The outer space is the expanse existing beyond the Earth; between celestial bodies. A naturally occurring physical structure, entity or association in the observable universe is called a celestial body. Going to a celestial body from the Earth (terrestrial) implies going into the outer space environment.
Conventionally, anything at an altitude of 100 km (62 mi) above the sea level is considered to be in the outer space (or in space). Technically, it is the Karman line that is used; anything that crosses the Karman line which is at an altitude of 100 km (62 mi) above the sea level is considered to be in the outer space (or in space). Of key note is that measurement of this specific distance above the sea level, not ground level. Somebody on the mountain top would also say that is his/her own ground level; while sea level gives a general platform for this measurement.
Some people think that surrounding the Earth is a solid circular material that would be needed to pass through to exit the Earth. It is not true. It is not true because we are on the Earth, and we are not in the Earth (Figure 1). Some people also make a mistake of thinking that the atmosphere around us (troposphere) is space. No, it is not also true (Figure 2). The Karman line demarcates the boundary between the atmosphere and the space region.
Figure 1. We are on the Earth not in the Earth (image not to scale)
Figure 2. Layers of atmosphere
There is no solid substance that surrounds the Earth. The solid substance is the Earth itself that we stand upon. What surrounds the Earth is layers of gases called atmosphere, that makes the Earth habitable. What makes it possible for everything on the Earth to remain on it (and not falling apart), is gravity. Gravity is always present on Earth. The two basic characteristics of the space region is the presence of radiation and microgravity. Only minute gravity is in action at this region, unlike on the Earth where gravity is always present.
Further
reading
Drake, N.
(2018). Where, exactly, is the edge of space? It depends on who you ask. National Geographic. https://api.nationalgeographic.com/distribution/public/amp/science/2018/12/where-is-the-edge-of-space-and-what-is-the-karman-line. Accessed 27.10.2020.
Oluwafemi, F. (2022). Space food on celestial bodies
and on the way there (Chapter 26). In: Future Foods: Global Trends,
Opportunities and Sustainability Challenges. Rajeev Bhat (Ed.). Elsevier
Sciences Inc.: Wiley Blackwell, Oxford, UK. pp. 451-468.
https://doi.org/10.1016/B978-0-323-91001-9.00012-8.
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