Keep in mind that they would inherently have shields given their size and importance, and In the current model of trans-newtonian physics they would be able to nullify the kinetic energy impact  by applying an opposite force, and they could otherwise just shoot them down (If the missile isn't destroyed on launch from the interaction with the atmosphere). Also keep in mind ground units are also able to be destroyed by space fire, hence why you have a navy/PDCs to keep space clear of hostiles.

I thought shields don't work in the atmosphere? That's why PDCs don't have them, right?

Re: point defense and atmosphere. Let's assume the missiles require LOS for a target lock. Max range/visiblity to the horizon is ~20km. Given that the US has launched a Mach 20 vehicle with pre-Newtonian tech ( http://www.space.com/12670-superfast-hypersonic-military-aircraft-darpa-htv2.html ), a Trans-Newtonian missile should be able to function at, say, 10,000km/s (revised downward from the previous 50,000km/s). That's still ~3kT KE. At 250 kilograms, the missile is about the size of an AGM-65 Maverick ( https://en.wikipedia.org/wiki/AGM-65_Maverick ). Too big to be man-portable, but you could mount 2 of them on a flatbed Humvee easily.

At 10,000km/s firing at a target 20km away, flight time is 2 milliseconds. So a War Machine's point-defense system would need to operate in the low-nanosecond range to detect, compute a firing solution, and engage a hypervelocity missile. But if point defenses were that good, space units in Aurora could probably shoot down railgun or gauss cannon projectiles.

Re: space fire (aka "ortillery"). If anything, the existence of space navies is another point against War Machines. A target the size of a football stadium is far easier to track and engage from orbit compared to a battalion of smaller tanks spread across several square kilometers of terrain.

The real question is:

1. How much energy absorption does 1 block of armor represent?

This would give us an idea of the velocities and masses of railgun/gauss cannon projectiles, and illustrate how much protection is actually needed/provided by ground vehicle TN armor in real terms.

So there was a lot of discussion about the new Titans units in the change log thread. One thing that wasn't much discusse however was "starting from what we know about TN elements, how would ground combat change"? We know that TN elements allow instantaneous change of speed and direction, as well as FTL detection of gravity waves, but also of heat and EM (And as such, FTL communication). Interestingly, it cannot send energy in a FTL manner (hence the range limit on energy weapons)

It also seems that the TN effects only work over a certain size, since you can't build a missile of less than 2,5 tons.

So, how would TN influence ground combat? I'd expect the 2,5 tons limits to be important. The agility and maneuvrability benefits are some important I'd expect militaries to devellop exosuits of that weigh range.

Instantaneous speed/direction would make for some very maneuverable anti-gravity tanks, but detection of gravity waves would also make them vulnerable to the gravitic equivalent of AGM-88 HARM.

Hence the multi hundered/thousand ton titans coming into existence. IRL MBTs are already ~50 tons so I would assume that it would be easy to convert those, and make bigger ones. And the smallest trans-newtonian engine in Aurora is 0.1MSP (so only 0.25 tons).

With precision guidance, short (compared to space) ranges, and lower speeds of the target vehicles, kinetic impact hyper-velocity missiles are viable weapons. Giant Ogre/Titan-style vehicles can be destroyed by massed fires from cheap (compared to the construction cost of oversized warmachines) ordnance.

KE = 1/2 * 250kg * (50,000 km/s)^2  <---0.25 ton missile flying at decent AMM speed
KE = 3.125e17 J
convert to tons of TNT = ~75MT

So there was a lot of discussion about the new Titans units in the change log thread. One thing that wasn't much discusse however was "starting from what we know about TN elements, how would ground combat change"? We know that TN elements allow instantaneous change of speed and direction, as well as FTL detection of gravity waves, but also of heat and EM (And as such, FTL communication). Interestingly, it cannot send energy in a FTL manner (hence the range limit on energy weapons)

It also seems that the TN effects only work over a certain size, since you can't build a missile of less than 2,5 tons.

So, how would TN influence ground combat? I'd expect the 2,5 tons limits to be important. The agility and maneuvrability benefits are some important I'd expect militaries to devellop exosuits of that weigh range.