Apa Itu Relativitas Umum Einstein?

Banyak prediksi relativitas umum yang berbeda dengan prediksi fisika klasik, utamanya prediksi mengenai berjalannya waktu, geometri ruang, gerak benda pada jatuh bebas, dan perambatan cahaya.

Contoh perbedaan ini meliputi dilasi waktu gravitasional, geseran merah gravitasional cahaya, dan tunda waktu gravitasional. Prediksi-prediksi relativitas umum telah dikonfirmasikan dalam semua percobaan dan pengamatan fisika.

Walaupun relativitas umum bukanlah satu-satunya teori relativistik gravitasi, ia merupakan teori paling sederhana yang konsisten dengan data-data eksperimen. Namun, masih terdapat banyak pertanyaan yang belum terjawab.

Secara mendasar, terdapat pertanyaan bagaimanakah relativitas umum ini dapat digabungkan dengan hukum-hukum fisika kuantum untuk menciptakan teori gravitasi kuantum yang lengkap dan swa-konsisten.

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Kunjungi Juga:

Memahami Relativitas Umum Einstein

Sumber: 

The University of Cambridge

Wikipedia

To Be Continued

Mekanika Orbit: Hukum-Hukum Astro Dinamika

"Hukum-Hukum Astro Dinamika Akan Menjadi Dasar Bagi Pengembangan IPTEK Antariksa Masa Depan Umat Manusia" 

*Arip Nurahma*

The fundamental laws of astrodynamics are Newton's law of universal gravitation and Newton's laws of motion, while the fundamental mathematical tool is his differential calculus.

Standard assumptions in astrodynamics include non-interference from outside bodies, negligible mass for one of the bodies, and negligible other forces (such as from the solar wind, atmospheric drag, etc.). More accurate calculations can be made without these simplifying assumptions, but they are more complicated. The increased accuracy often does not make enough of a difference in the calculation to be worthwhile.

Kepler's laws of planetary motion may be derived from Newton's laws, when it is assumed that the orbiting body is subject only to the gravitational force of the central attractor. When an engine thrust or propulsive force is present, Newton's laws still apply, but Kepler's laws are invalidated. When the thrust stops, the resulting orbit will be different but will once again be described by Kepler's laws. The three laws are:

1. The orbit of every planet is an ellipse with the sun at one of the foci.
2. A line joining a planet and the sun sweeps out equal areas during equal intervals of time.
3. The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits.

Escape velocity

The formula for escape velocity is easily derived as follows. The specific energy (energy per unit mass) of any space vehicle is composed of two components, the specific potential energy and the specific kinetic energy. The specific potential energy associated with a planet of mass M is given by

while the specific kinetic energy of an object is given by

Since energy is conserved, the total specific orbital energy

does not depend on the distance, r, from the center of the central body to the space vehicle in question. Therefore, the object can reach infinite r only if this quantity is nonnegative, which implies

The escape velocity from the Earth's surface is about 11 km/s, but that is insufficient to send the body an infinite distance because of the gravitational pull of the Sun. To escape the solar system from the vicinity of the Earth requires around 42 km/s velocity, but there will be "part credit" for the Earth's orbital velocity for spacecraft launched from Earth, if their further acceleration (due to the propulsion system) carries them in the same direction as Earth travels in its orbit.

Formulae for free orbits

Orbits are conic sections, so, naturally, the formula for the distance of a body for a given angle corresponds to the formula for that curve in polar coordinates, which is:

.

The parameters are given by the orbital elements.

Circular orbits

Although most orbits are elliptical in nature, a special case is the circular orbit, which is an ellipse of zero eccentricity. The formula for the velocity of a body in a circular orbit at distance r from the center of gravity of mass M is

where G is the gravitational constant, equal to

6.672 598 × 10⁻¹¹ m³/(kg·s²)

To properly use this formula, the units must be consistent; for example, M must be in kilograms, and r must be in meters. The answer will be in meters per second.

