# How close can a spacecraft get to the Sun if it is limited solely by passive cooling?

The Parker Solar Probe’s trajectory will take it within 8.5 solar radii of the sun’s photosphere. Its instruments, hiding in the shadow of the alumina-coated composite sun shield, will bask in 29 °C comfort, even without a cooling system. (Only exposed solar panels are cooled).

The JWST uses the same strategy (at an orbital distance of 214 solar radii) to attain a temperature of 27 K in the shade.

From sketchfab.com,

If a probe were equipped with radiators on the entire anti-solar surface, how close could a probe approach the sun?

Porous alumina has a reflectivity of 99.0% for visible light and 99.4% for IR. On the radiator side, there are materials with emissivity of 97.0-98.5%.

This means a spacecraft which is highly reflective on the sunward side and highly emissive on the anti-sunward side should come to thermal equilibrium somewhere between the solar surface temperature of 5800 K and the cosmic background temperature 2.7 K, a rather large range. Any idea how to calculate this equilibrium temperature for a given solar distance?

As an example, this cube-shaped spacecraft has a 0.414 solar radii perihelion. It has heat pipes to keep the interior temperature the same as the radiator panels. The sunny side is 99% reflective and the radiators 98% emissive. What would be the temperature of the interior?

Or, conversely, beyond what perihelion could the interior temperature be compatible with living astronauts? Functioning space-hardened electronics?

• Could you clarify what you would consider "active cooling" and "passive cooling" in a spacecraft context? Jan 2 at 16:01
• @Dragongeek ... "active"=consumes power or expends mass Jan 2 at 16:29
• Are we allowed to postulate a sun-shield that has a larger radius than the sun itself? Jan 4 at 16:50