Space enthusiasts look to the day when ordinary people, as well as professional astronauts and members of Congress, can leave Earth behind and head for a space station resort, or maybe a base on the moon or Mars. The Space Transportation Association, an industry lobbying group, recently created a division devoted to promoting space tourism, which it sees as a viable way to spur economic development beyond Earth.
The great stumbling block in this road to stars, however, is the sheer difficulty of getting anywhere in space. Merely achieving orbit is an expensive and risky proposition. Current space propulsion technologies make it a stretch to send probes to distant destinations within the solar system. Spacecraft have to follow multi-laver, indirect trajectories that loop around several planes in order to gain velocity from gravity assists. Then the craft lack the energy to come back. Sending spacecraft to other solar systems would take many centuries.
Fortunately, engineers have no shortage of inventive plans for new propulsion systems that might someday expand human presence, literally or figuratively, beyond this planet. Some are radical refinements of current rockets or jet technologies. Others harness nuclear energies or would ride on powerful laser beams. Even the equivalents of "space elevators" for hoisting cargoes into orbit are on the drawing board.
"Reach low orbit and you are halfway to anywhere in the Solar System," science-fiction author Robert A. Heinlein memorably wrote, and virtually all analysts agree that inexpensive access to low Earth orbit is a vital first step, because most scenarios for expanding humankind’ s reach depend on the orbital assembly of massive spacecraft or other equipment, involving multiple hunches.
The need for better launch systems is already immediate, driven by private and public sector demand. Most commercial payloads are destined either for the now crowed geo-stationary orbit, where satellites jostle for elbow room 36,000 kilometers above the equator, or for low-Earth or bit, just a few hundred kilometers up. Low-Earth orbit is rapidly becoming a space enterprise zone, because satellites that close can transmit signals to desktop or even handheld receivers. Scientific payloads are also taking off in a big way. More than 50 major observatories and explorations to other solar systems’ bodies will lift off within the next decade. The pressing demand for launches has even prompted Boeing’s commercial space division to team up with RSC—Energia in Moscow and Kvaerner Maritime in Oslo to refurbish an oil rig and create a 34,000—ton displacement semi-submersible launch platform that will be towed to orbitally favorable launch sites.
The major difficulty in developing space travel is that______.

A:there is no clear destination in space B:it is too risky and expensive for people to go beyond the orbit C:the current space propulsion systems can not meet the needs for the long space travel D:there is not enough energy for spacecraft to gain velocity

Vegetarianism A strict vegetarian is a person who never in his life eats anything derived from animals. The main objection to vegetarianism on a long-term basis is the difficulty to getting enough protein, the body building elements in food. If you have ever been without meat or animal foods for some days of weeks(say, religious reasons) you will have noticed that you tend to get physically rather weak. You are glad when the fast is over and you get your reward of a succulent meat meal. Proteins are built up from approximately twenty food elements called "amino-acids", which are found more abundantly in animal protein than in vegetable protein. This means you have to eat a great deal more vegetable than animal food in order to get enough of these amino-acids. A great of the vegetable food goes to waste in this process and from the physiological point of view there is not much to be said in favor of life-long vegetarianism. The economic side of the question, though, must be considered. Vegetable food is much cheaper than animal food. However, since only a small proportion of the vegetable protein is useful for body-building purposes, a consistent vegetarian, if he is to gain the necessary 70 grams of protein a day, has to consume a greater bulk of food than his digestive organs can comfortably deal with. In fairness, though, it must be pointed out that vegetarians claim they need far less than 70 grams of protein a day. Whether or not vegetarianism should be advocated for adults, it is definitely unsatisfactory for growing children, who need more protein than they can get from vegetable source’’s. A lacto-vegetarian diet, which includes milk and milk products such as cheese, can, however, be satisfactory as long as enough milk and milk products are consumed. Meat and cheese are the best sources of usable digestible and next come milk, fish and eggs. Slow and careful cooking of meat makes it more digestible and assists in the breaking down of the protein content by the body. When cooking vegetables, however, the vitamins, and in particular the water-solube vitamin C, should be lost through overcooking. From the passage, we know that______.

A:to gain enough protein, one must consume much more vegetable food than animal food B:cooking vegetables for long time makes it more digestible C:milk is the best source of usable animal protein D:the most common deficiencies in Western diets are those of vitamins

{{B}}第三篇{{/B}}

A:to gain enough protein, one must consume much more vegetable food than animal food B:cooking vegetables for long time makes it more digestible C:milk is the best source of usable animal protein D:the most common deficiencies in Western diets are those of vitamins

