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Posts Tagged ‘Astronaut’

As anyone who hasn’t been living under a rock knows, the International Space Station (ISS) orbits the Earth with (typically? always?) six astronauts on board. It has been doing this for something like the last fifteen years. One of the astronauts currently on board is the Disunited Kingdom’s first Government-funded astronaut, Tim Peake.

The first British person to go into space was Helen Sharman, but she went into space in a privately funded arrangement with the Russian Space Programme in 1991. Other British-born astronauts have gone into space through having become naturalised Americans, and going into space with NASA. But, Tim Peake has gone to the ISS as part of ESA’s space programme, and his place is due to Britain’s contribution to ESA’s astronaut programme. So, he is the first UK Government-funded astronaut, which is why there has been so much fuss about it in these lands.

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Official NASA portrait of British astronaut Timothy Peake. Photo Date: August 28, 2013.

Anyway, I digress. This blog is not about Tim Peake per se, or about the ISS really. I wanted this blog to be about whether Tim Peake is getting older or younger whilst in orbit. Of course everyone is getting older, including Tim Peake, as ‘time waits for no man’ as the saying goes. What I really mean is whether time is passing more or less quickly for Tim Peake (and the other astronauts) in the ISS compared to those of us on Earth.

As some of you might now, when an astronaut is in orbit he is in a weaker gravitational field, as the Earth’s gravitational field drops off with distance (actually as the square of the distance) from the centre of the Earth. Time will therefore pass more quickly for Tim Peake than for someone on the Earth’s surface due to this effect. This is time dilation due to gravity, a general relativity (GR) effect.

But, there is also another time dilation, the time dilation due to one’s motion relative to another observer, the time dilation in special relativity (SR). Because Tim Peake is in orbit, and hence moving relative to someone on the surface of the Earth, this means that time will appear to move more slowly for him as observed by someone on Earth. Interestingly (at least for me!), the SR effect works in the opposite sense to the GR effect.

Which effect is greater? And, how big is the effect?

Time dilation due to SR – slowing it down for Tim Peake

As I showed in this blog, the time dilation due to SR can be calculated using the equation

t^{\prime} = \gamma t \text{ where } \gamma = \frac{ 1 }{ \sqrt{ ( 1 -v^{2}/c^{2} )} }

If he is in orbit at an altitude of 500km (I guessed at this amount, according to Wikipedia it is 400km, but it does not alter the argument which ensues) then his distance from the centre of the Earth (assuming a spherical Earth) is 6.371 \times 10^{6} + 500 = 6.3715 \times 10^{6} metres. The centripetal force keeping him in orbit is provided by the force of gravity, and in this blog I showed that the centripetal force F_{c} is given by

F_{c} = \frac{mv^{2} }{r}

where m is the mass of the object in orbit, v is its velocity and r is the radius of its orbit.This centripetal force is being provided by gravity, which we know is

F_{g} = \frac{ GMm }{ r^{2} }

where G is the universal gravitational constant, and M is the mass of the Earth. Putting these two equal to each other

\frac{ mv^{2} }{ r } = \frac{ GMm }{ r^{2} } \rightarrow v^{2} = \frac{GM}{r}

Putting in the values we have for the ISS, where r=6.3715 \times 10^{6}, G=6.67 \times 10^{-11} and M= 5.97237 \times 10^{24}, we find that

v^{2} = 6.2522 \times 10^{7} \rightarrow v = 7.907(067129) \times 10^{3} \text { m/s} = \boxed{ 7.907(067129) \text{ km/s} }

But, this is the motion relative to the centre of the Earth. People on the surface of the Earth are also moving about the centre, as the Earth is spinning on its axis. But, we cannot calculate this speed as we have done above; people on the surface are not in orbit, but on the Earth’s surface. For something to stay e.g. 1 metre above the Earth’s surface in orbit it would have to move considerably quicker than the rotation rate of the Earth.

The Earth turns once every 24 hours, so for someone on the equator they are moving at

v_{se} = \frac{ 2 \pi \times 6.3715 \times 10^{6} }{ 24 \times 3600 } = 463.348(5554) \text { m/s}

where v_{se} refers to the speed of someone on the surface of the Earth. Someone at other latitudes is moving less quickly, at the poles they are not moving at all relative to the centre of the Earth. The speed of someone on the surface will go as v_{se} \cos (\theta) where \theta is the latitude. This is why we launch satellites as close to the Earth’s equator as is feasible; we maximise v_{se} and thus get the benefit of the speed of rotation of the Earth at the launch site to boost the rocket’s speed in an easterly direction.

The difference in speeds between the ISS and someone at the equator on the surface of Earth is therefore

7.907(067129) \times 10^{3} - 463.348(5554) = \boxed { 7.443(718574) \times 10^{3} \text { m/s} }

Referring back to my blog on time dilation in special relativity that I mentioned at the start of this section, this means that the time dilation factor \gamma, using this value of v, is

\gamma = \frac{ 1 }{ \sqrt{(1 - (v/c)^{2})} } = \frac{ 1 }{ 0.9999999997 }
(where c is, of course, the speed of light).
This value of \gamma is equal to unity to 3 parts in 10^{10}, so it would require Tim Peake to orbit for about 3 \times 10^{9} seconds for the time dilation factor to amount to 1 second. 3 \times 10^{9} seconds is just over 96 years, let us say 100 years.

