You can charge the Astro from anything you can plug the included USB cable into: wall socket, computer, your car, or a portable solar charger. Best of all, you can charge three devices at once.
If you want to save money and weight, check out the Anker Asto Mini 3000 mAh. It does not charge as fast as the 2nd Gen Astro and only charges a smartphone 1-2 times, but it comes with a flashlight and is about a 1/3 the weight and cost.
Another option if you are away from a power source for a long time is the Bushnell Bear Grylls SolarWrap Mini. It has a small internal battery but charges but can be charged from the sun when needed and is relatively light.