Why doesn't the New Horizons probe fly any nearer than 10,000 km from Pluto?

Question

New Horizons is intended to pass within 10,000 km of Pluto on its flyby on July 14. This will give a maximum resolution on the surface of 50m per pixel. This seems to me a bit of a waste, when it could have passed much nearer to get a higher resolution of some areas of Pluto.

Answer 1

The long distance to the Sun mandates long exposure times. The New Horizons spacecraft needs to be relatively stable and its pointing accurate throughout these long exposure times.

New Horizons does not have a scan platform. The cameras and other science instruments are fixed with respect to the vehicle. The satellite has to turn as a whole to keep the its scientific instruments pointed at Pluto. New Horizons also doesn't have control moment gyros or reaction wheels. All attitude control is via attitude thrusters.

The vehicle has to rotate by 180° from well before closest approach to well after closest approach. With a somewhat remote flyby, this 180° turn is spread out a bit. With a close-in flyby, this 180° turn has to happen rather quickly, right at flyby. The constant on/off thrusting that would be needed for a very close approach would do significant damage to the quality of the close-in imagery. There's little value to a close-in flyby if all that one sees is fuzz. A close-in flyby would also require considerably more fuel than a more remote flyby.

Answer 2

The Pluto flyby is tricky to plan: after the launch of New Horizons, astronomers discovered the four smaller moons of Pluto, and mission planners began to worry that the system could be filled with even smaller moons and dusty particles. New Horizons will fly through the system at 14 km/s, so a particle the size of a grain of sand could destroy the spacecraft. See http://guinan.space.swri.edu/nhepo/archive/2015/05/15 (thanks @TildalWave).

From what I understand, the trajectory is planned to pass through space where Charon was a few days before, so that hopefully it has cleared out the dust in that region. From the slide it looks like they are also planning a trajectory where the spacecraft passes behind both Pluto and then Charon as seen from Earth (a set of 'occulations'), probably for radio measurements of the two bodies.

Source: I saw a talk by James Green, director of NASA's Planetary Science division a few weeks ago http://www.pppl.gov/events/colloquium-worlds-seen-first-time-ceres-and-pluto -- a really good presentation!

Slides from a similar talk (showing a possible trajectory on page 29) are here: http://astronautical.org/sites/default/files/goddard/2015/goddard_2015-03-11-1715_green.pdf

Answer 3

In 2008, the team made a list of observations they wanted to make of Pluto and its 3 moons. They tried to find a date for the flyby that have the best possible combination of observations (which region of Pluto to fly over, what the distances to the moons would be, etc). This led to an encounter date of July 14.

Then the team looked for an optimal flyby distance, in a range of 3000-20,000 km. Each of the 4 science teams indicated what the optimal distance would be for their observations.

• the plasma science team favored a really close approach
• for the cameras, a larger distance would be better, to reduce the chance of smearing.
• they were planning a radio occultation experiment which needed NH to be visible from 2 DSN sites

They settled on an approach distance of 12,500 km.

(from Chasing New Horizons, by Alan Stern and David Grinspoon, page 186)

Then in 2011, a new moon was discovered, followed by another one in 2012. Small moons can generate rings: due to their low gravity, a meteoroid impact can blast debris into Pluto's orbit. (page 200) This whas when the alternate trajctories were devised (below).

The nominal flyby distance is in a region that simulations showed would be kept clear by Charon.

Two contingency plans (SHBOT) were made.

1. a flyby using the original trajectory, but keeping the high-gain antenna pointed in the direction of flight (ram direction). This increases survivability (any debris will hit the antenna first, which can take some damage without compromising the mission). This meant a small loss in observations because the spacecraft would be less free to maneuver.

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.

2. a flyby at a much smaller distance to Pluto, skimming the upper atmosphere (this region would be kept clear of debris by the atmosphere). This trajectory would mean a larger loss of observations compared to the baseline.

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.

Ultima Thule is much smaller than Pluto, so it's less likely to have collected a large number of moons, so a smaller chance of debris.

Alan Stern said the Ultima Thule flyby distance is close to the minimum distance:

He indicated that {a closer flyby - Hobbes} was not possible due to New Horizon's inability to tracked with it's camera systems any closer then the current 3500 km stand off distance.