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Comment by masklinn | Hacker News Reader

> I'm talking about just flying the entire Orion ship to Earth, loaded with propellant bombs.

Er… yes?

> Sure, you can try to take it out during the journey, but that may be complicated by it being the highest g ship in existence for the Mars-Earth distance.

That's not really a complication, it's still taking months to make the trip, and while it might be "the highest g ship in existence" that's linear acceleration, it's not a nimble ship performing a tailside in space to avoid projectiles.

> Once/if it arrives, you have the same strategic problems you have if you build it in orbit.

With the pretty damn big difference that you've had several months head start knowing the think was incoming at high speed instead of it just dropping of your head more or less instantly.

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Since Orion doesn't need to accelerate constantly, it's in principle quite possible to turn it and fire bombs in the direction of any interceptor, and the thick pusher plate makes it actually pretty durable from some aspects. What happens when two Orion ships duel, I don't know, but shooting it down isn't quite as simple as for most other spacecraft.

> Since Orion doesn't need to accelerate constantly

If it's coasting along it's significantly increasing transit time and thus available window to consider and prepare counter-measures.

> it's in principle quite possible to turn it and fire bombs in the direction of any interceptor

Sure but that means less payload to deliver and even more time for the target to prepare counter-measures.

> and the thick pusher plate makes it actually pretty durable from some aspects.

No matter how thick your pusher plates are they're not going to stand up to the kind of kinetic impact involved.

Er… yes?

Sorry, I didn't understand why you were talking about bothering to do that and then crashing it into the atmosphere.

If you're crashing it into the atmosphere you're not delivering anything, the ship and its payload will just disintegrate, possibly bouncing back in the process. Even if the payload does detonate on impact, the only purpose an explosion at atmospheric boundaries would have would be EMP.

Indeed, I didn't understand why you thought I would suggest something pointless. I wasn't suggesting it.

If you want to argue that crashing into the atmosphere would be an inevitable result of the trip, okay, but I don't understand what else it would have to do with my initial comment.

> If you want to argue that crashing into the atmosphere would be an inevitable result of the trip

That's not what I'm arguing FFS, I'm saying that'd be the result of going with the fastest possible trip, which would still leave the target significant time to set up countermeasures.

Here's the rest of the paragraph of my original answer since you apparently stopped at the first period:

> If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss.

The point is that no matter how you slice it, "the difference between building on Mars and building it in orbit" is the target gets a warning several months in advance in the former case, a few minutes or seconds in the latter. One gives ample opportunity to set up and deploy countermeasures, the other not so much.

So why not build the countermeasure before you build it in Earth orbit? what's the difference then.

[deleted some pointless back and forth]

Because there's no way you'll be able to detect the launch and deploy your countermeasures, even assuming said countermeasures actually work which is unlikely: it's doubtful we can counter ballistic missiles as-is, a payload starting from orbit can launch much closer to the target (rather than half a world away) and is not fighting gravity for half the trip.

Galileo safely aerobraked into Jupiter at 47km/s. It takes one hell of a heat shield, but it can be done.

> It takes one hell of a heat shield

That's quite the understatement, the galileo probe was half heatshield by mass. And it took a 230g hit to the face.

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