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A gravity assist is a spacecraft flyby that uses the gravity of a moving planet or other massive body to change the spacecraft's trajectory and its orbital energy relative to another body, such as the Sun. In the ideal unpowered two-body encounter, the incoming and outgoing speeds far from the planet are equal in the planet's reference frame. Their directions differ. Adding the planet's motion converts that directional change into a possible speed change in the Sun's reference frame.^[ref]

Write the planet's nearly constant velocity during the encounter as $\mathbf{V}$, and the incoming and outgoing planet-relative velocities as ${\mathbf{u}}_i$ and ${\mathbf{u}}_o$. Then ${\mathbf{v}}_i=\mathbf{V}+{\mathbf{u}}_i$, ${\mathbf{v}}_o=\mathbf{V}+{\mathbf{u}}_o$, and $|{\mathbf{u}}_i|=|{\mathbf{u}}_o|$. Subtracting the squared speeds gives the local change in specific orbital energy:For a hypothetical $90^{\circ }$ turn, choose $\mathbf{V}=(10,0)$, ${\mathbf{u}}_i=(-5,0)$ and ${\mathbf{u}}_o=(0,5)$, all in $\mathrm{km}{\mathrm{s}}^{-1}$. Planet-relative speed stays $5$, while Sun-relative speed changes from $5$ to $\sqrt{125}\approx 11.18\mathrm{km}{\mathrm{s}}^{-1}$. The specific-energy gain is $50\mathrm{k}{\mathrm{m}}^2{\mathrm{s}}^{-2}$. Whether a given turn is physically available depends on the planet's mass, the incoming speed and the permitted closest approach.

Conservation of energy is maintained because the planet and spacecraft exchange momentum and energy. The planet's change is extremely small because of its much greater mass. A different encounter geometry can remove orbital energy or mainly change orbital inclination; Ulysses spacecraft used the latter possibility to reach high solar latitudes.^[ref]

Solar Orbiter is an ESA-led spacecraft with strong NASA participation, launched in February 2020 to connect observations of the Sun with measurements of the solar wind. Its heliocentric orbit and gravity assists allow observations from different distances and increasing solar latitudes, rather than restricting every view to the plane from which Earth sees the Sun.^[ref]

Its ten instruments divide into six for remote sensing and four for sampling the surrounding environment. The remote sensing instruments observe emitted or scattered light from the Sun and its surroundings. The local instruments measure plasma, energetic particles, electric fields, magnetic fields and waves. A magnetometer and particle analysers therefore answer different questions from an image: one measures conditions at the spacecraft, while the other helps locate the remote source.^[ref]

A scientific comparison can ask whether a feature observed near the Sun is associated with a later change in the velocity distribution or composition of locally sampled particles. Identifying the connection requires accounting for the material's travel time and the changing geometry; simultaneous timestamps alone do not establish a common origin. The mission's combined apparatus makes such tests possible. A 2024 analysis, for example, separated proton and alpha particle populations in its solar wind data rather than representing all ions by one mean speed.^[ref]

The Ulysses spacecraft was a joint ESA–NASA solar probe launched on 6 October 1990 to investigate the space environment above and below the Sun's poles. A gravity assist at Jupiter on 8 February 1992 bent its trajectory into a strongly inclined heliocentric orbit around the Sun. It sampled southern polar regions in 1994 and northern polar regions in 1995, extending direct measurements far beyond the solar latitudes accessible near Earth's orbit.^[ref]

The payload included instruments for solar wind particles, magnetic fields, energetic particles, radio and plasma waves, and dust. The central experiment was to change observing location: measurements near the solar equator alone cannot determine whether the same conditions persist over the poles. During the first southern pass, the instruments found faster solar wind than in equatorial regions.^[ref]

Its observations also provided a test of the energy cascade in magnetohydrodynamic turbulence. Investigators combined velocity and magnetic field fluctuations into Elsässer variables and measured how mixed third-order statistics varied with separation. The 2007 analysis used selected intervals of high-speed polar wind from the first seven months of 1996. This illustrates how a mission designed for a new vantage point can later support a precise statistical experiment, with assumptions about translating measurement times into spatial separations.^[ref]

Voyager 2 is a robotic spacecraft launched on 20 August 1977 to investigate the outer Solar system. Its encounters with Jupiter in 1979, Saturn in 1981, Uranus in 1986 and Neptune in 1989 made one mission a comparative study of four giant planets. A gravity assist changes the spacecraft's trajectory and energy relative to the Sun by exchanging momentum with a moving planet, allowing a sequence of encounters that would require much more onboard propulsion if accomplished by engine firings alone.^[ref]

Voyager 2 later sampled the transition from the solar wind to the surrounding interstellar plasma. On 5 November 2018, its plasma instrument recorded a steep decline in the outward solar wind flow. The instrument inferred plasma properties from electrical currents collected from particles arriving in different directions. Changes in energetic-particle measurements supplied independent evidence: particles associated with the heliosphere declined while galactic cosmic rays became more prominent. The combined observations identify the heliopause crossing, rather than defining the boundary by a predetermined distance.^[ref]

This is a moving-laboratory experiment: the apparatus travels through regions that cannot be sampled from Earth, while several instruments test the interpretation of a sharp change. Crossing the heliopause means leaving the region dominated by the Sun's outward-flowing plasma; solar gravity does not suddenly end there. The crossing date and the December announcement date are different events.