[Magdalen] Philae has landed!

[Roger Stokes]

On 13/11/2014 03:02, Jon Egger wrote:
> What I'm wondering is the time issues involved.  Surely the landing isn't
> "live" as in happening right now, given the distance the signals must
> travel to Earth to receive them.  Yes, the probe landed there, but how long
> ago given the time constraints.
>
> Does that make any sense?

It makes perfect sense.  Currently a radio signal to or from the 
spacecraft will take about 30 minutes for the one way trip. That means 
that if data is sent from Rosetta (the orbiting craft) to Earth, 
processed here and then a command sent back it takes an hour for the 
command to get to the spacecraft.  There needs to be a degree of 
"artificial intelligence" on the two craft so they can respond to data 
in real time.

All the scientists on earth can do is anticipate what is going to happen 
if there is no interbention, calculate what adjustment is needed and 
then send a command allowing enough time for it to be implemented at the 
right instant.  The manouevering has to be incredibly precise given the 
small size of the comet.  The gravity is so weak that the escape 
velocity, the speed needed to escape its gravitational pull, is 
estimated at 3 ft/sec.  That is less thanthe speed we need to achieve to 
jump a couple of inches into the air. Getting Rosetta in orbit around 
the comet was another complex piece of fine adjustment of the craft's 
paramaters, not helped by the fact that the comet is not a regular shape.

Roger