Waste heat is a worldwide energy problem most people have probably never heard about. Every machine and power station, even renewable energy like wind and solar, creates heat that is simply lost in the atmosphere. Thankfully, scientists and engineers are devoted to capturing heat and transforming it into useful electricity.

Thermophotovoltaic devices (热光电设备) are one promising way to use this waste heat. University of Michigan engineers have created a precisely machined thermophotovoltaic cell that could be used to generate electricity from surplus renewable power. The cells would use heat stored in molten salt and turn it into electricity on demand, in a form that is much cheaper than batteries, say its creators.

One of the factors important to any device's thermoelectric efficiency is its material. If higher electrical energy can be created with smaller temperature differences, then the materials are said to have a high ZT value. And a new material created by scientists in Vienna has broken the ZT value world record.

Any new thermoelectric material has to have a distinct advantage, and research is identifying a number of promising products. Pyroelectric films (热电薄膜) are aimed at waste heat emitted at less than 100 degrees Celsius (212 F) and can generate electricity when a material is either heated or cooled.

Low-temperature generation includes computers and cars, making pyroelectrics particularly useful for squeezing more energy out of electronic systems.

Another relative advantage is inexpensive materials. More abundant elements like tin, selenium, and magnesium are cheap and abundant, with high ZT values when combined in the right way. Their use could lead to greener car engines and industrial facilities.

And when it comes to personal electronics, thermoelectric could eventually see off traditional charging hardware. Chinese researchers have built a wristband that gathers body heat to power a LED and may be able to power smartwatches or other mobile devices in the future.

You run into the grocery store to pick up one bottle of water. You get what you need, head to the front, and choose the line that looks fastest.

You chose wrong. People who you swear got in other lines long after you are already checked out and off to the parking lot. Why does this always seem to happen to you? It turns out, it's just math working against you; chances are, the other line really is faster.

Grocery stores try to have enough employees at checkout to get all their customers through with minimum delay. But sometimes, as on a Sunday afternoon, the system gets particularly busy. Any small interruption—a price check, a chatty customer—can have downstream effects, holding up an entire line.

If there are three lines in the store, delays will happen randomly at different registers. Think about the probability: The chances of your line being the fastest are only one in three. So it's not just in your mind: Another line probably is moving faster.

Researchers have a good way to deal with this problem. Make all customers stand in one long, snaking line—called a serpentine line —and serve each person at the front with the next available register. With three registers, this method is much faster than the traditional approach. This is what they do at most banks and fast-food restaurants. With a serpentine line, a long delay at one register won't unfairly punish the people who lined up behind it. Instead, it will slow down everyone a little bit but speed up checkout overall.

So why don't most places encourage serpentine lines? It takes many registers to keep one line moving quickly, and some stores can't afford the space or manpower. So wherever your next wait may be: Good luck.

Space is becoming more crowded. On December 3, a Falcon 9 rocket made by Space X thundered into the sky. On board were 64 small satellites, more than any American company had launched before in one go. They have a variety of uses, from space-based-radar to the monitoring of radio-frequency-emissions. They will be working in their orbits(轨道).

These objects are part of the latest breed of low-Earth-orbit (LEO) satellites. This launch is just taste of what is planned. Space X and One Web, a communications firm, plan to launch satellites in their thousands, not hundreds. The pair are set to double the total number of satellites in orbit by 2027. 

That promises to change things dramatically on Earth. LEO satellites can bring internet connectivity to places where it is still unavailable or unaffordable. This will also be a lasting source of new demand for the space economy. Morgan Stanley, a bank, projects that the space industry will grow from $350 billion in 2016 to more than $

1.1 trillion by 

2040. New internet satellites will account for a half this increase.

For that to happen, however, three worries must be overcome. Debris (碎片) is the most familiar concern. As long ago as 1978, Donald Kessler, a scientist at NASA, proposed a situation in which, when enough satellites were packed into low-Earth orbits, any collision could cause a chain reaction which would eventually destroy all space craft in the same orbital plane. The syndrome which bears Mr. Kessler's name weighs heavily on the minds of executives at the new satellite firms. Debris could cause entire tracts (广阔的一片) of space to be unusable for decades.

Solutions exist. One is to grab malfunction satellites and pull them down into Earth's atmosphere. Another is to monitor space more intensively for debris; a US Air Force program me called Space Fence is due to start in

2019. But technology is only part of the answer. Rules are needed to govern the safe disposal of old satellites from low-Earth orbit.   

Cyber-security is a second, long-standing worry. Hackers could take control of a satellite and seal intellectual property, redirect data flows or cause a collision. The satellite industry has been slow to respond to such concerns. But as more of the world's population comes to rely on the infrastructure of space for access to the internet, the need for action intensifies.

The third issue follows from the first two. If a simple mistake or a cyber-attack can cause a chain reaction which wipes out hundreds of billions of dollars of investment, who is liable? Underwriters (承保商) are studying the plans of firms (quite a lot) that wish to operate large numbers of satellites. But there is a long way to go before the risks are well understood, let alone priced.

As space becomes more commercialized, mind-bending prospects open up: packages moved across the planet in minutes by rocket rather than by plane, equipment sent to other small planets, passengers launched into orbit and beyond. All that and more may come, one day. But such activities would raise the same questions as LEO satellites do. They must be answered before the space economy can truly develop.