Have you ever wondered what happens to all of the old objects launched into space, such as rockets and satellites? It's called space junk, and much of it is still out there now. According to NASA, the definition of space junk is "any man-made object in orbit (轨道) around Earth that no longer serves a useful function".

One large piece of space junk, a disused rocket, hit the dark side of the moon on March 4. The rocket's origin is not clear, but scientists know that it was massive – weighing about 3 tons. It must have left a large crater (坑), estimated 10 to 20 meters wide, on the moon's surface. It won't be visible right away, but scientists are trying to locate it. "We will find the crater, eventually, " Mark Robinson, lead investigator for NASA Lunar Reconnaissance Orbiter Camera, told USA TODAY. "Depending on its location, it could take as long as 28 days. "

What's significant about this space junk crash is that it draws attention to some of the potential dangers of space junk. First, the accumulation of space junk in Earth's orbit means that satellites and the International Space Station (ISS) are at risk. In 2016, tiny space debris (残骸), smaller than a millimeter, caused a tiny chip in the ISS' window. Imagine the damage that a larger piece of debris could cause. The European Space Agency website states that objects "up to 1 cm in size could disable an instrument or a critical flight system on a satellite" while objects over 10 cm "could shatter a satellite or spacecraft into pieces". If this sounds terrifying, that's because it is. Any collision with larger pieces of debris could be especially dangerous for manned spacecraft.

In addition to potential damage to spacecraft and satellites, space junk also poses a threat to the Earth itself. Crashes can send debris into the atmosphere where it can remain for many decades. Some powerful collisions can even send large debris hurtling (猛冲) toward Earth's surface where it can affect the local environment. Leftover rocket debris can contain toxic materials that are harmful to plant and animal life. Some people in Siberia have even reported serious health problems after rocket debris landed in their regions in 2012, a local doctor told the BBC.  

Researchers from China and the United States have found what makes ginkgo (银杏) trees live for more than 1, 000 years, according to a newspaper report. Researchers from Beijing Forestry University, the University of North Texas and some other Chinese research institutions reported in the U. S. magazine Proceedings of National Academy of Sciences that they analyzed 34 ginkgo trees and divided them into three groups according to their ages. Compared with adult trees, the ancient trees, aging from 193 to 667 years, have thinner cambium (形成层) cell layers(细胞层). Cambium cells are layers of stem cells between the inside wood and outside bark changing into other tissue.

The old trees have a higher level of IAA, a plant growth regulator and growth-stimulating hormone, and a lower level of ABA, a growth-resisting hormone. However, the average cross-sectional area of trees at breast height in the ancient trees group has been increasing with aging, which shows the continuous cambial cells division.

Little difference was found when it comes to their ability to photosynthesize(光合作用), grow leaves, grow seeds or resist disease compared to younger trees, showing that the ancient trees group are actually in a healthy and mature state.

Lin Jinxing of Beijing Forestry University and an author of the study said that trees usually die from outside stress like pests. They found disease resistance-associated genes kept high expression in old trees, along with genes associated with the synthesis(合成) of protective compounds, showing that the old trees do not lose their ability to defend against outside stressors. The researchers concluded that the longevity(寿命)of ginkgo trees is a balance between the growing and aging processes, rather than being regulated by a single gene.

You are what you think. Whatever you are doing right now is determined by your thinking. All your emotions and desires are influenced by your thinking. The problem is that when you are not aware of your thinking, you have no chance of correcting it if it is poor. When thinking is subconscious, you are in no position to see any problems in it. And, if you don't see any problems in it, you won't be motivated to change it. What's worse, few people realize the powerful role that thinking plays in their lives, so they don't gain significant command of it. As a result, most people are in many ways victims of their thinking.

Consider your success as a student. The single most significant variable in determining your success as a student is the quality of your thinking. Your instructors will play a role in your learning, but even the best teachers cannot get into your head and learn for you. If you lack the intellectual skills necessary for thinking well through course content, you will not be successful in school.

Here is the key question we are putting to you in this book. If the quality of a person's thinking is the single most significant determinant of both happiness and success, why not take the time to learn from the best thinkers?

This book will inform you of the tools that the best thinkers use and will illustrate the activities and practice you can use to begin to emulate them, and take control of your destiny as a thinker. Each chapter focuses on a different aspect of these thinking aids, helping you discover the power of your own mind and of your potential to think systematically about your thinking.

This book, as a whole, introduces you to the tools of mind that will help you reason well through various challenges you face. If you take these ideas seriously and practice using them, you can improve the quality of your thinking and ultimately, the quality of your life.