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The Interstellar and Interplanetary Medium
Published in Ivan G. Draganić, Zorica D. Draganić, Jean-Pierre Adloff, Radiation and Radioactivity on Earth and Beyond, 2020
Ivan G. Draganić, Zorica D. Draganić, Jean-Pierre Adloff
A main reason for the present interest in cometary studies is the widely accepted belief that they contain the best preserved primordial material of presolar nebula. According to the Dutch astronomer Jahn H. Oort, as many as 100,000 million comets are circulating at the periphery of the Solar System, in a cold reservoir that remains from the time the Sun and the planets were formed some 4600 million years ago. Oort’s cometary cloud has not yet been observed, but astronomers do not doubt that it exists at a distance of 30,000 to 100,000 astronomical units from the Sun. An astronomical unit represents the distance between the Sun and the Earth and corresponds to about 150 million kilometers. It is surmised that occasional disturbances, such as the gravitational tug of a distant passing star, could “snatch” comets and force them into a cigar-shaped orbit with Oort’s cloud at one end and the Sun at the other. Such an event would offer scientists a fine opportunity for examining relics which date from the birth of the Solar System.
The Earth–Sun Relationship
Published in Matt Fajkus, Dason Whitsett, Architectural Science and the Sun, 2018
Figure 2.1 shows several images of the varying surface activity on the sun. The mean sun-Earth distance is approximately 150,000,000 km, varying at any given time by an average of 1.7% due to the elliptical eccentricity of the earth’s orbit. This distance is known as an astronomical unit and is the basis for measuring distances in the solar system. At a diameter of 1,390,000 km, the sun is so large that, even at this extraordinary distance from Earth, its angular diameter is still 0.53° (Figure 2.2). Because the sun is so far away and its angular diameter relatively small, we assume for the purposes of solar geometry calculations that it is a point source of light with all of its rays arriving at the surface of the Earth parallel to one another. Table 2.3 shows important distances in the solar system and on Earth in relation to one another.
Case Study: Interplanetary Networks
Published in Aloizio Pereira da Silva, Scott Burleigh, Katia Obraczka, Delay and Disruption Tolerant Networks, 2019
Aloizio P. Silva, Scott Burleigh
Communication in space moves slowly compared to communication on Earth. There are several reasons for this: Distance: On Earth, people are only a fraction of a light second apart, making Earth communication nearly instantaneous over the Internet. The most common unit of measurement for distances within the solar system is the astronomical unit (AU). One AU equals the average distance from the sun to Earth, about 150,000,000 km. Another option to indicate distances within the solar system is terms of light time, which is the distance light travels in a unit of time at the rate of 300,000 km per second. As an object moves farther out into space, there is a delay of minutes or hours in communicating with it because light has to travel millions of miles, instead of thousands of miles, between transmitter and receiver.Line of sight: Electromagnetic transmission, including light, generally travels in a straight line. The rays or waves may be diffracted, refracted, reflected, or absorbed by atmosphere and obstructions with material and they generally cannot travel over the horizon or behind obstacles. Anything that blocks the space between the signal transmitter and receiver can interrupt communication.Mass: The cost of launching a satellite increases with its mass. The high-powered antennas that would improve communication with deep space probes may be too heavy to send on a cost-constrained space mission.
How AD can help solve differential-algebraic equations
Published in Optimization Methods and Software, 2018
John D. Pryce, Nedialko S. Nedialkov, Guangning Tan, Xiao Li
In the DETEST model the time unit (TU) is 100 days. Distance is measured in astronomical units (AU), where 1 AU is the mean radius of the earth's orbit. The task is to integrate from given initial values up to t=20 TU; at tolerance we get agreement with DETEST's reference solution to around 12 decimal places.