To be read after Lesson 11

Space Cooling

A new method of cooling that can generate cryogenic tempera­tures of 200 °C below zero without the use of electricity and with almost no moving parts has been tested at the Jet Propulsion Labo­ratory in Pasadena, California. The refrigerator used for the pur­pose was recently tested to — 253 °C, only 20 degrees above absolute zero, the lowest possible temperature.

In space such cooling system could increase the life of future space station refuelling ports by cooling the large liquid-hydrogen fuel tanks which are likely to be in service.

In future earth applications it could be used for cooling hydro­gen-powered cars and planes, as well as for cooling superconduct­ing motors and computers.

According to the JPL (Jet Propulsion Laboratory) experts the key lies in the use of hydrides, materials that interact with hydro­gen. These materials absorb tremendous amounts of hydrogen gas at room temperature. The engineers of the JPL have taken advan­tage of this property to build a series of devices that act as compres­sors and provide a continuous cooling stream of liquid hydrogen.

The system saves weight in space since it can use direct solar heat instead of electricity from heavier, inefficient electric systems. Because it has so few moving parts and uses the same supply of gas in a closed cycle, it could operate for many decades. Because of its long potential lifetime, the system could be used to cool infrared sensors during missions to the other planets, which may take 10 years or more to complete.

The Propulsion Challenge¹

Magsails are a form of solar sails that use a completely different type of physical interaction with the Sun. Magsail is a simple loop (петля, контур) of high-temperature superconducting wire carry­ing a persistent² current. The charged particles in the solar wind are deflected³ by the magnetic field, producing thrust. Although the thrust density in the solar ion wind flux is 5,000 times less than the thrust density in the solar photon flux⁴, the mass of a solar sail goes directly with the area, whereas the mass of the magsail rises with the perimeter of the enclosed area.

The effective cross-sectional area of the magnetic field around the magsail is about a hundred times the physical area of the loop. As a result, preliminary calculations show the thrust-to-weight ra-

tio of a magsail can be an order of magnitude (порядок величины) better than a solar sail. Recent thermal balance calculations indi­cate that a properly Sun-shielded⁵ cable can be passively main­tained at a temperature of 65 К in space, well below the supercon­ducting transition point for many of the new high temperature su­perconductors.