# The Naked Scientists Forum

### Author Topic: how will Earth appear through my rear view mirror ?  (Read 1155 times)

#### pippystardust

• Jr. Member
• Posts: 15
##### how will Earth appear through my rear view mirror ?
« on: 03/04/2012 00:43:08 »
If the Earth is travelling through space at about 4000 MPH   then if I launch a space ship from earth in the opposite dirrection of orbit AND..... if I   have rear view mirror  ...would the earth appear to be travelling away from me at 4000mph Plus my speed of travel in the opposite direction?  also   if i traveled on a spacecraft out of the Earth's gravitational field  and I wanted to return to Earth!  then it wouldnt be where I left it !  so do I have to hang around in space until it comes passed and then "jump" onto a planet travelling at 4000mph???

PS   If I haven;t got the orbit speed right then Im sorry  ..it doesnt matter  its a hypothetical question!  and if you are really pedantic then just imagine you are on an Earth that orbits the Sun at 4000mph  then do the maths !

#### Kalopin

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• obligated to examine the percentage of possibility
##### Re: how will Earth appear through my rear view mirror ?
« Reply #1 on: 03/04/2012 01:10:36 »
No math required, yes, you got it! :)

#### JP

• Neilep Level Member
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##### Re: how will Earth appear through my rear view mirror ?
« Reply #2 on: 03/04/2012 02:51:59 »
If the Earth is travelling through space at about 4000 MPH   then if I launch a space ship from earth in the opposite dirrection of orbit AND..... if I   have rear view mirror  ...would the earth appear to be travelling away from me at 4000mph Plus my speed of travel in the opposite direction?  also   if i traveled on a spacecraft out of the Earth's gravitational field  and I wanted to return to Earth!  then it wouldnt be where I left it !  so do I have to hang around in space until it comes passed and then "jump" onto a planet travelling at 4000mph???

Yep, it'll look like it's moving away from you at your speed + whatever it's moving.  Just like if you pass a car driving towards you on the highway, it looks like it's moving away from you in your rearview mirror at the sum of your two speeds.  (You wouldn't be able to just add the values due to relativity if you were traveling at nearly the speed of light past the earth, but 4000 mph is really far from the speed of light).

As for meeting up with the earth again, you'd have to time it so you arrived at the right point in it's orbit when it was there.  This is something the folks at NASA get paid big bucks to solve, so it's not easy.  Then ideally you'd match speeds with it so you could orbit and land.

#### CliffordK

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##### Re: how will Earth appear through my rear view mirror ?
« Reply #3 on: 03/04/2012 06:10:26 »
You can't really "hang out" in a point of space as you would find yourself having to counteract the gravity of the sun, and using fuel to keep still.

A large portion of planning is to use natural movements of the planets as part of the acceleration of the rockets, and procedures such as aerobraking for deceleration of the rockets.

Because, the earth turns west to east, most rockets are also launched west to east to take advantage of the Earth's rotational velocity of 1674 kph.  Thus Cape Canaveral is in Florida, and not California (also launching over the ocean to avoid populated areas).

Earth orbits the sun at about 29.78km/s, or about 107,000 kph, or 66,000 mph.

It is much better to launch in the orbital direction of the Earth, to use the orbital speed as part of one's initial velocity.  To launch in the solar plane, one just has to wait for the part of the year that Earth's movement is in the right direction.

Likewise, when returning to Earth, it would be easier to catch it more or less in the same direction as the orbit, or an intersection of that movement.

Going in the opposite direction of Earth's orbital movement, one would have to conduct an aerobraking maneuver at greater than Mach 100 which would be very difficult, and dangerous.  It would be far preferable to time the return so that one would be moving in the same direction of earth, and catch it from ahead, or behind.

#### The Naked Scientists Forum

##### Re: how will Earth appear through my rear view mirror ?
« Reply #3 on: 03/04/2012 06:10:26 »