Space Environment 1

7/27/2019 Space Environment 1

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Corso di Laurea Specialistica in Ingegneria AerospazialeOrientamento: SPAZIO

LEZIONE N.5

The space environment and

.



Space operations and

spacecraft life cycleSpacecraft operation is characterized by its remoteness

from the Earth and thus the loss of the Earth’s protectiveshield, namely the atmosphere. This atmosphere evidently

provides a suitable stable environment in which the humanspecies has been able to evolve. Coupled with thegravitational force of the Earth, “the one-g environment”, itprovides familiarity in design and its removal hassignificant and sometimes unexpected implications.

S. Corpino - SISTEMI SPAZIALI 2

Space travel is hampered not only by the difficulty ofgetting a spacecraft into orbit, but also by the fact thatthe spacecraft must be designed to operate in environmentsthat are quite different from those found on the Earth’ssurface.

The different phases in the life of a space vehicle, namelymanufacture, pre-launch, launch, space operation, all havetheir own distinctive features. Although a space vehiclespends the majority of its life in space, it is evident thatit must survive the other environments for complete success.



Where is space?

If space is a place, where is it?

We know space begins somewhere above our heads, but how far?

If you get into a powerful jet fighter plane and push the envelope ofits ability, you can barely make it to a height where the sky takeson a purplish colour and stars become visible in the light of day.But even then, you are not in space.

Only by climbing aboard a rocket you can escape entirely aboveEarth’s atmosphere into the realm we normally think of as space*.


You can be addressed as an “astronaut” only if you get higher than100 km.

The line between where the atmosphere ends and space begins is by nomeans clear. There is no universally accepted definition of preciselywhere space begins.

For our purposes, space begins at the altitude where an object inorbit will remain in orbit briefly (only a day or two in some cases)before the wispy air molecules in the upper atmosphere drag it backto Earth. This occurs above an altitude of about 130 km.

* In June 21st

2004 the Space Ship One demonstrated that you can get to space with a vehicle other than a rocket,but in any case you can not be put into orbit.



The atmosphere and above…

The Earth is characterized by:

• Magnetic field

• Gravitational field

• Presence of the atmosphere(composition, density, and

pressure)


Going into space means to loosethe protective shield

represented by those factors,and at the same time it allows

us to get rid of them.

Only one space environment ormore?



What is in space?

Space is a perfect vacuum?

NOThe type of “space” that is encountered by an orbiting spacecraftmay contain significant amounts of

neutral molecules,

charged particles,


µm-sized particulates,and electromagnetic radiation.

Each of these environments has the potential to cause severeinteractions with spacecraft surfaces and/or subsystems and may,if not anticipated, severely impact mission effectiveness.

Studies indicate that approximately 25% of all spacecraft failuresare related to interactions with the space environment.

NASA, ESA, ISO and United Nations have recognized the importanceof the field of space environment and are dealing with it under

official programs.



The space environment

These lessons seek to bridge the gap between space physics andastronautical engineering by presenting an introduction to the spaceenvironment with an emphasis on those facets of the environment thatmay degrade spacecraft subsystems.

The objective is to obtain an understanding of the relationship betweenthe space environment and spacecraft, or space instruments, operatingprinciples, and design alternatives.

The first roblem encountered in the stud of s ace environment effects


is that of defining the various environments. For our purposes, we willgroup space environment effects into five categories:

PLASMA

NEUTRAL

MMOD

RADIATION

VACUUM

SPACECRAFT



The space environment

PLASMA

NEUTRAL

RADIATION Total Dose Effect (Electronics degradation, Crew safetyhazard), Single Event Effects (upset, latchup, burnout)

Spacecraft charging (shift in electrical potential),

Electrostatic discharging (dielectric breakdown,electrostatic discharge), Enhanced sputtering, reattraction

of contamination

Mechanical effects (Aerodynamic drag, Physical sputtering),


MMOD

VACUUM Pressure differentials, Solar UV degradation, Contamination

em ca ec s a om c oxygen a ac , spacecra g ow

MMOD: MicroMeteoroid/Orbital Debris.

Hypervelocity impact damage

Depending on the spacecraft orbit, the magnitude of these interactionsmay range from negligible to mission threatening.

Synergistic interactions between the various environments could resultin a total degradation that is worse than the sum of the parts.



