Page 94 - Intro to Space Sciences Spacecraft Applications

ULTRA- INFRA- Spacecraft Environment 81
VIOLET RED

3l’Hz 314Hr 3000Hz 300MHz 300KHz frequency
I I I I I I I
I I I I I I I
10-12m 1 nm lpm lmm lm d m 16, wavelength
Figure 4-3. Electromagnetic spectrum. The major categories of
electromagnetic radiations are indicated.

power of the body’s absolute temperature. In terms of power output per
unit area, the Stefan-Boltzmann relationship is:

where the temperature T is in OK and 0 represents the Stefan-Boltzmann
constant (o = 5.76 x 1W8 W/m2 OK4). The temperature of the sun’s photo-
sphere is approximately 6,000 OK which, when substituted into the above
relationship, results in a value E, = 74.7 x IO6 W/m2 indicating that each
square meter of the sun’s surface radiates more than 70 megawatts!

Solar Constant. We can estimate the total power output of the sun by mul-
tiplying the number just obtained from the Stefan-Boltzmann relationship
by the surface area of the sun. Using the radius of the sun at the photosphere
(R, = 696,000 km), this total energy (power) can be found from:

P, = E, x 4nR; (W) (4-3)

Assuming negligible energy losses in space, the same amount of total
energy must pass through any two spheres drawn around the sun. The
earth circles the sun at an average distance of 149.5 x IO6 km (represent-
ed by re, this has been adopted as a unit of distance measurement known
as one astronomical unit or 1 A.U.). Using equation 4-3 to equate the
energy emitted by the solar surface to that passing through a sphere of 1
A.U. radius, we get: