NASA's Messenger craft SMASHES into Mercury: See ya later, alien crater

Re: building our cities underground

This would also be handy for hiding from Shelks; I'm pretty sure we could hang on down there until Tumithak turns up to free us from the domination of Venus.

I think you mostly get Tetrahedra from Mercury. Underground cities are not required to deal with them, you just have to wait until it rains :-)

Re: Let's hope that there was nobody down there!

"the chances of anything coming from Mercury"

We're in trouble if it's 1,000,000:1

Re: Dear engineers,

Did you accept and install all those updates to 'improve your experience'?

Re: Ran out of fuel and crashed?

Presumably it will deorbit for the same reason that objects in Low Earth Orbit will. Things like solar wind and gravity perturbations eventually decay the orbit.

I think I read that they did in fact deliberately lower Messengers orbit so it would crash too.

The real universe is not like KSP where you can just sit in a 75km orbit for ever. ;)

Re: Ran out of fuel and crashed?

"I don't understand that bit. If it ran out of fuel, surely it would stay in orbit round Mercury for ever, Mercury having no atmosphere to drag it down. "

That is true if Mercury is a perfectly even sphere and you don't include the influence of other gravity sources in the solar system: the Sun especially, but I'd also look at Venus, Earth, and Jupiter for long-term tidal effects. Then there's light pressure and the solar wind.

The problem was first observed in the 1960s when the orbits of various Lunar orbiters tended to go awry. Luna, like Mercury, is not a perfectly even sphere. It has large mass concentrations (mascons) that send most satellites wandering off course over time.

Short wiki version (key words: "These anomalies are significant enough to cause a lunar orbit to change significantly over the course of several days"):

http://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects

Historical version:

https://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=2325

Uneven mass concentrations and external gravity loads can send satellites wandering. Apollo 16's PSF-2 lunar satellite was only in orbit 35 days. Geosynchronous satellites around Earth have to constantly use stationkeeping fuel to avoid drifting, and the non-spherical shape of Earth is a common consideration for orbital stability in other satellites. MESSENGER was becoming unpredictable; NASA wanted to crash it in a controlled fashion where they had the final say on its landing site for future science.

Re: Ran out of fuel and crashed?

"I don't understand that bit. If it ran out of fuel, surely it would stay in orbit round Mercury for ever"

Messenger was in an eliptical orbit which required a boost from its engine every few months to keep stable (due to the gravitational effect of the Sun.) Rather than deliberately crashing the probe into Mercury, mission controllers actually extended mission time by several weeks after all fuel was expended by venting Helium (previously used to pressurise the fuel supply) through the engine exhaust.

Re: Why crash on the far side?

It wasn't particularly maneuvarable, plus it's not always possible to predict precisely where it will hit in advance (the impact was the result of a gradual decay, not that they pointed the engines 'downwards' and plummeted straight in). There was presumably more science in prolonging the mission - the ony thing we didn't see is the impact. There also wouldn't have been any data transmissions (it's not a real-time camera broadcasting back, it takes pictures, processes, then transmits at the appropriate time).

Spectral data of impact - I assume we'd have needed it to be "side-on" to the planet from our perspective, and possibly in front of the sun to have something to read.