Thread by @anderssandberg

#FridayPhysicsFun – “Launch him/it into the sun!” My husband was griping over breakfast about the expression: surely this is not a good way of disposing of anything?
tvtropes.org/pmwiki/pmwiki.…

Is it actually possible to make something that will hit the “surface” of the sun?

The first problem of throwing something into the sun is that you need to hit it. Earth is moving 30 km/s, so you need to give the object this velocity in the opposite direction to make it fall straight in. That is a lot more than Earth escape velocity.

This is why the Parker Solar Probe needed several gravity assists to get into a close orbit. link.springer.com/article/10.100… en.wikipedia.org/wiki/Parker_So…

Obvious definition problem: what counts as the surface?

As one approaches the sun through the corona, the density of the plasma starts going up rapidly as one passes through the transition region into the chromosphere. It is still thinner than Earth air, and pretty transparent. history.nasa.gov/SP-402/p2.htm

In the underlying photosphere the density becomes roughly air-like. This is usually what is referred to as to as the surface, mostly because this is where the sun becomes opaque enough to block light.

The photosphere temperature is the light temperature we experience, 5772 K (since we see the blackbody radiation emitted by it through the upper layers). It is actually colder than the corona and the underlying layers.

Anyway, hitting the photosphere surely counts as dropping something into the sun.

Getting within about 3 solar radii is usually bad news for loosely held together objects like comets and planets since it is inside the Roche radius and they will start to pull apart as they swing by the sun.
link.springer.com/article/10.100…

Tidal forces are not serious problems for a spacecraft, at least not for the sun.

Being evaporated by the sunlight is another matter. The comet C/2007 M5 SOHO likely sublimated before hitting the photosphere.

The equilibrium temperature scales as 400(1-A)^(1/4)r^(-1/2) Kelvin where A is the albedo and r heliocentric distance. Dark objects hence reach 1260 K at 0.1 AU and 4000 K at 0.01 AU. A 99.9% effective mirror would be at 711 K at 0.01 AU, slightly cooler than surface of Venus.

There are some complications here: when you are very close you do not see the entire hemisphere of the sun (see appendix in iopscience.iop.org/article/10.384… ) but there is also a lot of bright plasma around you.

Stuff falling into the sun moves close to the local escape velocity, 615 km/s. So traversing the last 2 solar radii from 0.01 AU takes about 20 minutes. A big mirrored craft with a lot of shielding mass to boil off would likely be able to reach the photosphere and make a splash.

Since there will be direct heat conduction from the 5777K photosphere at this point, temperature and density only go up, and the rapid motion will create a lot of aerodynamic heating this is likely the limit.

Using a blunt body design the shock wave may be directed away from the vehicle (heating is surprisingly inversely proportional to drag). But now the vehicle is completely surrounded by radiating hot plasma.
web.archive.org/web/2015101318…

Great, we *can* launch annoying things or people into the sun, provided we do it in big disco balls filled with a mantle of water or hydrogen to boil off that we launch from Earth orbit at 30 km/s or do multiple gravity assists. Easy and cheap.

Another method is to wait. Eventually it is fairly likely the sun’s expansion into a red giant will cause it to engulf the Earth, and then everything on Earth gets thrown into the sun. Technically it is not launching anything, but I doubt anybody cares.

In this case the Earth survives almost a century in the red giant atmosphere since it is so tenuous. adsabs.harvard.edu/pdf/1987A&A...… academic.oup.com/mnras/article/…

So I think we can agree that as a disposal method launching things into the sun or waiting for 5+ billion years have some pretty severe drawbacks.