Slingshot Science: How Gravity Assists Propel Space Probes to Interstellar Space

StarTalk Explainers explains that most space probes do not rely solely on the rocket that launched them; instead, they ride gravitational slingshots around planets to gain energy. Using Pioneer 10/11, Voyager 1/2, and New Horizons as examples, the hosts show how gravity assists allow probes to escape the solar system by stealing orbital energy from planets rather than requiring free fuel. The episode also discusses the Pioneer plaque and its 21-centimeter hydrogen signature as a yardstick for distance, along with the idea that these missions carry a cosmic return address for potential extraterrestrial observers. In short, gravity assists are key to reaching interstellar space.

Overview

In this StarTalk Explainers episode, Neil deGrasse Tyson and Chuck discuss orbital mechanics and why rockets alone do not determine a space mission's fate. They introduce the gravity assist, or slingshot maneuver, and illustrate how a spacecraft can gain energy by passing behind a planet.

How Gravity Assists Work

The hosts describe launching a craft so that it falls toward a planet, gaining energy from the planet's gravity and then leaving on a faster trajectory. This process effectively transfers orbital energy from the planet to the spacecraft, enabling a boost without expending additional fuel. The conversation emphasizes that while it seems like a free lunch, the energy comes at the cost of the planet's orbital energy in a very tiny amount, preserving the conservation of energy in the system.

First Probes to Escape the Solar System

The narrative traces the early milestones: Pioneer 10 and Pioneer 11, followed by Voyager 1 and Voyager 2. Each probe carried a version of the iconic plaque with a pulsar map and a schematic of human figures, intended as a message to potential extraterrestrials. Gravity assists, notably with Jupiter, enabled these spacecraft to reach escape trajectories, marking the first successful forays into interstellar space for human-made objects.

Pioneer Plaques and Contact with Aliens

The plaque designs include the hydrogen spin flip as a 21-centimeter yardstick for distance and size, and the relative scale of humans to the spacecraft. The spider diagram pointing to pulsars serves as a galactic map for triangulation. Tyson and his co-host discuss whether publishing such instructions was wise, and reflect on the idea of aliens identifying our planet by our solar system’s configuration.

Pluto, Pluto Demotion, and New Horizons

The discussion moves to Pluto’s status and the New Horizons mission. Launched to study Pluto with rapid transit aided by gravity assists, New Horizons became the fastest spacecraft to reach Pluto, completing its primary objective quickly due to efficient mission design and the use of gravity assists. Alan Stern is highlighted as a central figure in Pluto’s science narrative. The episode connects these ideas to broader questions about how scientists design experiments to conclude within their lifetimes.

Corollaries and the Next Explainer

The conversation touches on the Coriolis effect as a separate orbital mechanic, and tees up a future StarTalk Explainers episode on sideways motion. The hosts reiterate that gravity assists are not free lunches but energy exchanges that enable ambitious journeys across the solar system and beyond.

Bottom Line

Gravity assists and planetary flybys are essential tools in space exploration, enabling missions to reach outer planets and interstellar space. The discussion showcases the clever interplay of orbital physics, spacecraft engineering, and the human drive to explore.