The time dilation due to GR – speeding it up for Tim Peake

For GR, the time dilation works in the other sense, it will run more slowly for those of us on the Earth’s surface; we experience gravitational time dilation which is greater than that experienced by Tim Peake. In this blog here, I derived from the principle of equivalence the time dilation due to GR, and found

\Delta T_{B} = \Delta T_{A} \left( 1 - \frac{ gh }{ c^{2} } \right)

where, in this case, \Delta T_{B} would be the rate of time passing on the Earth’s surface, \Delta T_{A} the rate of time passing on the ISS,  g = 9.81 (the acceleration due to gravity at the Earth’s surface) and h is the height of the orbit, which we have assumed (see above) to be 500 km = 500 \times 10^{3}.

Plugging in these values we get that

\frac{ \Delta T_{B} }{ \Delta T_{A} } = 1 - 5.45 \times 10^{-11}

So, the GR effect is about one part in 10^{11} (100 billion). In six months, the number of seconds that Tim Peake will be in orbit is about 1.6 \times 10^{7} seconds, so a factor of about 10,000 less than for the GR effect to amount to 1 second. Tim Peake would need to be in orbit for about 5,000 years for the GR effect to amount to 1 second of difference!

Conclusions

In conclusion, the SR effect on how quickly time is passing for Tim Peake is about 3 parts in 10 billion, in the sense that it passes more slowly for Tim Peake. The GR effect is even smaller, about one part in 100 billion, but in the sense that time is passing more quickly for him. The SR ‘time slowing down’ effect is greater than the GR time ‘passing more quickly effect’, by roughly a factor of 300.

Tim Peake is therefore actually ageing more slowly by being in orbit than if he were on Earth. But, he would need to orbit for nearly 100 years for this difference to amount to just 1 second! And, none of this of course takes into account the detrimental biological effects of being in orbit, which are probably not good to anyone’s longevity!

 

 

 

 

 

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Later this year, on the 3rd of November, there will be a Solar eclipse which will pass across the Atlantic ocean, across Africa and end in Somalia. The eclipse is actually termed a hybrid eclipse, because it starts as annular (where the Moon is a little further from the Earth and so does not block out all of the Sun), but changes to a total eclipse during the event, as the distance between the Moon and the Earth gets less. The figure below shows the path of the eclipse.


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I am part of a planned expedition to go and see the eclipse in Kenya. The expedition is run by the International Space Schools Education Trust (ISSET – a registered UK charity), and is part of their Astronaut Leadership Experience. I will go as the trip’s astronomy expert, and in addition to Chris Barber of ISSET, who will organise and lead the expedition, there will be two people from NASA, Ken Ham who has commanded Space Shuttle missions, and his wife Michelle who is an astronaut trainer.

The path of the eclipse in Kenya is shown in the figure below, it passes right across Lake Turkana which is in the northern part of Kenya near its borders with Uganda and Ethiopia and South Sudan.


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This area is very beautiful and very interesting. It is part of the Great Rift Valley, and on its shores Lucy was found, the oldest humanoid ever discovered. Lake Turkana also boasts a volcanic island, which is shown in one of the slides below. The plan is to fly to Kenya on the 26th of October, and go up to the Lake Turkana region towards the end of the week for the eclipse, which is on the 3rd. Before going to the north, we will go to the Masai Mara National Park, Mount Kenya (which straddles the equator), and explore more areas of the Great Rift Valley.


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The trip is open to anyone; to find out the costs and day-by-day itinerary you can visit the website here. The costs include return (round trip) flights to Kenya, and all accommodation, food and transport whilst in Kenya. If it is anything like the week we spent in the Gobi Desert in June for the Transit of Venus, it will be a truly memorable week full of activity, learning, exploration and fun.

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On Wednesday (27th of February) Dennis Tito announced ambitous plans to send human beings to Mars by 2018. The idea is to send a couple on what is called a free return trajectory trip to Mars. That is to say, the rocket would travel to Mars, then use a swing around the other side of Mars to come back to Earth. There are no plans in this mission to land on Mars, or even to go into orbit about the red planet. But still it is to me a very exciting project.


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For those of you who have watched the episode 1968 of the series From The Earth To The Moon, which I blogged about here, you may recall that a free return trajectory is mentioned by Frank Borman’s wife when she is worried about her husband as he makes his way towards the Moon. The Apollo 8 mission did not use a free return trajectory, because NASA wanted to put the command module into orbit about the Moon.

No such manouevre is planned for this proposed trip to Mars, the space craft would simply sling-shot around Mars and come back towards Earth. The entire trip would last 501 days, and because of the necessary favourable alignment required, another opportunity for such a short sling-shot mission would not arise until 2031.

Below is an artist’s impression of the proposed rocket and capsule. The company Tito is setting up to operate this mission is called Inspiration Mars Foundation. They say they will only be using already existing technology, which they will buy from NASA and other aerospace companies and manufacturers. Should this trip take place, it will be the first time a private organisation has accomplished something in space not previously accomplished by a Government agency.


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I mentioned this news item to my 11-year old daughter a few days ago, and she asked me what the point of going was if they did not intend to land on Mars. I wonder how many people share her point of view. To me, although I hope we will of course one day land on Mars and explore it in a way robots and rovers cannot, even the idea of going on a trip to Mars is an exciting one. Yes we have sent dozens of probes to Mars, but as Apollo 8 showed the World, human beings going somewhere we have not been before can inspire a new generation, and give us a perspective on our exploring that a space probe cannot.

Only two people will go on this proposed journey. Inspiration Mars Foundation say they are looking for a “mature” couple, so that they know each other well from having spent decades together, and can give each other companionship during this long trip. So, who said adventure is only for the young?

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