# Can we go to the Mars and come back safely?

#### Saint

Valued Senior Member
Can we go to the Mars and come back safely?
I read that we may be able to go there, but not able to come back to here, why?
Not enough fuel?
According to the law of physics, isn't it in vacuum, or very less resistance, an object will continue to move in straight line with constant velocity?
If the spacecraft can move towards the earth with constant velocity, why the astronaut cannot come back?

I read that we may be able to go there, but not able to come back to here, why?

You read that and there was no further explaination of the claim ?

It's an engineering problem, not a science issue.

Not enough fuel?
According to the law of physics, isn't it in vacuum, or very less resistance, an object will continue to move in straight line with constant velocity?
If the spacecraft can move towards the earth with constant velocity, why the astronaut cannot come back?
It's not about air resistance or vacuum, but about gravity. Even if there were no air, such as on the moon, if you throw something up then, in the presence of gravity then it will come back down. To go any significant distance you need to achieve a significant velocity.
To escape the pull of earth's gravity you need to achieve c.11 km/s (if launched from earth). If on Mars then to escape Mars's gravity it's c.5 km/s. So you need fuel.
A lot of it.
Either you carry that fuel with you Mars, which requires even more fuel to transport it, or you create the fuel when on Mars.

But basically it's not the interplanetary phase of the journey that's the issue, but with getting into orbit from the surface in the first place.

And as said, it's an engineering problem, not a science issue.

Correction to Sarkus' post. The interplanetary phase is an issue. To get to Mars, you need at least an additional 3 km/sec delta v in order to put yourself into the solar orbit that will intersect with Mars' orbit, and since that orbit is more elliptical than Mars' orbit, you need another change of velocity in order to match speeds with it. To return, you'll have to do this in reverse, changing from Mar's orbit to one intersecting Earth's orbit, and then matching velocity with Earth when you arrive.
On top of that, in order to make use of such a minimum velocity change trip, you have to wait until Earth and Mars are in the right positions (And contrary to what you might think, this is not when they are closest. For this to work, Mars has to be on the opposite side of the Sun from where the Earth was when you left when you arrive at Mars, and the same is true for the return trip.) So once you get to Mars, you can't just turn around and come back, you'll have to stay for months waiting for the proper return window.
Then there is the fuel issue. Say it takes X amount of fuel to make the Earth orbit to Mars orbit trip one way. It will take over 7 times that to turn it into a round trip. This is because you need to burn fuel to accelerate the fuel you'll burn later. So you add more fuel, which adds more mass, which means you need more fuel...

When space shuttle traveling in outer space, how does it navigate direction?
There is no GPS.
If you are flying to the MARS, how to make sure you are in the correct direction?
As you know, Mars is not static, it is moving too.
How do you navigate ?

Why need to burn fuel, can we use nuclear power?

When space shuttle traveling in outer space, how does it navigate direction?
There is no GPS.
If you are flying to the MARS, how to make sure you are in the correct direction?
As you know, Mars is not static, it is moving too.
How do you navigate ?
We know where the Earth is in it orbit around the Sun at any moment, we know where Mars is in its orbit and where it will be on any future date. We know what trajectory a spacecraft launched from Earth in a particular direction and speed will take, including how long it would take to reach Mar's orbital distance. We just work out when we need to leave Earth at that speed and direction in order to arrive at Mar's orbit at the the same time as Mars is at that point of its orbit. Any small course corrections can use the same method that the sailors of old did, the stars.

Why need to burn fuel, can we use nuclear power?
Even a nuclear powered spacecraft needs to carry a reaction mass ( something to throw out the back of the ship to make it move forward.)
The same 7 fold increase holds for that reaction mass. What nuclear power can do is reduce the total reaction mass needed to reach any given velocity, by increasing the rocket's exhaust velocity.
There is a catch however. For example if you doubled your exhaust velocity, you could make the round trip with the same amount of fuel as the round trip with the lower exhaust velocity. However, it takes 4 times the energy to double the exhaust velocity. If you wanted to triple the exhaust velocity, you need 9 times the energy.
As of now, we don't have any nuclear powered spacecraft that would fit the criteria needed for a manned Mars mission. (The US was working towards one in the 1970's, but funding for its development was cut.)

If the spacecraft can move towards the earth with constant velocity, why the astronaut cannot come back?
Because when he gets to Mars he has to, you know, stop.

When space shuttle traveling in outer space, how does it navigate direction?
Once in orbit it doesn't need to "navigate direction." It's in a stable orbit, and most shuttle missions just remained in that orbit the entire flight. Then when they wanted to come back they used their engines to slow down a little. That dropped the lower (into the atmosphere) which then allowed friction to slow them down the rest of the way.

To find the location to start the deorbit burn generally they use stellar navigation (looking at stars) inertial navigation (gyros and accelerometers) and even GPS, which works acceptably well in low Earth orbit.
As you know, Mars is not static, it is moving too.
How do you navigate ?
By doing the math to figure out where it will be when you get there. The orbits of the planets are VERY stable and well understood.
Why need to burn fuel, can we use nuclear power?
We don't need to burn fuel but we do need reaction mass as Janus mentioned. For slow probes we now use SEP - solar electric propulsion. We use solar panels to heat and accelerate ions of cesium, xenon, iodine, bismuth or even krypton to very high velocities. Since it takes a LOT of energy to do this we can't accelerate much at once. That means very high efficiency (lots of acceleration for a given unit of reaction mass) but very low thrust (low acceleration totals.)

For example, the Dawn space probe used an ion engine - that enabled it to visit two different asteroids and take measurements before it ran out of fuel. It was VERY slow to accelerate (0-60mph in 4 days) but it could thrust for a long time, and even low accelerations add up over time.

Do you mean when the space craft is in the orbit of Mars, it will revolve around the Sun due to gravity and does not need thrust?
Only when it wants to change velocity or direction, then it will need thrust?
Newton's law?