New Horizons: Encounter Planning Accelerates

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The PI's Perspective: Encounter Planning Accelerates
Dr. Alan Stern
May 16, 2013

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 our spacecraft at closest approach and devise
mitigation strategies to make sure the encounter with Pluto would be successful.

We've now largely completed that work and presented the results to both
an independent, NASA-appointed technical review team, led by the Jet Propulsion
Laboratory's Keyur Patel, and then to senior executives at NASA Headquarters.
Both groups have concurred with our findings, which I can summarize as
follows:

o 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.

o 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 clear debris. Another important factor is that when we tested
spacecraft components against high-velocity impacts using gun ranges in
New Mexico and Ohio, we found the spacecraft shielding is considerably
"harder" - that is, more resistant to impacts - than preflight estimates
indicated.

o The baselined 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 we're 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.

Thanks to these three findings, we're much more confident that unless
something new and unexpected comes up between now and encounter, our current
plan, which has been optimized to maximize the encounter science return,
is to the best level of current knowledge, safe.

But to be still more prudent, we're also implementing plans during the
final weeks of approach in summer 2015 for New Horizons itself to search
for hazards that we can't see from Hubble or Earth-based telescopes. And
we've 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 our current estimates are wrong
and we do lose New Horizons 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 - we are also prudently planning two alternate encounter
sequences that we can upload to New Horizons as late as 10 days before
closest approach, in the unlikely event that our 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.

Our 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, we repoint the spacecraft so its dish antenna can shield it
from impacts. This pointing attitude, called Antenna to Ram (or ATR),
would cost us some science because we 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.

Our 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
we go close enough, we 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 that it's both the best choice if late-breaking news tells
us the nominal trajectory is unexpectedly riskier than we're comfortable
with, and losing some science to execute Deep Inner SHBOT is far better
than losing the mission to a lethal impact.

What I've briefly summarized here resulted from thousands of hours of
work by many people on New Horizons and in the scientific community. They've
been carefully scrutinized by the independent technical panel NASA set
up at our request to peer-review the hazards analysis and mitigation plans
we've worked on since late 2011, and approved by NASA Headquarters.

We're excited to have these plans on the books and to begin testing both
SHBOTs before the encounter begins so that they are ready to use if needed.
When we launched we never imagined w'�d be planning three separate encounters
with Pluto, but that's what we're doing!

Waking Up New Horizons for Summer 2013!

Before I close this column out, I also want to update you on mission status
and other project news.

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 we've been traveling since launch!

New Horizons is healthy and on course, with all systems and science sensors
working. On May 21, we'll 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 our fault protection software with the next-to-last planned
set of enhancements before we start the Pluto encounter in January 2015
- just over 19 months from now.

This summer's wakeup will also include our 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. This rehearsal follows up on our successful encounter-day practice
from last summer.

After the nine-day rehearsal, we'll 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
4 1/2 months, while the team works on SHBOT encounter sequencing.

Also this summer, we'll be close enough to resolve Pluto from its large
moon Charon using our 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 - we'll be taking our first images of Charon
at the same time of year that the moon was discovered, back in 1978!

One thing we won't be doing this summer is a course correction. As in
2011 and 2012, our spacecraft navigation team has determined from tracking
data that we're on course and that there is no need to spend any fuel
this way - a good thing!

Our mission team can feel the increased pace of activity as we draw closer
and closer to 2015, and many of us are working much longer hours on this
project than we did in early- and mid-cruise. To prepare for encounter
operations to start in January 2015, we'll add new staff to our science
and operations teams. In fact, we've already made one very important addition
by bringing in a deputy project manager, Peter Bedini, of the Johns Hopkins
Applied Physics Laboratory. Peter was most recently the project manager
for APL's MESSENGER Mercury orbiter. New Horizons is privileged to have
someone as talented and experienced as Peter on our team. So, welcome
Peter to a spacecraft and encounter target that are a lot cooler than
Mercury - in fact, about 600 degrees Celsius cooler.

As I close this update, I'll just say thanks to all of you for following
our journey across the deep ocean of space, to a new planet, and a truly
new frontier. I'll plan another update as we complete this summer's intensive
mission operations. In the meantime, be on the lookout for a news notes
on our website related to this summer's activities.

Until I write again, I hope you'll keep on exploring - just as we do!