1.0Trade Study Description

1.0Trade Study Description

A satellite for the DINO team is going into a low earth orbit at the ISS level. It will be exposed to the space environment and precautions must be taken for the satellite to operate under these conditions. Research was conducted and a few concerns were found such as Atomic Oxygen, high energy protons and electrons, debris, micrometeors and UV radiation. These were weighted against each other and a plan of action was devised to take all of these into account.

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Atomic Oxygen:

Atomic Oxygen (AO) is a large concern for a satellite operating at low earth orbit. The amount of AO erosion on a surface greatly depends on the solar cycle, amount of sun exposure and angle of incidence with earth. The easiest way to deal with this problem is to have the satellite in orbit for a short span of time. DINO will be in orbit for only 6 months and therefore AO will not be a factor in construction.

High Energy Protons and Electrons:

High energy protons and electrons are a great danger to the outer exposed surfaces of satellites. A thin layer of material would stop the plasma, so it would not endanger electronics. Again, the satellite is only in orbit for 6 months and the small amount of decay caused by the protons and electrons would not be a problem.

Debris:

There is a good amount of debris in ISS orbit. The material is traveling at high speeds and can cause great damage to the satellite. The thickness should account for impacts that create a few centimeter indent. Aerogel (SiO2) would help decelerate the projectile and thus decrease the size of the indent. The MLI blanket will take care of any impacting debris.

Micrometeors:

Micrometeors are a hazard to orbiting satellites. They can strike the satellite and dent the outer material. These dents can harm the inner workings of the satellite. Aerogel (SiO2) could be used to slow down the high speed micrometeors, minimizing the dents size. The Aerogel would only need to be used if the inside of the satellite was exposed. Again the MLI blanket will stop these impacts.

Radiation:

The amount of radiation felt by a satellite in ISS orbit depends on many factors. The Solar Activity cycle and the density of the atmosphere are important factors. There is a power law function that describes the amount of radiation dependent upon the density at the specified altitude. The density raised to the negative two thirds power [rho^-(2/3)] is the amount of trapped radiation inside a spacecraft within 10%, throughout the solar cycle at ISS orbit. The relationship is shown below in figure 1.

Figure 1: Atmospheric Density vs. Trapped Dose Rate

The other important factor is the strength of the solar cycle at the time of the mission. The solar cycle is 22 years long. The strength of the sun during this cycle is determined by the amount of sunspots, which is shown below in figure 2.

Figure 2: Number of Sunspots per Year

The mission will be launched in 2005 and only last for 6 months. This period is during the solar minimum of its 23rd cycle and equates to a smaller amount of radiation on the satellite. These cycles also have a pattern and every 50 years the Maunder Minimum occurs which is due around the year 2000. These two factors joined together show that the satellite and its components will work during the mission as long as it’s enclosed and not exposed to the environment.

The forms of damaging radiation are: Trapped Protons and Ions, Galactic Cosmic Ray Ions, Solar Flare Protons and Solar Flare Heavy Ions. Solar Flare Protons and Ions only occur during the solar maximum, which is avoided during the DINO launch. Trapped Protons have higher levels during the Solar Minimum. All of these do not affect the satellite at low earth orbit as shown in figure 3 below.

Figure 3: Radiation effects at different altitudes

From this figure the only factors concerning the DINO mission are Inner Zone Electrons, Trapped Protons and Galactic Cosmic Ray Ions. The Inner Zone Electrons are the most dangerous at 20,000km where satellites do not last very long. These electrons adversely affect satellites in their range. The DINO satellite will not be at this radius and therefore won’t affect the mission as shown in figure 4.

Figure 4: InnerZoneElectronRange

Trapped Protons are another adverse affect of radiation. These also do not affect the DINO satellite because it will be operating at an orbit below 1 Earth Radius, as shown below in figure 5.

Figure 5: Trapped Proton effects at altitude

Cosmic Radiation is an atoms nucleus which has lost its electrons due to the high velocity at which they travel at. Galactic Cosmic Ray Ions intensity varies with the solar cycle as shown in figure 6.

Figure 6: Flux of Cosmic Ray Particles Varying with Solar Activity

(where LET is Linear Energy Transfer – The amount of energy transferred to a material per unit length)

The mission will occur during the solar minimum which is the maximum amount of flux. It has the most adverse affects on manned missions because the DNA is attacked as shown in figure 7.

Figure 7: The Cosmic Ray attack on Astronaut DNA

This form of radiation is not as strong as the others. The radiation will not destroy any of the circuits.

Conclusion:

The Atomic Oxygen exposure will not be a problem due to the small duration of the mission. The affect of high energy protons and electrons will also not be a factor due to the short lifespan of the satellite. The reflective MLI blanket and outer composite material will take care of the adverse affects of the micro meteors, high and low temperature reservoirs and debris in ISS orbit. Radiation is the largest concern for the DINO mission.

The best way to protect against all the radiation factors mentioned above is to increase the thickness of the aluminum wall. The thickness of .25 in. would help decrease the adverse effects of the radiation environment as shown below in figure 8.

Figure 8: The effect of shield thickness against radiation

The first line is the suggested .125 in. aluminum wall and the second line is the .25 in. aluminum wall. Due to the great decrease is radiation dose I would suggest that the larger wall be constructed, which corresponds to a radiation dose of 1.3E5 rad(Al). This smaller dose rate is worth the extra aluminum thickness, due to the adverse affects the radiation would produce inside the satellite.

References:

1.

Low& High Temperatures

2.

Collision

3.

Solar Activity Graphs of Cycle

4.

Trapped radiation and dose rate plots

5. Brownsberger, Kenneth R. and Snow, Theodore P. Universe Origins and Evolution. Wadsworth Publishing Company, BelmontCA, 1997.

6.

Damaging Radiation Forms

7.

Analyzing Damaging Radiation Forms