversary of Charon’s discovery and – entirely by coincidence – the spacecraft will be taking our first images of Charon at the same time of year that the moon was discovered, back in 1978. There won't be a course correction this summer as in 2011 and 2012. The spacecraft navigation team has determined from tracking data that New Horizons is on course and that there is no need to expend any fuel over this issue. The mission team will feel the increased pace of activity as the spacecraft draws closer and closer to 2015, and many members of the team are working much longer hours on this project than they did in early- and mid-cruise phases of the mission. To prepare for encounter operations to start in January 2015, new staff will be added to the science and operations teams. In fact, one very important addition was added by bringing in a deputy project manager, Peter Bedini, of the Johns Hopkins Applied Physics Laboratory.
The New Horizons team studied numerous alternate flybys, called SHBOTs, before recommending to NASA a pair of backups to protect New Horizons from possible impact hazards in the Pluto system.
All exploration comes with both rewards and risks:
Back in 2005 and 2006, when Pluto’s second and third moons (Nix and Hydra) were discovered, searches by astronomers for still more moons didn’t reveal any. So the accidental discovery of Pluto’s fourth moon by the Hubble Space Telescope in mid-2011 (during a search for Plutonian rings) raised the possibility that the hazards in the Pluto system might be greater than previously anticipated. Those concerns were amplified when Hubble discovered a fifth moon in 2012. As a result of those discoveries, the New Horizons science and operations teams began to more carefully scrutinize the true level of hazards to the spacecraft at closest approach and devise mitigation strategies to make sure the encounter with Pluto would be successful.
A survey was completed by NASA and an independent group-a NASA approved technical review team, led by the Jet Propulsion Laboratory’s Keyur Patel, and then by senior executives at NASA Headquarters. Both groups have concurred with the team's findings, which can be surmised as follows:
1. New Horizons benefits from its approach trajectory because that trajectory is steeply inclined to Pluto’s satellite plane and associated debris hazards that models show should lie close to the satellite plane. As a result, most of the risk New Horizons faces occurs only at closest approach, when the spacecraft is very near the satellite plane.
The New Horizons trajectory (red line) is steeply inclined to Pluto’s satellite plane, thereby restricting satellite debris hazards – which lie near the satellite plane – to the short time near closest approach.
2. The Pluto system appears to be far safer than early fears and initial calculations indicated when the new moons began popping up. In fact, the best current models predict a 0.3% (1-in-300) chance of a mission-ending impact near closest approach on the nominal trajectory. Much of the reason for this lowered risk assessment is that more sophisticated dust-impact models revealed a decrease (by about a factor of 100) in lethal impact probability for trajectories that fly into the region where New Horizons is aimed now – a region where the gravitational effects of Pluto’s largest moon Charon clears debris. Another important factor is that when the team tested spacecraft components against high-velocity impacts using gun ranges in New Mexico and Ohio, it was discovered that the spacecraft shielding is considerably “harder”– that is, more resistant to impacts – than preflight estimates indicated.
3. The base lined New Horizons closest-approach aim point is one of the safest possible aim points – if not the safest aim point – in the Pluto system. This is because New Horizons is headed to a closest approach in the region that Pluto’s Texas-sized moon Charon efficiently clears of debris. In fact, Charon offers such a good hazard-removal service that even if a recent impact onto a small moon created debris near Charon’s orbit just months before encounter, Charon would clear almost all of it by the time the spacecraft arrives.
But to be still more prudent, the team is also implementing plans during the final weeks of approach in summer 2015 for New Horizons itself to search for hazards that can’t be seen from Hubble or Earth-based telescopes. Then the team also added “fail safe” data downlinks just two days and one day before the encounter to send home the best images and spectra stored on the spacecraft’s recorders, just in case the current estimates are wrong and New Horizons is lost at closest approach. It’s always better to plan this way, just as the Apollo astronauts collected contingency samples right after stepping onto the moon in case they had to make a hurried getaway before their moonwalks could be completed. And – just as every space shuttle mission included (but never used) plans to land after just one orbit of Earth if the spacecraft wasn’t healthy enough to continue – there are two alternate encounter sequences that can be uploaded to New Horizons as late as 10 days before the closest approach, in the unlikely event that hazard observations on final approach raise new cautions.
Safe Havens:
These backup encounter sequence plans are called SHBOTs, an acronym for Safe Haven By Other Trajectory. The first SHBOT is called GIS, for Generic Inner SHBOT. It continues on the nominal trajectory and aim point, but for three hours near closest approach, then the spacecraft is repointed so its dish antenna can shield it from impacts. This pointing attitude, called Antenna to Ram (or ATR), would cost some science because the spacecraft won’t be as free to point the science instruments toward Pluto system targets during those three hours. But tests and modeling show this provides a factor of three to four times increase in success probability, and reduces the estimated loss of mission probability to about 1 in 1,000.
If necessary, the high-gain (“dish”) antenna on New Horizons can be used to shield most of the spacecraft from dust particle impacts during the Pluto encounter.
The second SHBOT is called DIS, for Deep Inner SHBOT. DIS also uses the ATR attitude. It also directs the spacecraft toward a much closer encounter with Pluto – just inside 3,000 kilometers from Pluto’s surface, compared to the nominal encounter close approach of about 12,500 kilometers from the surface. Why go closer, not farther, to avoid hazards? Because if the spacecraft goes close enough, it can benefit from the fortuitous “drag clearing” of debris particles from Pluto’s extended upper atmosphere. DIS has more severe science impacts than does GIS, but there is a strong consensus among the team members that it’s both the best choice if late-breaking news tells them the nominal trajectory is unexpectedly riskier than anticipated, and losing some science to execute Deep Inner SHBOT is far better than losing the mission to a lethal impact. When launched it was never imagined that the team would be planning three separate encounters with Pluto, but that is what has happened.
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