Vegetarianism A strict vegetarian is a person who never in his life eats anything derived from animals. The main objection to vegetarianism on a long-term basis is the difficulty to getting enough protein, the body building elements in food. If you have ever been without meat or animal foods for some days of weeks(say, religious reasons) you will have noticed that you tend to get physically rather weak. You are glad when the fast is over and you get your reward of a succulent meat meal. Proteins are built up from approximately twenty food elements called "amino-acids", which are found more abundantly in animal protein than in vegetable protein. This means you have to eat a great deal more vegetable than animal food in order to get enough of these amino-acids. A great of the vegetable food goes to waste in this process and from the physiological point of view there is not much to be said in favor of life-long vegetarianism. The economic side of the question, though, must be considered. Vegetable food is much cheaper than animal food. However, since only a small proportion of the vegetable protein is useful for body-building purposes, a consistent vegetarian, if he is to gain the necessary 70 grams of protein a day, has to consume a greater bulk of food than his digestive organs can comfortably deal with. In fairness, though, it must be pointed out that vegetarians claim they need far less than 70 grams of protein a day. Whether or not vegetarianism should be advocated for adults, it is definitely unsatisfactory for growing children, who need more protein than they can get from vegetable source’’s. A lacto-vegetarian diet, which includes milk and milk products such as cheese, can, however, be satisfactory as long as enough milk and milk products are consumed. Meat and cheese are the best sources of usable digestible and next come milk, fish and eggs. Slow and careful cooking of meat makes it more digestible and assists in the breaking down of the protein content by the body. When cooking vegetables, however, the vitamins, and in particular the water-solube vitamin C, should be lost through overcooking. From the passage, we know that______.

A:to gain enough protein, one must consume much more vegetable food than animal food B:cooking vegetables for long time makes it more digestible C:milk is the best source of usable animal protein D:the most common deficiencies in Western diets are those of vitamins

Vegetarianism A strict vegetarian is a person who never in his life eats anything derived from animals. The main objection to vegetarianism on a long-term basis is the difficulty to getting enough protein, the body building elements in food. If you have ever been without meat or animal foods for some days of weeks(say, religious reasons) you will have noticed that you tend to get physically rather weak. You are glad when the fast is over and you get your reward of a succulent meat meal. Proteins are built up from approximately twenty food elements called "amino-acids", which are found more abundantly in animal protein than in vegetable protein. This means you have to eat a great deal more vegetable than animal food in order to get enough of these amino-acids. A great of the vegetable food goes to waste in this process and from the physiological point of view there is not much to be said in favor of life-long vegetarianism. The economic side of the question, though, must be considered. Vegetable food is much cheaper than animal food. However, since only a small proportion of the vegetable protein is useful for body-building purposes, a consistent vegetarian, if he is to gain the necessary 70 grams of protein a day, has to consume a greater bulk of food than his digestive organs can comfortably deal with. In fairness, though, it must be pointed out that vegetarians claim they need far less than 70 grams of protein a day. Whether or not vegetarianism should be advocated for adults, it is definitely unsatisfactory for growing children, who need more protein than they can get from vegetable source’’s. A lacto-vegetarian diet, which includes milk and milk products such as cheese, can, however, be satisfactory as long as enough milk and milk products are consumed. Meat and cheese are the best sources of usable digestible and next come milk, fish and eggs. Slow and careful cooking of meat makes it more digestible and assists in the breaking down of the protein content by the body. When cooking vegetables, however, the vitamins, and in particular the water-solube vitamin C, should be lost through overcooking. From the passage, we know that______.

A:to gain enough protein, one must consume much more vegetable food than animal food B:cooking vegetables for long time makes it more digestible C:milk is the best source of usable animal protein D:the most common deficiencies in Western diets are those of vitamins

Vegetarianism A strict vegetarian is a person who never in his life eats anything derived from animals. The main objection to vegetarianism on a long-term basis is the difficulty to getting enough protein, the body building elements in food. If you have ever been without meat or animal foods for some days of weeks(say, religious reasons) you will have noticed that you tend to get physically rather weak. You are glad when the fast is over and you get your reward of a succulent meat meal. Proteins are built up from approximately twenty food elements called "amino-acids", which are found more abundantly in animal protein than in vegetable protein. This means you have to eat a great deal more vegetable than animal food in order to get enough of these amino-acids. A great of the vegetable food goes to waste in this process and from the physiological point of view there is not much to be said in favor of life-long vegetarianism. The economic side of the question, though, must be considered. Vegetable food is much cheaper than animal food. However, since only a small proportion of the vegetable protein is useful for body-building purposes, a consistent vegetarian, if he is to gain the necessary 70 grams of protein a day, has to consume a greater bulk of food than his digestive organs can comfortably deal with. In fairness, though, it must be pointed out that vegetarians claim they need far less than 70 grams of protein a day. Whether or not vegetarianism should be advocated for adults, it is definitely unsatisfactory for growing children, who need more protein than they can get from vegetable source’’s. A lacto-vegetarian diet, which includes milk and milk products such as cheese, can, however, be satisfactory as long as enough milk and milk products are consumed. Meat and cheese are the best sources of usable digestible and next come milk, fish and eggs. Slow and careful cooking of meat makes it more digestible and assists in the breaking down of the protein content by the body. When cooking vegetables, however, the vitamins, and in particular the water-solube vitamin C, should be lost through overcooking. From the passage, we know that______.

A:to gain enough protein, one must consume much more vegetable food than animal food B:cooking vegetables for long time makes it more digestible C:milk is the best source of usable animal protein D:the most common deficiencies in Western diets are those of vitamins