Not knowing Stargate much, this might have been explained, but if someone (or something) travels towards a closed iris, does it get sent back, or is it horribly squished against the facade?
Horribly squished. Stargate travel is one way, there is no option to get sent backwards once something has fully entered the event horizon.
More on how the Iris works:
The iris rests a few micrometers in front of the event horizon, preventing matter from reintegrating as it arrives through the wormhole. Objects are destroyed, and living beings are killed instantly upon "impacting" on the iris. Known substances capable of passing through the gate despite the iris include radio waves and accelerated particle beams (capable of moving at near-light velocities). The iris has only been breached by Tollan technology, capable of passing through any solid matter. From Gateworld
To quote O'Neil,
It'll be like bugs on a windshield.
Though more technically from Carter:
Pure titanium, less than 3 micrometers from the event horizon. It won't even allow matter to fully re-integrate.
So squished at the atomic level.
Quotes are from SG-1, 1x02, The Enemy Within
So, we know a few things.
- The matter does not re-integrate.
- There is at least the possibility that some kind of signature is left behind. The Tok'ra were given an item that they could toss through the gate, and it would leave a signature that lets Earth know that the Tok'ra were trying to communicate with them.
- There are several cases in which after objects re-entered, they were checked for radiation.
- There isn't some pile of matter increased when an object appears.
It seems that radioactive particles can re-integrate, but are more or less stuck. The matter seems to just disappear. There have been a few issues in which regard to matter not appearing in the destination, basically lost in transit. It seems likely that either that happened, or the incoming object was converted to pure energy, used to power the gate. Either way, not a pleasant experience...
Any object larger than a particle (3 μm) is crushed, "horribly squished" as you put it.
Radio signals and other forms of energy can pass through the iris though:
Definitely Squished! The best example I can think of this actually happening is:
In The Eye episode of Stargate Atlantis, Sheppard raises the shield on the stargate when the invading Genii reinforcements are coming through the Stargate. Only 5 of 60 make it, and if I remember correctly, you hear a series of "zaps" (like bugs hitting a zapper) as the unsuspecting Genii forces hit the shield.
Note: They technically use a Shield rather than an Iris in Atlantis, but the concept is the same: