Space is measured by km, AU, and ly. AU is the distance from the sun to the Earth and is used to measure the distance between planets and other masses in the solar system. As for kilometer, which is used to measure smaller things on Earth mostly. A ly (light year), is the distance light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km. More precisely, one light-year is equal to 9,500,000,000,000 kilometers. We use light years to measure objects in our universe much much farther away than anything we can imagine if we measure the distance with anything but a light year.
The distance from Mercury to the Earth varies as both the planets orbit the Sun. When closest to the Sun, Mercury is about 77 million kilometers from the Earth. When its the farthest, it's about 222 million kilometers. It would take about 61/2 years with the speeds we can reach today. Humans have never traveled to Mercury. I doubt they ever will until technology is greatly improved and we can build a space shuttle that can withstand such heat and is light enough to lift off of Earth.
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