The Sun is the only star humans will ever study at close range. It is also the single most important input to everything we build in space: spacecraft lifetimes, power budgets, crew radiation doses, and communications reliability are all set, in the end, by the Sun's behaviour. A small fleet of heliophysics missions is currently doing work that no Earth-based observation can match.
Why We Study It
There are three main reasons to fly missions to the Sun rather than just observe it from Earth or low orbit.
First is space weather. The Sun ejects high-energy particles in coronal mass ejections (CMEs) that can, when directed at Earth, induce currents in power grids, degrade satellites, interrupt high-frequency radio, and force polar-flight rerouting. Severe CMEs are rare but not unprecedented: the 1859 Carrington event would, if repeated today, do multi-trillion-dollar damage. Understanding and predicting space weather is the most commercially consequential branch of solar physics.
Second is the corona problem. The Sun's visible surface, the photosphere, is about 5,500 °C. The corona, the tenuous outer atmosphere above it, is between 1 and 3 million degrees — hundreds of times hotter. Nothing in everyday physics explains this. Heat should decrease with distance from the heat source, not increase by orders of magnitude. The corona is heated by some combination of magnetic reconnection events and wave processes, and the details are still being worked out by missions now in flight.
Third is stellar physics. Every other star humans characterise is a point of light. The Sun is the only one we can map in detail, resolve in time, watch flare, and compare with the rest. Every exoplanet habitability model depends on assumptions about the host star's activity that are ultimately calibrated against the Sun.
The Modern Heliophysics Fleet
Two missions define the current era of solar science.
- Parker Solar Probe (NASA). Launched 2018. Parker is, by a very wide margin, the closest a spacecraft has ever come to the Sun. Its perihelion in December 2024 took it to about 6.1 million km from the solar surface — inside the solar corona itself, through plasma whose behaviour was, until Parker got there, known only through remote sensing. Parker is protected by a carbon-composite heat shield and a carefully managed solar-energy budget: solar panels retract behind the shield at closest approach. It is the first spacecraft ever to "touch" its star.
- Solar Orbiter (ESA/NASA). Launched 2020. A complementary mission: not as close as Parker, but uniquely able to see the Sun from outside the ecliptic plane. Solar Orbiter's orbit is inclined to give it direct views of the solar poles, which are inaccessible from Earth and crucial for understanding the global solar magnetic field.
Between them, Parker and Solar Orbiter are observing the Sun across essentially all the angles and altitudes that matter. Both are well into their primary missions and are expected to continue through the current solar cycle maximum, which gives the programme its richest scientific harvest.
The Solar Cycle
The Sun's magnetic activity rises and falls on roughly an 11-year cycle, driven by a dynamo in the solar interior that is still not fully understood. Solar maximum — when sunspot counts, flare rates, and CME frequency all peak — happened for the current cycle (Cycle 25) around 2024–2025. That happens to coincide with both Parker and Solar Orbiter being operational, plus a range of other observatories, which is why the 2020s are an unusually productive decade for solar physics.
The next solar maximum is due around 2035. Between now and then, the Sun will quiet down, flares and CMEs will become rarer, and the observational focus will shift to long-term dynamo studies and pole-to-equator flows.
Space Weather as Infrastructure
"Space weather" sounds abstract, but it is a real and funded operational discipline. The U.S. Space Weather Prediction Center, part of NOAA, issues daily forecasts of geomagnetic activity used by grid operators, airlines, and satellite operators. ESA runs the SOSMAG and related services. The forecasts rely on a combination of Sun-facing observatories (DSCOVR at L1, SOHO, SDO) and empirical models of how CMEs propagate through the heliosphere.
The gap most forecasters would like to close is lead time. The current state of the art gives about 30 to 60 minutes of warning from when a CME is detected by L1 monitors until it hits Earth's magnetosphere. That is not enough to do more than put satellites into safe mode. A future dedicated mission at Lagrange point L5 (trailing Earth in its orbit around the Sun) could extend warning times to several days by seeing the Sun from the side, catching CMEs earlier in their trajectory. NOAA's proposed Space Weather Follow-On L5 mission is the best-known candidate.
Why This Matters
Every crewed mission beyond low Earth orbit is ultimately limited by solar particle events. An Apollo-era mission that happened to be between the Earth and Moon during a major flare would have exposed its crew to potentially lethal radiation. Future Mars missions face a much worse version of the same problem, because the cruise is seven months long and the spacecraft cannot easily return to a safer orbit.
Understanding the Sun is not one branch of space science among others. It is the limiting constraint on nearly everything ambitious humans want to do off Earth. Parker and Solar Orbiter are the most important missions most people never hear about.