NASA’s New Horizons examines solar winds at the solar system’s edge

Deep space observations from NASA’s New Horizons spacecraft have provided new, quantitative insights into how the solar wind slows down as it approaches interstellar space

The study confirms that the sun’s supersonic particle stream is being actively braked by material bleeding into our solar system from deep space.

The research, published in The Astrophysical Journal, was led by Dr. Heather Elliott at the Southwest Research Institute (SwRI) in San Antonio, Texas.

The mechanics of cosmic drag

The solar wind consists of charged particles ejected by the Sun at supersonic speeds—roughly 1 million miles per hour. As this plasma stream expands outward through the heliosphere (the bubble of space controlled by the Sun’s magnetic field), it runs a gauntlet against neutral interstellar gas particles leaking in from the galactic neighbourhood.

This encounter triggers a specific cosmic brake:

• Charge exchange:

  • Supersonic solar wind ions collide with incoming neutral interstellar atoms.

• Ionisation and mass loading:

  • The neutral galactic atoms lose electrons and become ionised.

• Deceleration:

  • The solar wind forces these newly ionised “pickup ions” to move along with it. This adds mass to the solar wind, diluting its kinetic energy and forcing the entire stream to slow down.

Mapping the slowdown (21 to 58 AU)

New Horizons is currently positioned roughly 66 AU from the Sun (where 1 AU represents the 93-million-mile distance between Earth and the Sun). By analysing data gathered by the spacecraft’s specialised Solar Wind Around Pluto (SWAP) instrument, researchers mapped exactly how the solar wind’s velocity changed between 21 AU and 58 AU.

The multi-point tracking reveals a steepening slowdown curve:

• 30 to 43 AU (Earlier Voyager and New Horizons Data):

  • 5% to 10% slower than at 1 AU near Earth.

• 58 AU (New Horizons’ latest milestone):

  • 13% to 15% slower than at 1 AU near Earth.

• 84 AU (Voyager 2 historical termination shock marker):

  • A sharp 46% sudden drop in speed, where solar particles rapidly drop below the local plasma speed of sound.

This gradual drag profile aligns tightly with theoretical models of interstellar gas infiltration, mapping exactly where and how the Sun’s dominant environmental influence begins to fade.

Protecting Future Astronauts from Cosmic Radiation

Quantifying the exact density and location of this deep-space boundary layer is not just an academic exercise—it is critical for the safety of crewed space exploration.

The physical shape and density of these heliospheric boundaries act as a primary shield for our solar system, deflecting high-energy Galactic Cosmic Rays (GCRs). Because GCRs pose severe radiation risks to electronics, hardware, and astronauts traveling to the Moon or Mars, understanding this boundary layer is crucial for predicting cosmic radiation exposure.

By combining New Horizons’ local measurements with broader data from active and upcoming missions—like Voyager, IBEX, and IMAP—heliophysicists can more accurately model this cosmic shield, a vital step toward planning humanity’s eventual journey into interstellar space.