The Solar System




The Solar System

The Sun has the biggest effect on the space environment of ourinterest.

The Sun is just one small, yellow star out of billions in the galaxy.Fuelled by nuclear fusion, the Sun combines 600 million tons ofhydrogen each second.

We are most interested in two by-products of the hydrogen fusionprocess:

Electroma netic radiation


Charged particles



Electromagnetic radiation

The energy released by nuclear fusion isgoverned by Einstein’s famous formula:

2mcE =

This energy is primarily in the form of electromagnetic radiation.

We classify the waves of radiant energy in term of wavelength λ , whichrepresents the distance between wave crests. The range of all possiblewavelength forms the Electromagnetic spectrum.

The EM spectrum spans from high energy gamma-rays at one end to long-


wave eng ra o waves a e o er. ra a on waves move a espeed of light (about 300000 km/s).

Solar radiation can be both helpful and harmful to spacecraft andhumans in space.



Electromagnetic spectrum


EM radiation from space is unable to reach the surface ofthe Earth except at a very few wavelengths, such as thevisible spectrum, radio frequencies, and some ultravioletwavelengths. Astronomers can get above enough of theEarth's atmosphere to observe at some infrared wavelengths

from mountain tops or by flying their telescopes in anaircraft. Experiments can also be taken up to altitudes ashigh as 35 km by balloons which can operate for months.Rocket flights can take instruments all the way above theEarth's atmosphere for just a few minutes before they fallback to Earth, but a great many important first results inastronomy and astrophysics came from just those fewminutes of observations. For long-term observations,

however, it is best to have your detector on an orbitingsatellite... and get above it all!



Waves

f c=λ

• Wavelength λ : distance from crest to crest [m]

• Frequency f: number of waves/cycles that go by in one second [Hz](10 Hz means 10 cycles in one second)

• Speed of light in vacuum, c 0 : 3 x 10 8 m/s

• Speed of light, c: c 0 /n

The following relationship exists:


Energy in EM radiation is related to the number of waves that hityou over a given time (ten waves hitting in one second – 10 Hz –will deliver twice the energy of five waves – 5 Hz).

This energy relationship can be expressed as: f hQ ⋅=

Where:Q: energy [J]

f: frequency [Hz]

h: Planck’s constant = 6.626 x 10 -34 Js



Waves’ energy

f hQ ⋅=

This equation shows more energy is available at higher frequencies,so higher frequency waves have more energy than lower frequency

waves. That is why we can walk through radio waves all day long, butone large dose of gamma rays can be lethal.

Everything above the temperature of absolute 0 K, emits EM radiation.All objects will emit energy at different wavelengths depending on


their material properties and temperature. The classical explanationfor this phenomenon is that thermal radiation begins withaccelerated, charged particles near the surface of an object. Thesecharges then emit radiation like tiny little antennas. The thermallyexcited charges can have different accelerations, which explains whyan object emits energy at many different wavelengths.

Max Planck refined this explanation and helped us to usher in thefield of quantum physics. He postulated that energy is emitted intiny bundles or “quanta” called photons. He has been able to developa model which related the amount of power given off at specificwavelengths as a function of an object’s temperature



Black body radiation

Planck used a black body as his perfect case.

A black body is an object that absorbs and re-emits all of the radiation thatstrikes it.

The hotter the object, the more EM radiation it will emit at shorterwavelengths, which also means higher frequencies and higher energy.

The peak output for the Sun is in the visible region of the EM spectrum.

Wien’s displacement law relates the wavelength (or the frequency) of maximumoutput for a given black-body radiator to its temperature:


[ ] [ ][ ]K T

K mm2898=µ λ

For the Sun, at 6000 K, λm

is 0.483 µm, whichis in the middle of the range of visible light(0.39-0.74 µm).

Using Wien’s law, we can determine the best frequency to use to see aparticular subject.

We can also determine the total power output of a body radiation, by means ofthe Stefan-Boltzmann equation:

4T E ⋅⋅= σ ε E = energy per square meter [W/m 2 ]

ε = emissivity [0< ε<1]

σ = Stefan-Boltzmann constant = 5.67x10 -8 W/m2K4

T = temperature [K]

This equation estimates the totalamount of power available overall wavelengths for a specifiedtemperature.



The solar spectrum

The Solar Spectrum approximates to that of a black body.

It is evident that it departsfrom the black-body spectrumat some wavelengths, thesediscrepancies arising in thesolar atmosphere. There aretwo primary regions of this.The lower, or chromosphere,extend to a few thousand


kilometres above thephotosphere and it is a regionof increasing temperature(peaking at 100000 K). It isresponsible for enhanced UVemission. The upperatmosphere, called the corona,

becomes more tenuous andextend to several solar radii.Its nominal temperature isaround 2x10 6 K and it emitssubstantial amounts of X-rays.

The nominal release of energy from the Sun is at a rate of 3.85x1026

W.



The radiation environment

One of the Sun’s main outputs is EM radiation. Most of thisradiation is in the visible and near-infrared parts of the EMspectrum. A smaller, but significant part of the Sun’s output isat other wavelengths of radiation, such us X-rays and γ-rays.

Spacecraft and astronauts are well above the atmosphere, so theybear the full brunt of the Sun’s output.


The effect on a spacecraft depends on the wavelength of theradiation. In many cases, visible light hitting the spacecraftproduces electrical energy through solar cells. This is a cheap,abundant and reliable source of power for a spacecraft. But thisradiation can also lead to several problems for spacecraft:

• Heating on exposed surfaces• Degradation or damage to surfaces and electronic components

• Solar pressure




The infrared (or thermal) radiation a spacecraft endures, leads to heatingon exposed surfaces which can be either helpful or harmful to thespacecraft, depending on the overall thermal characteristics of itssurfaces.

Electronics in a spacecraft need to operate at about normal roomtemperatures (20 °C). The vacuum of space is normally very cold (-200 °C).

In some cases, the Sun’s thermal energy can help to warm electroniccomponents. In other cases, this solar input, in addition to the heat


,

spacecraft too hot.We must design the spacecraft’s thermal control system to moderate itstemperature.

Normally, the EM radiation in other regions of the spectrum have littleeffect on a spacecraft. However, prolonged exposure to ultravioletradiation can begin to degrade spacecraft coatings. This radiation isespecially harmful to solar cells, but it can also harm electroniccomponents, requiring them to be shielded or hardened.

In addition, during intense solar flares, bursts of radiation in the radioregion of the spectrum can interfere with communication equipment onboard.




When you hold your hand up to the Sun, all you feel is heat!

However, all that light hitting your hand is also exerting a verysmall amount of pressure.

One of the way we can look at the EM radiation is in terms oftiny massless bundles of energy, called photons, which move atthe speed of light. These photons strike your hand, exertingpressure similar in effect to atmospheric drag. This solar


pressure s muc , muc sma er an rag.

The solar pressure is only 5 N for a square kilometre of surface.

While that may not sound like much, over time this solar pressurecan disturb the orientation of spacecraft and cause them to pointin the wrong direction.

On the other hand, it is also possible to use the solarpressure’s effects to sail around the solar system.



Charged particles

Scientists model atoms with three building-block particles:protons, electrons and neutrons. Protons and electrons areknown as charged particles, positive and negative respectively.

Perhaps the most dangerous aspect of the space environment isthe pervasive influence of charged particles.

Three primary sources for these particles are:


Solar Particle Events (SPEs), which are energetic protonsthat are emitted during coronal mass ejections.

Galactic Cosmic Rays (GCRs), which consists of energeticnuclei, mostly protons, originating outside the solar system(presumably they are produced by nova o supernova explosions,or are particles accelerated by the interstellar fields).

Van Allen radiation belts, which are energetic particles,mostly electrons and protons, confined to gyrate around theEarth’s magnetic field lines.



Charged particles:

solar wind and flaresDuring the fusion process, intense heat is generated in the Sun’s interior (morethan 1.000.000 °C). At these temperatures, a fourth state of matter exists:plasma (the state at which atoms themselves break into their basic particles).Inside the Sun, we have a swirling hot soup of charged particles.

These charged particles don’t stay put, because all charged particles respond toelectric and magnetic fields. The Sun has an intense magnetic field, soelectrons and protons shoot away from the Sun at speeds of 300 to 700 km/s. Thisstream of charged particles flying off the Sun is called the SOLAR WIND . AtEarth the speed of the wind is 450 km/s, its density is 9 protons/cm 3 , and its


kinetic temperature is 100000 K.

Occasionally, areas of the Sun’s surface erupt in gigantic bursts of chargedparticles called solar particle events or solar flares . These flares aresometimes so violent that they extend out to Earth’s orbit. Fortunately, suchlarge flares are infrequent and concentrated in specific regions of space, sothey usually miss the Earth entirely.

The charged particles from solar wind and solar flares pose a kind of problemsto humans and machines in space.



Charged particles:

Galactic Cosmic Rays

Galactic cosmic rays (GCRs) are high-energy particles which reach the vicinity ofthe Earth from outside the Solar System. Theses particles are similar to those

found in the solar wind and flares, but they originate outside the solar system.We can say that GCRs represent the “solar wind” from distant stars. In many cases,however, GCRs are much more massive and energetic than particles of solar origin.Ironically, the very thing that protect us on Earth from these sources of chargedparticles creates a third source potentially harmful to orbiting spacecraft andastronauts the Van Allen radiation belts.


GCRs pose a serious hazard because a single particle can cause a malfunction incommon electronic components such as random access memory, microprocessors, andhexfet power transistors. When a single passing particle causes this malfunction,we call radiation effects single-event phenomena (SEP).

GCRs can also generate background noise in various satellite subsystems such assensors, infrared detectors, and components employing charge-coupled devices. Inaddition to increased noise signals, these rays create spurious events which canmasquerade as real signals. The spurious signals can affect satellite subsystemsdepending on the genuine signals’ frequency of occurrence, time duration, andrepetition, as well as the sophistication of the sensor system.



Charged particles:

single-event phenomenaA galactic cosmic ray loses energy mainly by ionization. This energyloss depends chiefly on the square of the particle’s charge, Z, andcan be increased if the particle undergoes nuclear interactionswithin an electronic component. Thus, lower-Z ions deposit as muchenergy in a device as less abundant, higher-Z ions. When the GCRleaves electron-hole pairs in a depletion region of an electronicdevice, the electric field in that region sweeps up the pairs.

Single-event phenomena include three different effects in electroniccomponents:


Bitflip , or single-event upset (SEU). It neither damages the partnor interferes with its subsequent operation.

Single-event latchup (SEL). In this case, the part hangs up, drawsexcessive current, and will no longer operate until the power to thedevice is turned off and then back on. The excessive current drawn in

the latched condition can destroy the device if the power supplycannot handle the current. When latchup demands to much current forthe power supply, it may drag down the bus voltage, or even damagethe supply.

Single-event burnout (SEB). This causes the device to fail

permanently.



To evaluate the frequency of SEP for a given part, we must know three things:

• The external environment

• How the incident energy spectrum and particle intensity change as a particlepasses through the spacecraft to the sensitive device

• How the electronic device responds to ionizing radiation.

These phenomena are very difficult to be evaluated, because of the complexinteraction between the radiation environment and the device’s circuit elements.

On-orbit failure rates can be redicted onl for sin le-event u sets in memor

Charged particles:single-event phenomena


devices, with well defined sensitive volumes, in which the galactic cosmic raysproduce electron-hole pairs.

A simple, even if not very accurate, equation developed by Petersen in 1983,expresses the upset rate R as follows:

m]mbs/[picoCoulolengthpathunitperGCRthebydeposited

charge,ortransfer,energylinearthresholdtheisL

]m[areasensitivesdevice'theis

daybitperupsetofnumbertheisR

:where,105

2

210

µ

µ σ

σ L

R−

⋅=



Adams in 1987 developed a more accurate method for predicting single-eventupsets. In this method, a series of computer programs accept the followinginputs:

• Orbital parameters and epoch of interest

• Amount of shielding around the device

• Minimum charge required to upset the device

• Dimensions of a particular transistor’s sensitive volume.

After the software calculates the u set rates for each transistor the rates are

Charged particles:

single-event phenomena


combined to give an overall rate for the device in question.

In conclusion, single-event upset rates in complex devicessuch as microprocessors, or single-event latchup orburnouts in any devices, cannot be reliably predicted.We must resort to educated guesses based on acceleratorobservations and flight performance of similar devices.

Space Environment 1

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