If you want stealth, hide behind a comet. It will be seen if someone is looking, but won't raise any warning bells - there are a lot of space rocks out there. If you could engineer the approach to be close, but far enough off that no preventive responses occur, all the better.
Better yet, a comet will release a gas trail, this will mask any cold-propulsive residue, and naturally act as interference for any radar or other electro-magnetic detection system. At worst such systems will recognise the ship, but will likely be dismissed as a false positive due to inconsistent observations at that and other frequencies.
Asteroids are known to approach the Earth/Moon system, aim your asteroid to pass behind the moon some distance away from the Earth so as to not draw Preventative responses.
There is no longer any need for subterfuge. At these distances any detection facility will identify your spacecraft as at least anomalous when detected. Due to how close it is, there is a reasonable chance that such news will be passed around various observatories quickly. Soon after confirmation, it will likely be brought to the attention of those who can respond.
While deception is useless now, that is no reason to announce your presence. Leave the cover of the asteroid while it is occluded by the moon.
Use the lunar gravity and hot propulsion to slingshot the vessel to ramming speeds. The moon is 384,400 km away, the Juno spacecraft had a max speed at 58km/s which could travel that within 2 hours. As the key is to reach the target prior to any interception by a counter measure the faster the better:
- 6406km/s takes 1 minute.
- 3203km/s takes 2 minutes.
- 1602km/s takes 4 minutes.
- 801km/s takes 8 minutes.
At these speeds the space ship does not need any fancy high-grade explosive. Though if you want that go ahead. Simply fill it with one or more tungsten metal cylinder and make a Kinetic Missile.
Mach 25 is about 8.5km/s so this spaceship will need to slow down enough that it simply does not explode in the atmosphere. As the ship itself can be a delivery mechanism, one method would be to fire smaller propelled rockets in the counter trajectory. The delta-v budget to counter 6000km/s is roughly the budget needed to launch a rocket from earth to the moon. (Irony abounds).
The tech to build such a rocket has existed since the early 1970's with a Saturn V. Obviously the rocket will need to be resized as not all the energy needs to bleed off, and the payload is about twice as massive as the lunar module+lander at ~16000kg vs a kinetic missile at ~33110kg (given a 6.1m Height 0.3m radius cylinder ~1.72m^3 of tungsten). With a slower lunar-earth approach the rocket can be significantly less powerful. Although this does jeopardise the success of such a vehicle attacking.
The kinetic missile literature indicates that a strike from orbit could occur anywhere on the surface within about 12-15minutes. As the spaceship is already moving at speed and presuming that suitable course corrections had already occurred, the optimal firing area will be close and aligned. Additionally as the ship is already moving at speed, there is no time required for acceleration (other than the burn to slow for atmospheric entry), this would drop the target time down to atmospheric entry time at top-speed + time to travel from moon.
The fastest known meteriod traveled at about 72km/s through the atmosphere exploding a little too soon at 100km above the ground. Most asteroids average ~20km/s in atmosphere. The Atmosphere at its thickest is somewhere between 7km and 20km high, although the top of the atmosphere stands as high as 100km. Presuming the later, a tungsten rod could travel that distance in 5 seconds, but the atmosphere really is in the way. It would take about 2-4 minutes with deceleration due atmospheric compression to reach the surface.
Thus we have a window of 5minutes to 2hours for an attack once the ship becomes detectable after leaving the occluding lunar shadow.
Counter Measures and Response
It takes time to identify, verify, inform, and respond.
Some military installations with dedicated missile defense systems, if active, and linked to a hot-trigger for orbital strikes, might, be able to identify, and respond to this weapon successfully if undetected till atmospheric-entry. But due to the amount of energy in the system (20km/s is no joke), the chances are that the best that could happen is an aerial detonation, which will produce concussive shock waves that will damage and potentially demolish buildings depending on how high the detonation occurred in the atmosphere, and the relative distance to those structures.
If the Earth had orbital defense systems, it might be possible to apply the force of MegaWatt/GigaWatt lasers/masers to overheat your spaceship on its approach to Earth. It would need to overwhelm the Cyrogenic hydrogen cooling system installed in your ship. But again as stealth is no longer a requirement, this could be more formidable if the surface of the ship had been replaced by a highly reflective material. Thus most of the heat energy generated by the lasers/masers would be reflected, reducing the effectiveness of such a system. Obviously speed is critical here, an approach taking handful of minutes will have to deal with less heat than an approach taking two hours.
An ICBM usually carries large explosives and is a rocket capable of sub-orbital flight. That is it can be launched from the surface and travel to space, but does not gain sufficient speed to enter orbit, and will (usually on purpose) crash back to earth. It takes 3-5 minutes for such a craft to boost to max. velocity, and stays up to 25minutes in space. If a similar rocket were constructed with the intention of intercepting solar missiles (such as the space ship) it would need somewhere between 10 and 25 minutes to launch and position itself for intercept.
If suitably targeted such an anti-solar missile could deflect and or sufficiently damage the space-ship so as to nullify its capacity for damage. Such a missile will however be vulnerable at its zenith (low velocity or capacity for maneuvering) its effective zone of control would be some 10-15 minutes prior to intercept. This leaves several counter-counter measures available.
- use a counter missile specifically to hit the anti-solar missile.
- alter course with a significant burn (you would need to do so anyway to reduce speed) after the missile had reached a critical point in the trajectory.
- confuse the anti-solar missiles guidance systems with something approximating chafe, or other disruptive mechanism.
Arguably the slowest aspect of any counter-measure are the humans. At best 1minute from identification to response, presuming an alert, active, and responsible group. The less coherent the group, such as relying on civilian/researchers, cross-site verification, bureaucratic involvement will slow such response times down. 15 minutes might be super fast in the worst of these scenarios. Conversely a fully automated system would be able to respond within seconds (the moon is about 1.3 light-seconds away). This of course presupposes that the counter-measure systems automatic or biological are looking for and have perceived a possible issue.
- Use subterfuge to close distance.
- Once at distances where detection is definitive reduce transit time.
- Forgo cloaking on attack run - ditch the cloaking layer in favour of a highly-reflective armour. If hoping for the best, ditch the cloaking on the attack run only after observing counter measures.
- Ensure sufficient coolant is available to prevent over-heating on the attack run.
- Attempt to keep lunar-earth approach time low.
- reducing the likely-hood of response.
- reducing the likely-hood of successful intercept by anti-solar missile
- minimising the time available to laser/maser defenses.
- Have anti-solar missile counter-measures.
- Use the deceleration process to avoid anti-solar missiles.
- Do not target a defensive location with counter-measures without employing a multiple missile strategy to saturate and overwhelm those counter measures.
In short, cloaking would actually be a handicap. Yes black paint, an advanced cooling system using hydrogen and rearward radiation would minimise detection chances particular at the periphery, but will fail quickly under scrutiny. The best bet is subterfuge, then a fast attack run.