| Nanopascals (nPa) | Terapascals (TPa) |
|---|---|
| 1 Nanopascal | 1 × 10-21 TPa |
| 2 Nanopascals | 2 × 10-21 TPa |
| 3 Nanopascals | 3 × 10-21 TPa |
| 4 Nanopascals | 4 × 10-21 TPa |
| 5 Nanopascals | 5 × 10-21 TPa |
| 10 Nanopascals | 1 × 10-20 TPa |
| 20 Nanopascals | 2 × 10-20 TPa |
| 25 Nanopascals | 2.5 × 10-20 TPa |
| 50 Nanopascals | 5 × 10-20 TPa |
| 100 Nanopascals | 1 × 10-19 TPa |
| Reference | Nanopascals (nPa) | Terapascals (TPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1.01325 × 1014 nPa | 0.000000101325 TPa |
| Healthy blood pressure (120 mmHg) | 1.6 × 1013 nPa | 0.000000016 TPa |
| A car tyre | 2.2 × 1014 nPa | 0.00000022 TPa |
| A racing bicycle tyre | 6 × 1014 nPa | 0.0000006 TPa |
The nanopascal is a unit of pressure equal to one billionth of a pascal. Its symbol is nPa. It marks the far end of the pressure scale, the region where the idea of pressure as a push on a surface stops being useful and becomes a statement about how few particles are present.
The clearest home for the unit is extreme high vacuum. Ordinary laboratory vacuum reaches millipascals; ultra-high vacuum, used for surface physics and for the beam pipes of particle accelerators, reaches micropascals. Below that lies extreme high vacuum at nanopascals and lower, and reaching it takes a sealed chamber, hours of baking at two hundred degrees to drive gas out of the metal itself, and pumps that trap molecules rather than push them.
At those pressures a chamber is not empty. A nanopascal still contains something like a quarter of a million molecules per cubic centimetre, which sounds like a great many until you compare it with the twenty-five billion billion in the same volume of room air. What matters is not the count but the mean free path: a molecule now travels thousands of kilometres before striking another, so it hits the walls long before it meets a neighbour.
That is exactly the point. Surface science needs a sample to stay clean for the length of an experiment, and at ordinary pressures a fresh surface is covered by a layer of adsorbed gas in about a nanosecond. At a nanopascal the same surface stays clean for days. The vacuum is not there to remove air but to buy time.
Space provides the natural comparison. Low Earth orbit is around a micropascal, still dense enough that the atmosphere drags on satellites and eventually pulls them down. Interplanetary space is nanopascals. Interstellar space is far lower still, roughly a femtopascal, which no terrestrial pump has ever matched — the best laboratory vacuums are still denser than the space between the stars.
Radiation pressure lands in similar territory. Sunlight falling on a perfectly absorbing surface at Earth's distance exerts about 4.5 micropascals, and at the distance of the outer planets it falls to nanopascals. Solar sails work with these numbers, which is why they must be enormous and light to gather a usable force.
One nanopascal equals 0.000000001 pascals, 0.001 micropascals, 1,000 picopascals, or about 0.000000000000145 pounds per square inch.
The terapascal is a unit of pressure equal to a thousand gigapascals, written TPa. It is ten million bar. Nothing built by engineers operates at this pressure, and the unit belongs instead to two quite separate corners of science: the stiffness of the strongest materials known, and the interiors of large planets.
Carbon nanostructures put it on the map. A single-walled carbon nanotube has a Young's modulus close to one terapascal, and a sheet of graphene the same, which makes them the stiffest materials ever measured relative to their weight. Diamond, long the benchmark, comes in at 1.2 terapascals. Those three numbers are the reason the unit appears at all in materials science.
Stiffness at this level is not the same as strength. A nanotube resists stretching enormously, but a real fibre made of many of them fails at a far lower stress because the tubes slide past one another. Confusing a terapascal modulus with a terapascal breaking strength is one of the commonest errors in popular accounts of these materials.
Planetary interiors reach genuine terapascal pressures. The centre of Jupiter is estimated at three to four terapascals, and the cores of larger gas giants beyond that. Under such conditions hydrogen behaves as a metal, which is what generates the planet's magnetic field, so the unit describes a state of matter rather than a load on a structure.
Laboratories can now reach it briefly. Laser-driven shock compression and pulsed magnetic techniques drive samples into the terapascal range for nanoseconds at a time, long enough to record how a material's density and structure respond. Those experiments are the only direct evidence available about matter under the conditions inside giant planets.
For scale, one terapascal is ten million times atmospheric pressure and about a thousand times the pressure at the centre of the earth divided by three. The number stops being something a person can feel and becomes a description of what atoms do when they are pushed close enough together to change their chemistry.
One terapascal equals 1,000,000,000,000 pascals, 1000 gigapascals, 10,000,000 bar, or about 145 million pounds per square inch.