Answer · confidence: partial

Do probes get free speed from planets? Where does that speed come from?

Research question: When a probe swings past a planet, does it gain speed for free, and where does that extra speed actually come from?

The records show that a gravity assist does not add energy for free: the spacecraft exchanges a small amount of energy with the planet, and the flyby bends its path ². The Europa Clipper example shows the exchange can run in either direction, since at Mars the probe was used to slow down relative to the Sun, approaching at about 15.2 miles per second and expected to leave at about 14 miles per second ⁵⁹. A later Earth flyby is expected to act as a slingshot that gives the spacecraft a velocity boost, so the same mechanism can add speed as well as remove it ¹². The records do not explicitly state where the energy comes from, so the planet's motion is only implied by the described exchange ². Without these assists, the spacecraft would need extra propellant or much longer travel times ¹³; the newest fact concerns an expected December 2026 event, which is still ahead of today's date of October 2026 ¹².

Checked 2026-10-18. Only documented and supported facts are used.

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Sources

[1] supported

Longuski developed the 'V-Infinity Leveraging' technique, which uses a deep space maneuver to increase or decrease the hyperbolic velocity at the next gravity-assist body.

"He also developed the technique and coined the term, “V-Infinity Leveraging,” which uses a deep space maneuver to leverage (increase or decrease) the hyperbolic velocity at the next gravity-assist body."

James Longuski

[2] supported

A gravity assist does not add energy for free; the spacecraft exchanges a small amount of energy with the planet, and the flyby bends its path.

"Meanwhile, we’re exchanging a small amount of energy with the planet, so we leave on a path that will bring us back past Earth."

NASA’s Europa Clipper Uses Mars to Go the Distance

[3] supported

Longuski and Nathan Strange of JPL developed the 'Tisserand graph' method, which is widely used for designing gravity-assist spacecraft trajectories.

"widely used for gravity-assist spacecraft trajectory design"

James Longuski

[4] supported

Lucy gained its first gravity assist from Earth about a year after its launch.

"It gained one gravity assist from Earth a year later on 16 October 2022"

Lucy (spacecraft)

[5] supported

At Mars, Europa Clipper uses Mars's gravitational pull to slow down ('pump the brakes') and reshape its orbit around the Sun, so it leaves Mars slower than it approached relative to the Sun.

"For about 12 hours prior and 12 hours after that time, the spacecraft will use the gravitational pull of Mars to pump the brakes and reshape its orbit around the Sun."

NASA’s Europa Clipper Uses Mars to Go the Distance

[6] supported

Longuski's mathematical gravity-assist method reduced trajectory calculations from months or years to hours or even days.

"Allowing for the calculation to go from months or years, to hours or even days."

James Longuski

[7] supported

Three possible trajectories were designed using two Mars gravity assists each.

"Nevertheless, 3 possible trajectories were designed, with two Mars gravity assists."

Vesta (spacecraft)

[8] supported

Lucy gained a second gravity assist from Earth in 2024, after its asteroid flyby of Dinkinesh in 2023.

"gained another gravity assist from Earth in 2024."

Lucy (spacecraft)

[9] supported

Europa Clipper approached Mars at about 15.2 miles per second relative to the Sun and was expected to leave at about 14 miles per second, a drop in speed.

"approaching it at about 15.2 miles per second (24.5 kilometers per second) relative to the Sun."

NASA’s Europa Clipper Uses Mars to Go the Distance

[10] supported

A single Mars swing-by was also possible, but using two gravity assists raised the spacecraft's mass budget by 30% and added about 1.8 years of travel time to the asteroid belt.

"A single Mars swing-by is also possible, but the double gravity assist increases the mass budget of the spacecraft by 30%, at the cost of an additional 1.8 year in travel time to the asteroid belt."

Vesta (spacecraft)

[11] supported

Lucy is planned to return to Earth in 2031 for another gravity assist that sends it toward the L5 Trojan cloud.

"Lucy will return to Earth in 2031 for another gravity assist toward the L 5 Trojan cloud"

Lucy (spacecraft)

[12] supported

The later Earth flyby is expected to act as a slingshot that gives the spacecraft a velocity boost.

"The flyby sets up Europa Clipper for its second gravity assist — a close encounter with Earth in December 2026 that will act as a slingshot and give the spacecraft a velocity boost."

NASA’s Europa Clipper Uses Mars to Go the Distance

[13] supported

Without the gravity assists from Mars and Earth, the spacecraft would need extra propellant, which adds weight and cost, or would take much longer to reach Jupiter.

"Without the assists from Mars in 2025 and from Earth in 2026, the 12,750-pound (6,000-kilogram) spacecraft would require additional propellant, which adds weight and cost, or it would take much longer to get to Jupiter."

NASA’s Europa Clipper Uses Mars to Go the Distance