Why did Voyager 2 receive a gravitational slowdown (as opposed to a slingshot) at Neptune?

Question

I was actually answering this question today regarding velocities of spacecraft, and it occurred to me I've never really looked at this graph in much detail before:

Heliocentric velocity of Voyager 2 plotted against distance from the sun

As you can see, after leaving Earth, the craft rapidly slowed down (an example of the inverse square law), and it remained below escape velocity until its encounter with Jupiter raised its velocity by over 10 km/s. It received another ~8-9 km/s slingshot from Saturn and another 1 km/s assist from Uranus.

However, the interesting event occured at Neptune: It appears to have had a gravitational slowdown of approximately ~2 km/s, as opposed to an assist. Why was this?

This indicates to me that it took the opposite trajectory of this graph:

Instead of approaching the planet from behind its velocity vector, it flew in from the front and out the back.

I am aware that gravitational slowdowns can have many interesting uses, ESA's mission proposal for a Pluto orbiter require a gravitational slowdown at Jupiter to reduce its velocity for Pluto orbit insertion, so I suspect this was either a result of a science or engineering constraint.

Is there any explanation for why this was done?

Answer 1

They wanted a close flyby of Triton. Triton's orbit is at a large angle to the ecliptic plane, and Triton was below Neptune at the time of Voyager's flyby, so they needed a course change that pointed "down" from Neptune.

From the Voyager Neptune travel guide (large PDF), page 118-119 of the PDF (page number 107-108 indicated on the page):

However, Voyager 1 passed (slightly) the leading hemisphere of Saturn, and Voyager 2 will pass (slightly) the leading hemisphere of Neptune. In these two cases, the spacecraft slowed down and the planets speeded up.

Neptune is Voyager 2's last planet. There being no next planet to seek (Pluto is not reachable; refer to Figure 6-2), Voyager 2 is not limited to passing Neptune through any particular gravity-assist corridor, and can instead concentrate on Neptune's large moon, Triton. Triton is as interesting to many planetary scientists as Neptune is. Triton is large enough to have an atmosphere. Its surface temperature and pressure are close to the triple point of nitrogen, raising the possibility of nitrogen clouds, frozen nitrogen pools, and snow/ice on the surface. In 1980, Andrey Sergeyevsky discovered that there was indeed a way to pass closely by both Neptune and Triton, thereby maximizing the scientific return from each. The means was a final application of the gravity-deflection concept. The spacecraft would pass very close to Neptune (within 4850 kilometers of the cloud tops) in order to bend its path by about 45 degrees to pass close by Triton 5.2 hours later (see Figure 6-1.) The close passage of Neptune occurs near its North Pole, and is just barely on the leading hemisphere. Voyager 2 will slow down slightly (and Neptune will speed up even more slightly) as a result of this final gravity assist.

Image from Voyager mission status report 91, via Wikipedia

Answer 2

The mission was to fly by the outer planets. Once it got to Neptune that mission was complete. From Wikipedia:

Because this was the last planet of the Solar System that Voyager 2 could visit, the Chief Project Scientist, his staff members, and the flight controllers decided to also perform a close fly-by of Triton, the larger of Neptune's two originally known moons, so as to gather as much information on Neptune and Triton as possible, regardless of Voyager 2's departure angle from the planet.

The probe was going into interstellar space no matter what by the time it got to Neptune (barring a collision with Neptune or a moon) and speed of getting there wasn't a concern, so they set up the pass to gain as much knowledge as they could.

Answer 3

I posted this image in the question where are the solid motors of Pioneer and voyager spacecraft? Jupiter gravity assit It helps explain when a spacecraft speeds up for a gravity assist and when it does not.Use this image along with Neptune/Triton flyby posted above.Objects that flyby in front of a planet orbital motion or polar regions slow down. The other option for Voyager 1 or 2 was to use a gravity assist to visit Pluto.Voyager 1 visited Titan instead of a closer Saturn flyby for a Pluto missionVoyager 1 Saturn trajectory Another citation of the Voyager astrodynamics at Saturn to set up the Uranus-Neptune tour Voyagers at Saturn an astrodynamics study