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Have any spacecraft been either launched or proposed which involve a survivable close encounter with the Sun? This would include solar observation missions as well as any missions designed to make a close pass as part of their trajectories en-route elsewhere, and by "close" I mean at least within the orbit of Mercury.

How close have spacecraft approached or have been proposed to approach, and how long were they / will they be within the most extreme thermal/radiation/etc. conditions that would be encountered? What features would be required / have been incorporated into any such spacecraft in order to survive those conditions?

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You seem to have a lot of questions here. Could you clarify what you want as an answer. –  Rory Alsop Apr 27 at 18:15
    
Going inside Venus' orbit is a suicide by cancer for manned craft. The MESSENGER was designed to reach Mercury. –  Deer Hunter Apr 27 at 18:43
    
I was curious about a few of points: (1) The greatest extreme a spacecraft has been designed to endure for a limited period (during perihelion) where it has the opportunity to recover from accumulating stress at greater distance (would presumably only apply to thermal stress). (2) The greatest extreme endurable for an extended period (closest possible nominally circular orbit) and the hazards and associated defences, and whether these hazards will inevitably kill the spacecraft anyway. (3) Is any spacecraft doomed to a short life when that close to the Sun? –  Anthony X Apr 27 at 20:43
    
If you've got enough unobtainium, you can probably graze at 1 Solar radius. Too much dependence on state of the art. –  Deer Hunter Apr 27 at 21:44
    
I'm sure unobtainium would make many things possible... some day. –  Anthony X Apr 27 at 23:23

2 Answers 2

Solar Probe, planned to launch in 2018, will get to within 8.5 solar radii of the Sun's surface. For comparison, Mercury gets no closer than 65 solar radii. Solar Probe will use a thick carbon-carbon, carbon foam shield when in close proximity.

The record so far is held by Helios-2 in 1976, which got just inside Mercury's orbit at 0.29 AU, about 61 solar radii above the surface of the Sun.

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The neatest, and the most prominent example of a "close encounter" with the Sun is the MESSENGER. Additional information may be gleaned from this proposal.

The main problem for unmanned craft is, of course, the Sun itself. Protection from overheating relies on a combination of design decisions:

  • a heat-resistant sunshade (on MESSENGER, made from ceramic cloth)
  • 3-axis stabilization to keep the instruments and the craft from pointing at the Sun outside of the sunshade's protection
  • solar panels (kept outside the sunshade) are rotatable, made from photovoltaic cells on the sun-facing side and from PV cells and optical reflectors (radiating excess heat into space) on the reverse side
  • providing for radiators' unfettered view of the cold space background
  • using state-of-the-art materials

If you consider the requirement to pass by Mercury on the hot side, you have to be prepared to tackle two extra problems:

  • a lot of extra heating "from below" (infrared) that's not caught by the sunshade -

    • using side radiators to pick up heat, transfer it via heat pipes and dump through radiators viewing cooler space
  • fast heating/cooling of the spacecraft (thermal cycling) -

    • Thermal cycling induces mechanical stress, so the main idea is to allow for flexible expansion and contraction of the structure.

    • Keeping the most cycling-sensitive items (electronics, batteries, thrusters etc.) physically de-coupled from the radiators and the shade.

    • Careful periapsis height selection to limit thermal change rate.

To achieve all this, one has to do a lot of modelling.

For manned spacecraft at the current technology level, going inside the orbit of Venus for any prolonged period looks like a suicide mission - instead of thermal constraints you're bound by radiation dose limits.

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