The quantity GM is often termed the standard gravitational parameter, which has a different value for every planet or moon in the solar system.

Once the circular orbital velocity is known, the escape velocity is easily found by multiplying by the square root of 2:

Sumber: Wikipedia

SPACE POWER REACTORS Part I

"Suatu hari nanti Umat manusia akan menyinggahi tempat-tempat asing dan Nuklir akan membawa mereka"

~Arip Nurahman~

This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).

Introduction:

After a gap of several years, there is a revival of interest in the use of nuclear fission power for space missions. While Russia has used over 30 fission reactors in space, the USA has flown only one the SNAP-10A (System for Nuclear Auxiliary Power) in 1965.

The SNAP-10A reactor. (Source:NASA)

From 1959-73, there was a US nuclear rocket program—the Nuclear Engine for Rocket Vehicle Applications (NERVA)—focused on nuclear power replacing chemical rockets for the latter stages of launches. NERVA used graphite-core reactors, heating hydrogen and expelling it through a nozzle. Some 20 engines were tested in Nevada and yielded thrust up to more than half that of the space shuttle launchers. Since then, "nuclear rockets" have been about space propulsion, not launches. The successor to NERVA is today's nuclear thermal rocket (NTR). 

Another early idea was the US Project Orion, which would launch a substantial spacecraft from the Earth using a series of small nuclear explosions to propel it. The project commenced in 1958 and was aborted when the Atmospheric Test Ban Treaty of 1963 made it illegal, but radioactive fallout could have been a major problem. The Orion idea is still alive as other means of generating the propulsive pulses are considered.

* Radioisotope power sources have been an important source of energy in space since 1961.

* Fission power sources have been used mainly by Russia, but new and more powerful designs are under development in the USA.

After a gap of several years, there is a revival of interest in the use of nuclear fission power for space missions.

While Russia has used over 30 fission reactors in space, the USA has flown only one - the SNAP-10A (System for Nuclear Auxiliary Power) in 1965.

Early on, from 1959-73 there was a US nuclear rocket program - Nuclear Engine for Rocket Vehicle Applications (NERVA) which was focused on nuclear power replacing chemical rockets for the latter stages of launches. NERVA used graphite-core reactors heating hydrogen and expelling it through a nozzle. Some 20 engines were tested in Nevada and yielded thrust up to more than half that of the space shuttle launchers. Since then, "nuclear rockets" have been about space propulsion, not launches. The successor to NERVA is today's nuclear thermal rocket (NTR).

Another early idea was the US Project Orion, which would launch a substantial spacecraft - about 1000 tonnes - from the earth using a series of small nuclear explosions to propel it. The project was commenced in 1958 by General Atomics and was aborted in 1963 when the Atmospheric Test Ban Treaty made it illegal, but radioactive fallout could have been a major problem. The Orion idea is still alive, as other means of generating the propulsive pulses are considered.

Radioisotope Systems - RTGs

So far, radioisotope thermoelectric generators (RTGs) have been the main power source for US space work over nearly 50 years, since 1961. The high decay heat of Plutonium-238 (0.56 W/g) enables its use as an electricity source in the RTGs of spacecraft, satellites, navigation beacons, etc and its alpha decay process calls for minimal shielding. Heat from the oxide fuel is converted to electricity through static thermoelectric elements (solid-state thermocouples), with no moving parts. RTGs are safe, reliable and maintenance-free and can provide heat or electricity for decades under very harsh conditions, particularly where solar power is not feasible.

So far 45 RTGs have powered 25 US space vehicles including Apollo, Pioneer, Viking, Voyager, Galileo, Ulysses and New Horizons space missions as well as many civil and military satellites. The Cassini spacecraft carries three RTGs providing 870 watts of power as it explores Saturn. Voyager spacecraft which have sent back pictures of distant planets have already operated for over 20 years and are expected to send back signals powered by their RTGs for another 15-25 years. Galileo, launched in 1989, carried a 570 watt RTG. The Viking and Rover landers on Mars in 1975 depended on RTG power sources, as will the 900 kg Mars Science Laboratory Rover due to be launched in 2011 (the two Mars Rovers operating 2004-09 use solar panels and batteries).

