From Earth to Mercury Help
From Earth to Mercury
Getting There Is Half The Trouble
Mercury lies much closer to the Sun than does Earth, and the little planet races around the Sun in only about one-quarter of an Earth year. At first thought it might seem like an easy task to hurl a space object inward toward the Sun, but it requires considerable energy. In order for an object to fall in toward the Sun from Earth orbit, that object must decelerate, and this takes as much energy as the acceleration necessary to achieve orbits more distant from the Sun than that of Earth. When you apply the brakes on a vehicle for a long time, they get hot; energy is expended slowing you down. Powerful retrorockets must be employed to begin our journey inward toward Mercury.
Along the way to Mercury, you cross the orbit of Venus. If you time things just right, you can use the gravitational field of Venus to help the ship attain a course for a rendezvous with Mercury (Fig. 5-4). This is known as a gravitational assist and has been a useful maneuver ever since the first interplanetary probes were launched by humankind back in the 1970s. Gravitational assists can either accelerate or decelerate a space ship and can help interstellar space vessels get underway out of the Solar System. In your case, a Venus nearmiss sends the ship plunging inward toward the Sun. Timing is critical; the slightest miscalculation might send you into an eccentric orbit between Mercury and Venus, missing Mercury by tens of thousands of kilometers. Or worse, it could put you on an irrevocable course into the Sun.
Even if you don’t fall into the Sun, there is danger in a trip to the inner planets. The Sun occasionally has “tantrums” called solar flares that eject large quantities of high-speed subatomic particles. These particles affect living tissue in much the same way as the high-intensity gamma rays produced by nuclear bombs. Earth’s atmosphere protects you from these particles when you’re on your home planet’s surface, but in Earth orbit or when traveling between Earth and the Moon, there is some risk. The peril increases according to the inverse-square law as you approach the Sun. If a solar flare were to take place while you were in orbit around Mercury, you would be subjected to almost 10 times the radiation from these particles as you would get if you were in orbit around Earth. For this reason, your itinerary planners saw to it that you should make your journey near the time of sunspot minimum, when solar flares are least likely to occur.
As you attain orbit around Mercury, you see the surface up close. It looks similar to Earth’s moon, although there are more cratered areas and less maria , or flat regions. The escarpments , or cliffs, produced when the crust collapsed around the cooling core long ago are vividly apparent, especially near the twilight line where the Sun shines down on the surface at a sharp angle, producing long shadows. The captain has selected a polar orbit around Mercury—one that passes over the north and south poles—so that you can land easily in one of the craters near the south pole. Your trip planners left nothing to chance. They found a crater and a specific spot within that crater where the Sun never shines. Thus, while you are on the surface, you’ll be safe even if a solar storm happens to take place. If humankind ever sees fit to put a permanent base on Mercury, it will be important that it be in a spot such as this. The temperature is far below zero; this will require that the landing craft’s heat generator work hard.
“There it is,” says the captain, and you look down to see a ring of mountain peaks lit up by the Sun, with shadows so black that the crater looks like a bottomless well. “We will land right there. But not on this pass; on the next one.” You don’t have long to wait. Mercury is a small world, and you are in a low orbit, having no worries about atmospheric drag. In less than an hour you’ll be on your way down.
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