From Earth to Mercury Help (page 3)
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.
Down And Back
The landing shuttle is named Eagle , after the Apollo 11 lunar landing module, and that’s a good name for this contraption. The Mercury lander looks very much like the Apollo mission landing craft. “I know what you’re thinking,” says the captain as you get into the little chamber. “But if it worked in 1969, it will work now. The biggest difference is that this one can deal with greater temperature extremes. And the communications and navigation equipment are a lot better than they were in 1969.”
The first officer will be our guide for the landing mission. The captain must stay with the Valiant . The first officer also will serve as our teacher during long periods of interplanetary travel. “One hour of general astronomy and cosmology training every day will make cosmic gurus out of you by the time this journey is over,” he says.
As you break free of the main vessel, you get a feeling like that of a first free dive when learning SCUBA diving. It’s one thing to practice SCUBA diving in a swimming pool or a small pond; it is entirely another to dive in the open ocean, miles from land. If something goes wrong while you’re isolated in this little landing craft, only the skill of the first officer and a good measure of luck will stand between you and disaster.
As you near the crater, the mountains loom. “That looks like water ice,” you say, noticing a grayish sheen inside the crater. “It might be exactly that,” says the first officer with a hint of a smile. The craft slows; the landing lights come on. Then suddenly you are in pitch darkness. The Sun has dipped below the rim of the crater. Stars flash into view as the Sun no longer dominates the sky. Your eyes adjust to the darkness; the first officer scans the surface below with high-resolution submillimetric and infrared radar. You hover. The craft seems to move a little. The glint of the Sun-scorched peaks on the far side of the crater lends some perspective. “Hmm,” says the first officer. Then again, “Hmm.” His eyes are glued to the screen and to a group of lights and dials. “Is this good or bad?” you ask. “It depends,” says the first officer. “If you are worried about liftoff, it is good. If you are looking forward to touchdown, it is bad.”
You begin the ascent back up to the Valiant . You will not land.
“I couldn’t find a suitable spot,” apologizes the first officer. “We have limited fuel, and that means we have limited time. The captain didn’t get the Valiant into the orbit she had hoped for. The orbit is a little too high; we had to travel a little farther to get down here than I was expecting.”
“Is that good or bad?” you ask.
“It is not good,” says the first officer. “But it could be worse. If our orbit had been a few kilometers higher or slanted by another degree with respect to the poles, I would not have attempted this trip at all.”
An Orbital Interlude
After you return to the Valiant , the captain has little to say. “This is the nature of life in outer space,” she says; “you take what you can get. Now settle in and get comfortable. We have to stay in orbit around Mercury for awhile.”
“Why?” you ask. But the captain has already left the room; she has more important business.
The first officer explains. “The captain selected a polar orbit when we made our rendezvous with Mercury. It was relatively easy to get into that sort of orbit. We drifted in and swooped under Mercury’s south pole. The planet’s gravitational field took care of the rest. But now we find ourselves in an orbit that is inclined 90 degrees to the plane in which the planets orbit the Sun.”
“So?” you ask.
“If we shoot out of this polar orbit in the planetary plane right now,” explains the first officer, “we won’t be heading for Venus. But we have to stay in the planetary plane. If we did not shoot out of Mercury orbit in the plane of the planets, we would find ourselves bound for interstellar oblivion. Our fuel would never be sufficient to get us back to the Solar System. Our ship must leave this orbit along essentially the same line from which it approached (Fig. 5-5). This greatly restricts our options.”
“So what do we do?” you ask.
“We wait,” says the first officer, “until Mercury is in the proper position with respect to Venus. Then we will fire our engines, shoot back out from under the south pole, and coast to Venus, where we will enter an equatorial orbit. That will be much less inconvenient to get out of properly.”
“How long do we have to wait here?”
“It is a good thing we have broadband Internet access on this ship,” you say.
“Yes,” says the first officer. “But the latency is horrible. We’re several light-minutes away from the hub in Dallas, Texas. I suggest that you watch old videos or listen to old music albums. We have a huge selection. And don’t forget to work your body out for two solid hours a day. Otherwise you will lose calcium from your bones.”
“Won’t the artificial gravity prevent calcium loss from bones?”
“To some extent. But remember that it’s less than one Earth gravity and will increase slowly because the geometry of our trajectory means that we will be en route to Venus for a long time.”
“How long?” you ask.
“You don’t want to know,” says the first officer.
Practice problems of this concept can be found at: Mercury and Venus Practice Problems
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