The latest RTG is a 290 watt system known as the GPHS RTG. The thermal power for this system is from 18 General Purpose Heat Source (GPHS) units. Each GPHS contains four iridium-clad Pu-238 fuel pellets, stands 5 cm tall, 10 cm square and weighs 1.44 kg. The Multi-Mission RTG (MMRTG) will use 8 GPHS units producing 2 kW thermal which can be used to generate some 110 watts of electric power. It is a focus of current research and will be used in the Mars Science Laboratory, which will be a large mobile laboratory, the rover Curiosity, which is about five times the mass of previous Mars rovers.

The Stirling Radioisotope Generator (SRG) is based on a 55-watt electric converter powered by one GPHS unit. The hot end of the Stirling converter reaches 650°C and heated helium drives a free piston reciprocating in a linear alternator, heat being rejected at the cold end of the engine. The AC is then converted to 55 watts DC.

This Stirling engine produces about four times as much electric power from the plutonium fuel than an RTG. Thus each SRG will utilise two Stirling converter units with about 500 watts of thermal power supplied by two GPHS units and will deliver 100-140 watts of electric power from about 1 kg Pu-238. The SRG and Advanced SRG have been extensively tested but has not yet flown. NASA plans to use two ASRGs for its probe to Saturn's moon Titan (Titan Mare Explorer - TiME) or that to the comet Wirtanen.

Russia has developed RTGs using Po-210, two are still in orbit on 1965 Cosmos navigation satellites. But it concentrated on fission reactors for space power systems.

As well as RTGs, Radioactive Heater Units (RHUs) are used on satellites and spacecraft to keep instruments warm enough to function efficiently. Their output is only about one watt and they mostly use Pu-238 - typically about 2.7g of it. Dimensions are about 3 cm long and 2.5 cm diameter, weighing 40 grams. Some 240 have been used so far by USA and two are in shut-down Russian Lunar Rovers on the moon. Each of the US Mars Rovers which landed in 2004 uses eight of them to keep the batteries functional.

The Idaho National Laboratory's (INL) Centre for Space Nuclear Research (CSNR) in collaboration with NASA is developing an RTG-powered hopper vehicle for Mars exploration. When stationary the vehicle would study the area around it while slowly sucking up carbon dioxide from the atmosphere and freezing it, after compression by a Stirling engine.

Meanwhile a beryllium core would store heat energy required for the explosive vaporisation needed for the next hop. When ready for the next hop, nuclear heat would rapidly vaporise the carbon dioxide, creating a powerful jet to propel the craft up to 1000 metres into the 'air'.

A small hopper could cover 15 km at a time, repeating this every few days over a ten-year period. Hoppers could carry payloads of up to 200 kg and explore areas inaccessible to the Rovers. INL suggests that a few dozen hoppers could map the Martian surface in a few years, and possibly convey rock samples from all over the Martian surface to a craft that would bring them to Earth.

Both RTGs and RHUs are designed to survive major launch and re-entry accidents intact, as is the SRG.

Sources:

1. Poston, D.I. 2002, Nuclear design of SAFE-400 space fission reactor, Nuclear News, Dec 2001.
2. Poston, D.I. 2002, Nuclear design of HOMER-15 Mars surface fission reactor, Nuclear News, Dec 2001.
3. Vrillon et al, 1990, ERATO article, Nuclear Europe Worldscan 11-12, 1990.
4. US DOE web site- space applications.
5. Space.com 21/5/00, 16/6/00, 22/7/00, 17/1/03, 7/2/03.
6. www.nuclearspace.com
7. Delovy Mir 8/12/95.
8. G. Kulcinski, University of Wisconsin material on web.
9. Kleiner K. 2003, Fission Control, New Scientist 12/4/03.

10. OECD 1990, Emergency Preparedness for Nuclear-Powered Satellites.
11.  NASA web site