That depends what you include under the term "objects". In general, those would be subatomic particles. Many of them are (in a sense) considered to have no volume. Some of them, such as photons and gravitons, have zero mass (rest mass; they will have some mass due to their energy).
cells
The Moon is the smallest of the three objects listed
The smallest objects - are meteors. Most of them are smaller than a Basketball.
The smallest size of objects that the human eye cannot perceive is typically around 0.1 millimeters, which is about the size of a grain of sand. These tiny objects are too small to be seen without the aid of a microscope.
Comets
actualy no two objects can physicly touch because the magnetic field between the objects caused by the atoms
Well, atoms are the smallest objects in the universe. Everything is made up of atoms. That means that bacteria is made of atoms!
Mercury is the smallest planet. Pluto, Ceres, and other trans Neptunian objects are now called dwarf planets, and don't count.
The very smallest "stars" in the galaxy are brown dwarfs. These are starlike objects that have failed to produce sustained nuclear fusion.
The smallest objects visible by an ordinary light microscope are typically around 200 nanometers in size. Objects smaller than this, such as viruses and individual molecules, require more powerful microscopes like electron microscopes.
That depends what you would accept as "objects". The interplanetary space includes not only specks of dust, but also gas atoms.
The smallest objects in quantum physics are "strings" which are 10^-35m in size which is comparable to the Plank length. At this size nothing is flat, measureable or quantifiable. Nothing can be measured smaller than this by any technique currently in use OR will ever be invented in the future... unless a new form of maths and physics is invented which alters our current perception of the very small...