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What is the function of the word upon in the sentence in the following paragraph? Which word can be replaced with it ?

On February 1st, 2003, the seven crew members of the Space Shuttle Columbia perished when the shuttle disintegrated upon reentry into Earth's atmosphere. But is there a way that NASA could have rescued the crew while the shuttle was still in orbit?

http://io9.com/how-nasa-might-have-been-able-to-rescue-the-columbia-cr-1531638458

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In that context, "upon" means: just after or as something happened. You could reword the sentence as follows:

On February 1st, 2003, the seven crew members of the Space Shuttle Columbia perished when the shuttle disintegrated as it re-entered Earth's atmosphere.

or:

On February 1st, 2003, the seven crew members of the Space Shuttle Columbia perished when the shuttle disintegrated just after re-entering Earth's atmosphere.

However, the word "upon" is a better fit here, for a variety of reasons explained below.

Why this makes sense to native speakers

The primary sense of "upon" is, as you might expect, "above something and on it", as in "A cloth was spread upon the table" and "A crown sat upon the queen's head." The primary sense is rare in English today, and sounds a little archaic, as "upon" has taken on many secondary senses, and the primary sense isn't really much different than plain ol' "on".

One of those secondary senses occurs in this sentence: "Once you reach the top of the hill, you can look upon all of Los Angeles." Here, the suggestion of being above applies to the person doing the looking: since you are up on top of a hill, you are higher than Los Angeles, so you can see the whole thing in front of you.

This sentence contains the primary sense and suggests that secondary sense without actually mentioning looking:

We stand upon the threshold of a new year.

This is a poetic way of saying that we are now at the boundary between the old year and the new year, and we are in a position to look ahead and see or imagine what the new year might bring. Literally, a threshold is the bottom of a doorway that enters into a room. It forms a common metaphor for being at the end of one time and the beginning of another. Here is a famous couplet:

Leaving the old, both worlds at once they view,

That stand upon the threshold of the new.

--Edmund Waller, 1686

The phrase "stand upon the threshold" strongly suggests that you are about to step forward toward the new state of affairs that you now see ahead of you. There is a natural analogy between moving forward in space and action unfolding in time.

Hopefully now it only requires a small step to understand the temporal sense of "upon" in your example. The new event (the shuttle's disintegration) follows immediately "upon" the triggering event (the re-entry). The triggering event is like a threshold that you step across, because it causes a transition from the old state (when the shuttle was OK) to the new state (the shuttle has fallen apart). Notice that re-entry is itself a transition. "Upon" suggests physical contact, like one's foot on the threshold as one enters the room. In your example, the disintegration happens just as re-entry happens: there is a sort of "temporal contact" between the two events.

  • Thank you very much your your comprehensive answer.Before your answer, I was thinking like it could replace with "just before" and "while" or as you said "as" , it seems as "just before" can also used? – Mrt Feb 21 '15 at 3:46
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    @Murat "Just before" is definitely not an accurate substitute. It would lose the "stepping forward" sense of "upon", and it would suggest the reverse sequence of events. "While" is an OK substitute, but it just means the two actions occurred simultaneously, not that one occurred "on the tail of" the other. "Upon" very concisely suggests the causal connection as well as the transition from one important state to another. "While" lacks the rich connotations described above. – Ben Kovitz Feb 21 '15 at 3:51
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    This fully answers the question as written and OP has indicated it comprehensively answers his concerns. – Matthew Read Feb 21 '15 at 6:49
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The definition of upon points to on.

Then I would use on sense 6 to get the meaning:

at or during the time of ⇒ "on entering, on the first day"

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In "upon reentry into the Earth's atmosphere" "upon" is used as a temporal preposition. You could say "when entering the Earth's atmosphere" instead.

"upon" is the same as "on", it is labelled "formal" in dictionaries - I would say it is elevated style.

Instead of "upon re-entry" you might also use "on + gerund":

  • on re-entering the Earth's atmosphere
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Many students learn that nouns are things and verbs are actions. This is not true. Many nouns describe actions or events. Here are some examples:

  • entry
  • departure
  • explosion
  • disappearance
  • murder
  • release
  • publication
  • reinstatement
  • arrival

Now imagine that you want to say:

  • Hand out the information packs when the students arrive.

Notice that when has a similar meaning to after in this sentence.

Next, imagine that you don't want to use a verb "arrive". You want to use the noun, "arrival":

  • Hand out the information packs _______ the students' arrival.

That gap [ ___ ] in the middle is for the preposition. You can't use the preposition when here. The preposition when only takes clauses. It cannot come before a noun. There must always be a clause after when (a clause is a mini-sentence with some form of verb). In the example above there is no clause after when. We used a noun to describe the event instead of a verb. When we want to say when before an event noun we use upon:

  • Hand out the information packs upon the students arrival.

Hope this is helpful

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