Astronautics: New Horizons

Understanding Astronautics: New Horizons

A mission to the far outer Solar System must solve a difficult energy problem. A rocket provides a powerful initial push, but the Sun keeps pulling the spacecraft back as it travels outward. Engineers therefore choose a path that gives the spacecraft enough solar escape speed.

Passing Jupiter changed the probe's motion without using much of its own fuel. Jupiter moved around the Sun, so the spacecraft could take a tiny amount of the planet's orbital energy and leave on a faster path.

The planet's own change was far too small to measure. This technique is called a gravity assist, and it depends on exact timing and a carefully chosen approach direction.

A Pluto encounter could not be treated like a slow visit. The spacecraft crossed the Pluto system at very high speed, so the most important observations had to be scheduled long before arrival. Cameras, spectrometers, particle detectors, and radio equipment each had short viewing opportunities.

The spacecraft had to point its instruments at Pluto, its moons, or the space around them at the right moments. Pointing away from Earth meant it could not send data home during some of the best observations.

Its computer followed stored commands because signals took many hours to travel across space. A small navigation error could have moved the best images far from their intended target.

The instruments measured more than surface pictures. Different wavelengths of light revealed materials such as nitrogen ice, methane ice, water ice, and complex dark compounds. Changes in brightness and color helped scientists map plains, mountains, craters, and possible glaciers.

Radio measurements showed how the spacecraft signal changed as it passed behind Pluto. That change gave evidence about Pluto's thin atmosphere. Images of the moons helped researchers compare their surfaces and histories.

A flyby mission gathers a narrow but detailed slice of information, unlike an orbiter that can return repeatedly. This makes every observation plan valuable.

Sending results back to Earth was another engineering challenge. The spacecraft used a limited electrical power supply from radioactive decay, since sunlight is very weak so far from the Sun. Its transmitter sent a faint radio signal received by large ground antennas.

Data arrived slowly, and the complete set of encounter observations took many months to return. Engineers had to decide which files to send first, then use compression and error checking so valuable information was not lost. Students can connect this mission to several physics ideas.

Speed equals distance divided by time helps describe long journeys. Kinetic energy equals one half of mass times speed squared explains why high speed greatly raises the difficulty of changing course. Momentum, gravity, energy transfer, light, and radio waves all work together in a real space mission.