Taken from the script I found on trekcore.com -
"
96 EXT. SPACE - THE BORG SHIP (OPTICAL) moving by Mars... unmanned pods attack it... the Borg ship destroys them easily... moves on... and now we reveal their final destination... the familiar shape of Earth straight ahead..."
I found this scene to be quite chilling actually. The real defence of Earth took place at Wolf 359, where most of the fleet was obliterated. This is all we had left to defend ourselves....
In my opinion the real defense of Earth was yet to come. I believe that Earth had a planetary defense system designed to hold off attacks by the total combined fleets of the Klingons, the Romulans, and a bunch of other potential enemy powers, and hold them off for hours, days, weeks, months, or years until the attacking fleet gave up and left or Starfleet could assemble a vast fleet from all over the Federation to crush the attackers.
In E.E. Smith's
Second Stage Lensman a galactic government sends a vast war fleet to invade another galaxy. Two vast space fleets with millions of space battleships fight and one is totally defeated. Then the victorious space fleet enters the Trallus system, but the planet Onlo, or Trallus VIII, has such powerful defenses that they don't dare to attack it.
And the Earth defense system should be like that - on a much smaller scale of course, designed to defend against mere hundreds or thousands or tens of thousands of space battleships instead of many millions.
So Starfleet has no worries about having only one starship available at Earth. Earth is not defended by whatever starships may happen to be in or near the Solar System at the moment, but by the Earth defense system designed to defend Earth from many hundreds or thousands of warships each as powerful as a starship.
So Starfleet sent the Enterprise to incercept and investigate V'Ger in TMP, but put its main faith in the Earth Defense System to protect Earth if V'Ger was hostile.
When V'Ger and the Enterprise reached Earth:
ILIA PROBE: V'Ger signals the Creator.
KIRK: Spock?
SPOCK: A simple binary code transmitted by carrier-wave signal. Radio.
KIRK: Radio?
DECKER: Jim, V'Ger expects an answer.
KIRK: An answer? I don't know the question.
ILIA PROBE: The Creator has not responded.
CHEKOV: All planetary defence systems have just gone inoperative.
UHURA: Sir, Starfleet says the devices are proceeding to equidistant positions orbiting the planet.
McCOY: They're the same things that hit us.
SPOCK: They are hundreds of times more powerful, Captain. From those positions they could devastate the entire surface of the planet.
No doubt the planetary defense system of Earth was vastly improved after the V'Ger incident. But it failed again during the whale probe incident, so no doubt Earth's planetary defense system was once again vastly improved and upgraded.
And in "The Best of Both Worlds Part 2":
[Bridge]
SHELBY: Captain, the Borg have entered Sector zero zero one.
(A little later, Riker and Worf enter)
SHELBY: The Borg have dropped out of warp, sir. Jupiter outpost nine two reported visual contact at twelve hundred hours, thirteen minutes.
RIKER: Planetary defences?
SHELBY: Responding. No reports on effectiveness but I can't believe that against the Borg
RIKER: Ensign Crusher, at their current speed, when will they reach Earth?
WESLEY: Twenty seven minutes.
RIKER: The soonest we could intercept?
WESLEY: Forty two minutes, sir.
RIKER: Riker to Data.
[Data's lab]
RIKER [OC]: What's your status?
DATA: The initial cybernetic connection into Captain Picard's neural net pathways has been established. Mister O'Brien is ready to process the Borg signal through the transport pattern buffer.
RIKER [OC]: Make it so. With dispatch, Mister Data.
DATA: Proceeding immediately, sir. Data out.
(Data has part of his hair removed to show various ports and flashing lights in his head. Picard is on the same platform Lal used. He is naked except for some tubes and implants)
DATA: The neural link will be established in three stages. Doctor, I suggest you closely observe Captain Picard's lifesigns, while at each stage Chief O'Brien monitors my positronic matrix activity. Counsellor, hopefully, you will be able to determine whether I am reaching Captain Picard.
O'BRIEN: At what point should I shut it down if there's a problem?
DATA: I do not know. I have never done this before. Initiating first neural link.
CRUSHER: The Captain's vital signs are stable.
O'BRIEN: Positronic activity unchanged.
DATA: First neural connection is confirmed. I cannot report any significant access to the Borg consciousness.
(And Troi shakes her head)
[Bridge]
(The cube is met by three ships, which are immediately destroyed)
WORF: It is confirmed. The Borg have broken through the Mars defence perimeter.
WESLEY: Enterprise now approaching Terran system, sir.
RIKER: Slow to impulse. Time to intercept?
WESLEY: Twenty-three minutes, fourteen seconds, sir.
When the Borg are reported to have entered the Solar System it is estimated that they will reach Earth 15 minutes before the Enterprise does. Fifteen minutes is an eternity for the Borg cube to transports gazillions of nanoprobes to Earth to begin turning everyone into Borg.
So why doesn't the episode end with the Borg in control of Earth and the mission a failure?
Well, if the Borg cube and the Enterprise didn't slow down after entering the Solar System, the statement that the Enterprise was 23 minutes and 14 seconds from catching up with the Borg should have been about 18.76666 minutes after the statement that the Enterprise was 42 minutes from catching up with the Borg. So it should have been about 18.76666 minutes after the statement that the Borg cube would reach Earth in 27 minutes and thus about 8.233334 minutes before the Borg were previously calculated to reach Earth.
But Riker told Wesley to slow to impulse right before Wesley calculated the time to intercept so I don't know how much that changed the calculations. Wesley's new calculation may indicate that they would catch up with the Borg about 14.766666 minutes after the Borg reached Earth, which would imply that the Borg cube and the Enterpsie slowed to impulse at about the same distance from Earth if their impulse speeds were similar.
A few minutes after this, while the Borg presumably hadn't reached Earth yet, their attempts to communicate with Picard result in:
[Bridge]
WORF: Sir, the Borg have halted their approach to Earth.
SHELBY: I think we got their attention.
RIKER: Time to intercept?
WESLEY: Two minutes, four seconds, sir.
So the Borg stop heading for Earth when they still haven't reached it, and only 2.066666 minutes before the Enterprise will now catch up with them. So apparently the Borg ship was travelling so fast through the solar system that the Enterprise using impulse drive wouldn't be able to catch up with the Borg until the Borg ship stopped at Earth and began attacking. Which makes me really wonder why Riker ordered the Enterprise to slow to impulse.
But anyway there seem to be only two possibilities.
One is that the Borg ship stopped to face the Enterprise before engaging the Earth defense systems and thus there is no evidence about how well the Earth defense systems might have worked against the Borg cube.
And the other possibility is that the Bog ship did begin attacking Earth and the Earth defense systems did stop the Borg attack for the seconds or minutes necessary for the Enterprise to arrive and destroy the Borg cube.
And the Earth defense systems seem to have been powerful enough to protect Earth from any danger from the exploding Borg cube.
Of course it is uncertain whether the G Borg Cube was just a few thousand miles from Earth or still tens of millions of miles away due to the possible use of a telephoto lens or the equivalent in scenes where the Borg cube exploded, So we don't now how dangerous the exploding cube would be to Earth.
But my answer here:
https://worldbuilding.stackexchange...ether-or-not-to-call-it-a-plane/151406#151406
Does point out that the mass and distance of an exploding object determine how dangerous it may be.
(Much better one was 
Good one!
I love Riker's 'No shit, Sherlock' expression as Worf says this
