Two particle instruments on the still-healthy New Horizons spacecraft face shutdown on Oct. 1 to save minimal resources. Those instruments are the only ones capable of measuring solar wind–interstellar interactions in the spacecraft’s current region of the outer heliosphere.
New Horizons remains healthy and can operate until about 2050, but NASA plans to turn off SWAP and PEPSSI on Oct. 1 to save a very small amount of money.
SWAP and PEPSSI uniquely measure particle energies and fluxes in the outer heliosphere, providing data that Voyager lacked and that are critical for understanding the heliopause and its effects on planetary environments.
# What's happening
Despite that longevity, NASA plans to shut down two science instruments — SWAP (Solar Wind Around Pluto) and PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation) — beginning Oct. 1. The stated reason is to save a very small amount of mission resources.
# What SWAP and PEPSSI measure
# Why those measurements matter
The heliosphere is the Sun's magnetic and plasma bubble that deflects or moderates galactic cosmic rays and interstellar particles. Its size and structure affect how much interstellar material reaches the inner Solar System.
Geologic evidence in ocean sediments records episodes when Earth's atmosphere was apparently exposed to interstellar material. Traces of the isotopes iron-60 and plutonium-244 date to two brief events roughly 2 million and 7 million years ago. Numerical models show the heliosphere can shrink dramatically if it encounters dense interstellar clouds, potentially compressing inside Mercury's orbit and exposing planets to enhanced neutral hydrogen and cosmic rays.
Those changes can increase atmospheric ionization and particle-driven chemistry and may affect cloud nucleation and climate. Measuring particle interactions at the heliopause and in the VLISM is therefore directly relevant to understanding past environmental changes on Earth and to basic questions about habitability and planetary evolution.
# The immediate tradeoff
Shutting down SWAP and PEPSSI pauses a unique, ongoing dataset of how the solar wind behaves at large distances. The instruments can be turned back on later, but only if NASA secures additional funds. Turning them off interrupts continuity of observations during a period when New Horizons is well positioned to study the heliospheric boundary and the approach to interstellar space.
# What to watch next
# Bottom line
New Horizons can keep providing unique particle measurements of the outer heliosphere, but SWAP and PEPSSI are slated for shutdown to save a small sum. Those measurements link directly to questions about how the heliosphere shields the planets and how past compressions of that shield may have affected Earth's climate and chemistry. The instruments are not lost, but continued funding is the only thing keeping that continuous, high-value record alive.
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