When it comes time for NASA's Psyche spacecraft to power itself through deep space, it'll be more brain than brawn that does the work. Once the stuff of science fiction, the efficient and quiet power of electric propulsion will provide the force that propels the Psyche spacecraft all the way to the main asteroid belt between Mars and Jupiter. The orbiter's target: A metal-rich asteroid also called Psyche.

The spacecraft will launch in August 2022 and travel about 1.5 billion miles (2.4 billion kilometers) over three and a half years to get to the asteroid, which scientists believe may be part of the core of a planetesimal, the building block of an early rocky planet.

When a fast solar wind stream erupts from a coronal hole (a cooler region in the Sun's atmosphere) and overtakes a slower moving solar wind stream, a stream interaction region (SIR) can form. In the SIR, a density "pileup" of compressed plasma develops upstream of the interface; typically there is a peak in pressure followed by a rarefaction region in the fast solar wind component. As the SIR propagates away from the Sun, to distances of one astronomical unit or beyond, the compression can form a shock that efficiently accelerates charged particles.

It seems like every week, another rocket is launched into space carrying rovers to Mars, tourists or, most commonly, satellites. The idea that "space is getting crowded" has been around for a few years now, but just how crowded is it? And how crowded is it going to get?

I am a professor of physics and director of the Center for Space Science and Technology at the University of Massachusetts, Lowell. Many satellites that were put into orbit have gone dead and burned up in the atmosphere, but thousands remain.