What could you do with your own, do-it-yourself satellite? Such things now exist—and at prices starting for as low as $8,000, including the blast-off into space. You could use it, say experts, to track migratory animals--whales, perhaps, or elk. You could have it tweet you now and then, just to say hello. It could send you pictures of a developing hurricane or take readings of changes in the earth's magnetic field.Imagine that you're at a cocktail party. "My goodness, look at the time," you say, glancing at your watch. "Sorry to go, but I've got to take a reading of the earth's magnetic field with my satellite." That alone could be worth $8,000.According to a story in Businessweek, about a dozen homemade satellites will be launched into space in the next year.