# Impossible To Navigate In Space Traveling At Near Light Speed?

#### rhinefire

DP Veteran
Spacecraft navigation (in the solar system) requires that one has a mathematical model for how gravity affects the motion of a spacecraft (and in the real world one must consider other forces right down to the pressure of sunlight). Part of this model must be the positions at a given moment of every body in the solar system that can have an effect large enough to significantly affect the spacecraft (i.e. how accurate do you want the navigation to be). In practice this is done by simply computing at a given instant, the force on the spacecraft from all such bodies and effects. This is basically application of the force equation for each body then add them up. Compute the position numerically from that a very short time later. This gives a new position, new forces (because everthing has moved), add them up and move the computed spacecraft position, then repeat the process for whatever span of trajectory you need to consider. This produces a predicted future trajectory, and it is all done numerically, step by step. This is a process of numerical approximations. It is analgous to estimating the area under a curve on a piece of paper by putting a series of adjacent boxes under it like a bar-graph, multiplying height by width for each and adding them all up. Use more boxes and you get a more accurate estimate. Then one waits for a time to see how accurate the predition was by observing the actual trajectory (using sophisticated instrumentation). If the agreement is good, one can rely on it. And the computation can be updated from that point. If it isn't, one looks at the navigation observations and checks such things as forces from gas jets on the spacecraft, solar flares, or anything else that might be causing trouble that one left out or got wrong. In this sense, celestial navigation is exacly analogous to any other navigation problem: observe positions over time, in the sea for example estimate/model currents, winds, engine power, predict the future positions, and refine them as needed.[/FONT]

After reading the above response to "How do we navigate a craft in space?" I thought fine but what about factoring velocity? If we are tp reach the stars the speed will have to be "substantial" and to factor in all the above seems to be somewhat of a challenge. Thoughts?

#### Rexedgar

##### Yo-Semite!
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Extend GPS satellites in ever expanding concentric coverage?

#### Phys251

##### Purge evil with Justice
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Short answer: If we can get to the moon using numerical arithmetic (no, that wording is not redundant) with less computational power than that of a first-gen iPhone, then we develop computers that could someday let us travel not much less than the speed of light.

A far bigger concern I have is all the space debris. Slamming into a 1-gram rock at half the speed of light can cause massive damage to even the most well-built spacecraft.

#### Lord Tammerlain

DP Veteran
Short answer: If we can get to the moon using numerical arithmetic (no, that wording is not redundant) with less computational power than that of a first-gen iPhone, then we develop computers that could someday let us travel not much less than the speed of light.

A far bigger concern I have is all the space debris. Slamming into a 1-gram rock at half the speed of light can cause massive damage to even the most well-built spacecraft.

I expect by the time we can develop engines that can get space craft to near light speeds, we will be able to develop energy shields as well

#### Visbek

##### Stuck In The Circle
DP Veteran
Short answer: If we can get to the moon using numerical arithmetic (no, that wording is not redundant) with less computational power than that of a first-gen iPhone, then we develop computers that could someday let us travel not much less than the speed of light.

A far bigger concern I have is all the space debris. Slamming into a 1-gram rock at half the speed of light can cause massive damage to even the most well-built spacecraft.
I concur. Navigation is a difficult but solvable problem -- assuming we have sufficiently accurate information on the positions of objects in transit.

Debris is one major issue. Energy is another; the amount of energy required to accelerate a spaceship to near-light speed would be incredible. Deceleration is another issue; if it takes you, say, 2 years to accelerate to 90% of c, you'll have to spend another 2 years decelerating. Another issue is the human lifespan; we'd probably be limited to trips maybe... 3-10 parsecs away? Very rough unscientific guess. If it takes us 30 years to get anywhere, that's a loooong time to be in space, lots of time for something to go wrong, and you're probably not coming back.

By the way, the fastest man-made object so far is the Parker solar probe, which travels at 430,000 mph, or 192,227 meters per second. Light travels at 299,792,458 meters per second. So, we are maybe 0.07% of the way there....

#### Phys251

##### Purge evil with Justice
DP Veteran
I concur. Navigation is a difficult but solvable problem -- assuming we have sufficiently accurate information on the positions of objects in transit.

Debris is one major issue. Energy is another; the amount of energy required to accelerate a spaceship to near-light speed would be incredible. Deceleration is another issue; if it takes you, say, 2 years to accelerate to 90% of c, you'll have to spend another 2 years decelerating. Another issue is the human lifespan; we'd probably be limited to trips maybe... 3-10 parsecs away? Very rough unscientific guess. If it takes us 30 years to get anywhere, that's a loooong time to be in space, lots of time for something to go wrong, and you're probably not coming back.

By the way, the fastest man-made object so far is the Parker solar probe, which travels at 430,000 mph, or 192,227 meters per second. Light travels at 299,792,458 meters per second. So, we are maybe 0.07% of the way there....

To paraphrase Carl Sagan, if we do not blow ourselves up, we may be only a few centuries away from star travel to nearby star systems.

But long-distance travel, even to not-so-distant galaxies such as Andromeda, is something that I think is a long, long ways away. For one thing, due to time dilation at such high speeds, a flight to Andromeda would almost certainly be a one-way trip. Maybe even a multi-generational trip, considering that it's currently on the order of a light-century away from Earth.

#### rhinefire

DP Veteran
How do the communicate with Earth at such far distances? Answer: They don't.