The following issues are involved.

1, the weight of the ship. To be used many times, the spacecraft must have the necessary living facilities for long-term stay in space, including food, water, oxygen and daily necessities, and must also have the energy needed for cruising in many places. I'm afraid these add up to thousands to tens of thousands of tons. You can imagine how big this big guy will be. Humans are still unable to launch thousands of tons of objects into space at a time. Is it necessary to accumulate so many items in the earth's orbit through multiple launches? Yes, green plants can make oxygen and food, but first, the technology is not mature enough. In case of failure or problems on the way, astronauts will either starve to death or suffocate; Second, in this way, the weight of the spacecraft is even greater.

2. The chemical fuels currently used are inefficient. At present, a spacecraft orbiting the earth (such as a small space station) is launched, weighing dozens of tons and using hundreds of tons of fuel. Since 1970s, human beings have used nuclear energy technology on spacecraft, but only used nuclear fuel cells for power supply and communication of spacecraft instruments. They have not mastered the technology of using nuclear technology for spacecraft propulsion, nor can they use nuclear engines for spacecraft propulsion. At least for decades, such technology has not been developed. In other words, if you use the chemical fuel used by the spacecraft at present, you can't carry so much chemical fuel if you want to run back and forth in the solar system.

3. The problem of the probe taking off again. At present, human alien probes only take off again on the moon. Taking this as an example, the manned lunar probe is divided into two parts: the landing section and the return section (or the return module). The landing section is connected with the return section, where * * * landed on the moon with the astronauts. After the completion of the lunar activity, the astronauts returned to the return section, turned on the rocket of the return section, and the return section rose to return to space. The landing part stays on the moon and is no longer used. In this way, sail to multiple planets at a time, or carry multiple landing sections and return sections on the mother ship, one for each landing planet; Otherwise, it is necessary to use a landing probe that can completely ascend back to the mother ship and can be refueled and reused many times. In the first case, the mother ship needs to carry multiple disposable detectors; In the second case, there is no such technology at present.

4. Time and economy. Going to several planets at a time, the speed of the spacecraft is the key. At present, the maximum speed reached by human space probes is Voyager of the United States, which is about 60,000 km/h, about17 km/s. At this speed, it takes about two years to reach Mercury at the nearest distance, and then it takes about one year to reach Venus. This is just an inner planet. It takes nearly a year to travel from the Earth to Mars, not counting the time required for accelerating and decelerating from the Earth to the target planet and conducting exploration activities on the target planet. I have to go back, and the time has almost doubled. Sailing in space for nearly 10 years, regardless of whether the astronauts can stand it psychologically and physically, even counting the food, water, oxygen and fuel they need, it is also a huge number.

So considering this problem now, one is that it is economically unbearable, and the other is that it does not have the corresponding technology.