In a way, Donald Trump's impeachment came out of nowhere.

It took less than a week for a story about a phone call with the President of Ukraine to become the catalyst for Democrats to launch an impeachment inquiry.

Which makes predicting the likelihood of what happens next to be almost impossible. The story changes by the day, sometimes by the hour.

But what is clear is that there are only five ways that impeachment ends for Mr Trump.

If you'd like to see a handy chart detailing the process, you can tap here and we'll show you one.

What you need to know about impeachment Broadly, impeachment is a two-step process in the US Congress

Broadly, impeachment is a two-step process in the US Congress The House of Representatives votes on whether to impeach the president. This needs a simple majority

The House of Representatives votes on whether to impeach the president. This needs a simple majority Then the Senate votes on whether the president should be convicted and removed from office. This needs a two-thirds majority

Then the Senate votes on whether the president should be convicted and removed from office. This needs a two-thirds majority If you want to dig deeper, we've broken it down here

Thanks to a vote last week, we've now got a roadmap for how the public phase of the investigation will play out.

Here's what could happen once it's over, starting with …

1. A formal impeachment vote is never pursued

At the moment the Democrats, who control the House, are just conducting an impeachment investigation. There is a world in which Democrats decide that their investigation didn't turn up enough evidence to hold any kind of official vote on impeachment.

Matthew Glassman is a senior fellow at the Government Affairs Institute at Georgetown University.

He said it's really important to keep in mind that impeachment is a political process, not a legal one. Which means the politics of it will have a big influence on how things play out.

"The politics could sour for Democrats," Dr Glassman said.

Though most polls show Americans are in support of impeachment, it's not an overwhelming majority. And some numbers suggest that impeachment is less popular in electorally critical swing states.

Chance of this happening?

Dr Glassman said it's still unlikely that the Democrats drop the vote altogether.

Despite stonewalling from the White House, the investigation keeps turning up new evidence.

And the roadmap Democrats passed for impeachment going forward signals that they're in this for the long haul.

2. An impeachment vote in the House fails

Nancy Pelosi and the Democrats control the House of Representatives. ( Reuters: Kevin Lamarque )

Before we get to the first vote in the House, the step right before is one of the most important to keep an eye on.

Molly Reynolds, a senior fellow with the Brookings Institute, says that's when the House Judiciary Committee assembles the findings of all the investigations into a formal document. This is the actual "Articles of Impeachment" you've probably heard about.

"And that looks, in a lot of ways, like another piece of legislation," Dr Reynolds said. But it gives us an answer to a key question: "What is the terrain that they set out for fighting this out in the House and the Senate?"

The House can choose to parse out issues into separate articles and vote to approve some and not others, as was the case in former president Bill Clinton's 1998 impeachment.

If no Articles of Impeachment can successfully pass the House with a simple majority, this doesn't go any further and Mr Trump remains president.

Chance of this happening?

Given the amount of precious policy-making time that Democrats will have put into drafting the articles at that point, it's pretty unlikely that they'd fail to pass them.

US representatives Rashida Tlaib, Ilhan Omar, Alexandria Ocasio-Cortez and Ayanna Pressley supported impeachment before the Ukraine scandal. ( Reuters: Erin Scott )

Dr Reynolds agrees "simply based on the number of Democrats who have come out in favour of the impeachment inquiry and that many of those Democrats did so before we had the additional information that we have now about the Ukraine call."

Oh, and if you're wondering when this vote could happen, your best bet is to just wait it out.

The Democrats, eager to avoid clashing with big election events, have said they'd wrap things up by December, but other reporting suggests that's too ambitious given the weight of the choices they'll need to make.

3. The House votes to impeach, the Senate acquits

Mitch McConnell and Republicans control power in the Senate. ( Reuters: Tom Brenner )

This is the outcome where Mr Trump's support with Republicans holds firm.

Dr Glassman said the Senate trial is likely to last for "weeks, not months", running six days a week. He said the constitution is very clear about impeachment, but it doesn't get into the nitty-gritty of the process.

It means Republicans will have a lot of control over the rules of the impeachment trial (like how long people can speak, what evidence can be presented etc.) because they hold a majority in the Senate.

Pretty much everything will be televised.

"The chief justice of the Supreme Court presides over the trial," Dr Reynolds said.

"We know that it would involve presentations of evidence from prosecutors from the House who are making the case for conviction. It'd also involve presentations by a defence team for the President."

As for the President himself appearing as a witness? Don't count on it.

"The idea that the President will be on the witness stand is a bit of a romantic notion," Dr Glassman said.

Republicans control 53 votes. As long as fewer than 20 decide to remove Mr Trump from office, he'll be acquitted.

He's still considered to be impeached (just like Bill Clinton), but as Dr Glassman points out:

"It functionally has no effect."

Chance of this happening?

Democrats control the House. Republicans control the Senate. And partisanship remains a major factor in American politics.

All of that means as things stand at the moment — this is the most likely outcome of this impeachment process, according to Dr Glassman.

"The most likely outcome at this point is a majority vote to impeach in the House, and the President is acquitted in the Senate," he said.

An overwhelming number of Senate Republicans still oppose impeachment. Of the 53 Republicans, 44 of them signed on to a resolution condemning the impeachment process so far.

If you want more of an idea of how partisanship is playing a hand in this process so far, look no further than the impeachment resolution that passed the House.

Every Democrat, except two, voted in favour. Every Republican voted against it.

Likewise, according to US poll analyst FiveThirthyEight, only 11 per cent of Republican voters support impeachment.

4. The House votes to impeach, the Senate convicts

This outcome is the end of the road for Donald Trump. ( Reuters: Leah Millis )

This is the worst possible outcome for Mr Trump. If 67 senators (that's every Democrat and independent, as well as at least 20 Republicans) vote to convict, he is removed from office.

It happens immediately, and Vice-President Mike Pence is sworn in as president shortly after.

"If they do choose to convict him there could be a subsequent vote that would prevent the President from seeking federal office again in the future," Dr Reynolds said.

That also needs a two-thirds majority vote, otherwise Mr Trump could run for office again in 2020.

Removal and barring someone from holding office in the future are the only punishments Congress can impose as part of the impeachment trial.

Chance of this happening?

Dr Glassman thinks this is very unlikely, but not for the reason you'd expect.

He said Mr Trump would catch wind of Republicans' plan to vote yes and resign before the Senate had a chance to vote, rather than lose a public vote and become the first president to ever be convicted.

5. Trump steps down (aka the Nixon approach)

Richard Nixon resigned after his support among Republicans collapsed. ( Reuters )

This could happen at any time. Or never.

If Mr Trump does decide to step down the impeachment process would stop, and Mr Pence would become president.

Dr Glassman said if (and remember it's a BIG if) this was going to happen, it would play out just like it did in 1974, when Republican leaders visited president Richard Nixon to tell him that his support had collapsed, and he would certainly be impeached.

"In 1974 Republicans ended up being sick of the president lying to them," he said.

"It will come down to the fraying of the relationship."

Chance of this happening?

Probably never ... as the facts stand at the moment.

"There's very little about Trump's personality that suggests to me that this would be something that is possible," Dr Reynolds said.

"But at the same time, we don't know what new information could come out."

TL;DR? Here's a chart explaining it all