into the solar system at high speed

[Ras72]

In the many movies the arrival of extraterrestrial beings or machines takes place in the form of a major ship retrofiring its main propulsion as it approaches the solar system.

In my view, a much more reasonable expectation, would be for robotic probes to evasively (out basic prudence, if for no other reason) enter our system using the sun and the more massive planets to manouver and lose speed before approaching our planet to deploy and activate on orbits closer to Earth and eventually on it. It is even plausible that the entire event would never involve any major vessel coming in our vicinity; it may not even involve any contact (manifest presence) at all.

 

This idea, after all, is neither new nor "alien"; the 1970s Daedalus study into a fusion propelled automatic mission to Barnard's star, already envisioned probes, from the ship, to arrive in the destination area years ahead. Actually all the considerations made for aliens arriving on Earth are suggestedly, equally valid with roles reversed.

Is there any merit to the scenario proposed?If each probe had a mass of 5 tons, how long would it take for it to deploy on Earth's low orbit from the moment it first enters our system travelling at 0.3c?

[Janus]

An object entering the Solar system at 0.3c would cross the orbit of Neptune in a little over 8 hrs. Any gravitational interaction with planets or the Sun would barely deflect its course let alone slow it down. It would take many many passes to slow it down by any significant amount, and it would just sail right through the system on the first pass. It would have to do most of its braking before it even entered the Solar system for it have nay chance of entering any type of orbit.

 

How far out depends on how much acceleration the probe can maintain. For instance, if the probe could maintain 10g, then it would have to start decelerating when it is ~10 days and ~270 AU away. ( ~15 times the width of Neptune's orbit.).

[Ras72]

An object entering the Solar system at 0.3c would cross the orbit of Neptune in a little over 8 hrs. Any gravitational interaction with planets or the Sun would barely deflect its course let alone slow it down. It would take many many passes to slow it down by any significant amount, and it would just sail right through the system on the first pass. It would have to do most of its braking before it even entered the Solar system for it have nay chance of entering any type of orbit.

 

How far out depends on how much acceleration the probe can maintain. For instance, if the probe could maintain 10g, then it would have to start decelerating when it is ~10 days and ~270 AU away. ( ~15 times the width of Neptune's orbit.).

And without doing any significant braking with propulsion systems, simply with orbital manouvers and aerobraking in the gas giants upper atmospheres, it would depend on the shielding. Right?

Is it possible to provide a rough estimate of how long that could take with just passive shields (best current materials tech)? If at all possible. Is it in the many years? 5? 20?

(Leaving aside that without 'exotic' shielding at 0.3c it would probably be destroyed by dust anyway)

 

 

[Janus]

And without doing any significant braking with propulsion systems, simply with orbital manouvers and aerobraking in the gas giants upper atmospheres, it would depend on the shielding. Right?

Is it possible to provide a rough estimate of how long that could take with just passive shields (best current materials tech)? If at all possible. Is it in the many years? 5? 20?

(Leaving aside that without 'exotic' shielding at 0.3c it would probably be destroyed by dust anyway)

 

It's not possible with just passive shields. Even if the planets were all perfectly lined up so that the probe passed through each one's atmosphere, it would not even come close to slowing the probe down enough. It would come in from one direction, pass through them all and keep going right out of the Solar system. You are talking about needing to shed ~2e19 joules of energy. This is almost the equivalent of a 5 gigaton nuclear bomb.

[Ras72]

As per my OP I thought that a hypothetical spaceship travelling at 0.3c and wanting to get a probe on or around Earth could do it without massive retrofiring of propulsion.

But this hinges on the probe having some unspecified, advanced shielding to make the fullest use of the gas giants upper atmospheres and to make tight turns around the sun.

So the scenario is not one susceptible to reasoned arguments.

[Janus]

As per my OP I thought that a hypothetical spaceship travelling at 0.3c and wanting to get a probe on or around Earth could do it without massive retrofiring of propulsion.

But this hinges on the probe having some unspecified, advanced shielding to make the fullest use of the gas giants upper atmospheres and to make tight turns around the sun.

So the scenario is not one susceptible to reasoned arguments.

 

The shields are not the issue. The issue is that such a probe simply couldn't lose enough speed by aerobraking, and definitely wouldn't be making tight turns around the Sun.

 

Look, your probe is moving at ~90,000 km/sec. In order for it to stay in the Solar system, it has to get its velocity down into the range of 10's of kilometers per second. If you don't do this in the first pass through the solar system, there won't be a second pass. And as I mentioned in my last post, even if all the planets were perfectly aligned so that it would pass through the atmosphere of each as it crossed the Solar system, it would come even close to slowing it down enough to keep it in the Solar system.

 

As far as tight turns around the Sun goes: To deflect the probe by as little as 1 degree, would take the full surface gravity of the Sun acting on it for ~1.5 hrs. But the probe is not going to be in the vicinity of the Sun for any where near that long. In 1.5 hrs it will traveled more than the diameter of Mars' orbit. This means that even when just skimming the surface of the Sun, the probe will have its course changed hardly at all.

 

The practical aspects of the problem don't even come into play, as the idea won't work even in theory.

[Ras72]

The shields are not the issue. The issue is that such a probe simply couldn't lose enough speed by aerobraking, and definitely wouldn't be making tight turns around the Sun.

 

Look, your probe is moving at ~90,000 km/sec. In order for it to stay in the Solar system, it has to get its velocity down into the range of 10's of kilometers per second. If you don't do this in the first pass through the solar system, there won't be a second pass. And as I mentioned in my last post, even if all the planets were perfectly aligned so that it would pass through the atmosphere of each as it crossed the Solar system, it would come even close to slowing it down enough to keep it in the Solar system.

 

As far as tight turns around the Sun goes: To deflect the probe by as little as 1 degree, would take the full surface gravity of the Sun acting on it for ~1.5 hrs. But the probe is not going to be in the vicinity of the Sun for any where near that long. In 1.5 hrs it will traveled more than the diameter of Mars' orbit. This means that even when just skimming the surface of the Sun, the probe will have its course changed hardly at all.

 

The practical aspects of the problem don't even come into play, as the idea won't work even in theory.

Ok now get it I was wrong by a few orders of magnitude

 

Thank you

[Airbrush]

An ETI capable of traveling at 0.3c can certainly also decelerate by its' own sophisticated power.