Ayer exploramos el espacio por si acaso.

Because for many -ar verbs, the nosotros form is the same in the present and the preterite:

• exploramos = we explore
• exploramos = we explored

So Spanish relies on context to tell them apart. In this sentence, ayer clearly shows that it is past.

This is very common:

• Hoy exploramos el espacio. = Today we explore the space.
• Ayer exploramos el espacio. = Yesterday we explored the space.

Spanish often drops subject pronouns because the verb ending already tells you who the subject is.

Here, exploramos already means we explored, so nosotros is unnecessary.

You could say:

• Ayer nosotros exploramos el espacio por si acaso.

But that would usually add emphasis, contrast, or clarity, such as:

• Ellos no fueron, pero nosotros exploramos el espacio por si acaso.

So the version without nosotros is the most natural in many situations.

Ayer is a time expression meaning yesterday. It tells you when the action happened.

It does not have to come first, but putting it first is very common because it sets the time frame right away.

All of these are possible:

• Ayer exploramos el espacio por si acaso.
• Exploramos el espacio ayer por si acaso.
• Exploramos ayer el espacio por si acaso.

The first one sounds especially natural because Spanish often likes to put time expressions near the beginning.

Por si acaso is a very common fixed expression meaning something like:

• just in case
• as a precaution
• in case it might be needed

It suggests that the action was done as a preventive measure, not necessarily because it was definitely needed.

For example:

• Llevé una chaqueta por si acaso. = I took a jacket just in case.
• Revisamos todo por si acaso. = We checked everything just in case.

So in your sentence, it shows the exploration was done as a precaution.

Because por si acaso is the standard expression.

In Spanish, por si is used to express doing something in case something happens:

• Trae agua por si hace calor. = Bring water in case it gets hot.

And por si acaso is the shortened idiomatic version when the possible situation is left more general:

• Trae agua por si acaso. = Bring water just in case.

Para si acaso is not the normal standard form here.

Because this si means if.

Spanish distinguishes:

• si = if
• sí = yes or an emphatic form like himself/herself/itself after a preposition in some structures

In por si acaso, the meaning is related to if, so it is written si, with no accent.

Spanish uses articles like el, la, los, las more often than English does.

Here, el espacio is simply the noun phrase serving as the direct object of exploramos.

Depending on context, el can make it sound like:

• a specific space already known in the conversation, or
• just the normal way of referring to that noun in Spanish

English often drops the article where Spanish keeps it.

It could mean either one, depending on context.

Espacio can refer to:

• space in the sense of outer space
• a physical space/area
• room or available space

So without more context, the phrase may be somewhat ambiguous.

If you want to make outer space very clear, Spanish can also say:

• el espacio exterior

If you want a more earthly meaning, a speaker might choose a more specific noun such as:

• la zona
• el lugar
• el cuarto
• el área

Because the personal a is mainly used before specific people and sometimes pets or personified beings.

Since el espacio is a thing, not a person, there is no personal a.

Compare:

• Exploramos el espacio. = no a
• Vimos a Carlos. = personal a because Carlos is a person

Yes. Spanish word order is fairly flexible, especially with adverbs and short phrases.

For example:

• Ayer exploramos el espacio por si acaso.
• Por si acaso, ayer exploramos el espacio.
• Exploramos el espacio ayer por si acaso.

The meaning stays close, but the focus changes a little:

• starting with ayer emphasizes the time
• starting with por si acaso emphasizes the precaution
• ending with por si acaso sounds very natural because it adds the reason at the end

The original sentence is a very normal, natural order.

It can work both ways.

1. As a fixed phrase by itself:

   • Llévalo por si acaso.
   • Ayer exploramos el espacio por si acaso.

2. As part of a longer structure with por si

   • clause:
   • Lleva una linterna por si se va la luz.
   • Exploramos el área por si había alguien allí.

So por si acaso is like a compact version when the possible problem is left unstated or obvious from context.