STEREO Orbits
Try making stereo pictures with SOHO images:
With Therese Kucera
SOHO has greatly advanced out understanding of the Sun. But there is something that even SOHO can't do; see the Sun in 3D. It would be really useful to get a stereo view of the Sun. We would be able to understand the dynamics of solar events much better. We could more accurately measure which direction Coronal Mass Ejections (CMEs) are headed, improving our ability to predict the magnetic storms they can create on Earth.
To see things in 3 dimensions you need at least two eyes. And these need to be separated by a distance comparable to the distance of the object you want to image. This results in each eye seeing a slightly different view.
Looking at things in the distance, like mountains or the Moon, doesn't give a 3D effect because their distance is too great compared to the distance between your eyes; each eye sees an identical view. But the Sun is 150 million kilometres away. To see it in 3D, your eyes need to be very far apart indeed!
In 2006 NASA will launch two identical spacecraft with the aim of imaging the Sun in true 3D. The pair of the Solar Terrestrial Relations Observatory (STEREO) spacecraft will orbit the Sun either side of the Earth to achieve the different views. I am the Deputy Project Scientist for the STEREO project.
In the picture above SOS developers Mike Cripps and Graham Colman watch the construction of one of STEREO's telescopes at Goddard Space Flight Centre in summer 2004. The telescope is called a coronagraph and is specially designed to image the Sun's atmosphere - the corona.
Above Graham is holding a part of the coronagraph. Over the centre of his eye is a dark spot. When in space, the telescope will place this spot over the Sun's disk. Blotting it out like the Moon does in an eclipse, so that the fainter corona becomes visible. The device sits inside the telescope tube at the point of focus. A central silvered cone is the same size as the solar disk. It reflects the disks light into the surrounding collar where it is absorbed. The remaining light from the corona passes through and is refocused onto the detector.
The SOHO's coronagraphs work in a similar way as shown in the above image of a CME. However the spot hides an area more than three times the diameter of the Sun's disk as can be seen in the above image. STEREO's coronagraphs eclipsing spots are much smaller so that they can see coronal features much closer to the 'surface' of the Sun.
Your challenge is to design the orbits for the STEREO spacecrafts
The spacecrafts will be launched together by one rocket. They then need to go away from the Earth in opposite directions. STEREO A will need to gain 22° ahead of the Earth each year whilst STEREO B will need to drift 22° further behind each year. After 2 years the spacecraft will be approximately 45° ahead, or behind, the Earth and be 90° apart from each other.
The way you get them to do this is to put them in slightly different orbits to the Earth.
How this is
possible?
There are three parts to designing STEREO's orbits
Part 1
Firstly you need to work out the sizes of the orbits
Remember that STEREO A spacecraft needs to drift ahead by 22° per year while the STEREO B spacecraft needs to drift behind by 22° per year.
How should the orbits of the two spacecraft look compared to the Earth? Draw a sketch of your answer.
Check your sketch here
At what distance will each spacecraft need to orbit the Sun?
For help with this click here.
To check your answers click here
Part 2
Now you need to work out how to get the spacecraft to their orbits
You may know that most orbits are not circular - they a squashed circles called ellipses. Exactly how squashed is called the orbits eccentricity. You can use elliptical orbits to more easily achieve the distancing of the spacecraft away from Earth. In fact we are going to whip the spacecraft around the Moon, using it to speed up STEREO A and to slow down STEREO B.
Click here to see an animation showing how the spacecraft will achieve their orbits either side of the Earth
In the animation the yellow arrow indicates the direction of the Sun, the central blue spot is the Earth and the orbiting white spot is our Moon.
You will see that by day 110 STEREO A is within the Earths orbit whilst STEREO B is out side of it.
Click here to see an animation showing how the spacecraft orbits will drift them away from the Earth over 2 years
This animation shows orbits of the inner solar system including Mercury, Venus, Earth & Mars together with the STEREO spacecraft orbits.
There are a few words you need to know to describe elliptical orbits. These are shown in the diagram below
You are going to launch both spacecraft from Earth at 1 AU from the Sun. You can make this the perihelion distance for one spacecraft and the aphelion distance for the other.
For which spacecraft would it be the perihelion? For which would it be aphelion? Draw a diagram of what the orbits would look like compared to the Earth's. You can draw the Earth's orbit as circular and greatly exaggerate the ellipses of the space craft.
For help with this click here
To check your answers click here
Part 3
Finally you need to work out the sizes of your elliptical orbits
What would be the perihelion and aphelion distances for each spacecraft?
What would be the eccentricity?
For
help with this click here
To check your answers click here
You have now designed the orbits for the STEREO space mission -Well done!
Designing orbits like this has enabled us to explore the solar system. You now know that we can send probes to any point we like because we can use mathematics to work it out in advance. And it's not just scientific exploration that benefits. In everyday life, you rely on precisely worked out orbits to get your TV signals, telephone calls, GPS and weather information from satellites.
I hope you enjoyed this challenge. You can follow the development of the STEREO project at stereo.gsfc.nasa.gov