The past year has been tumultuous for the stock market, to say the least. But even the worst downturns are only temporary, so it's only a matter of time before we enter a new bull market.

When exactly that will happen is unclear. The market can be unpredictable in the short term, so even the experts don't know what will happen over the coming weeks or months. But if you wait to invest until the market is already thriving, you may miss out on the lucrative recovery period.

Right now, then, can be a fantastic time to buy. Many stocks are still priced at a discount, and you'll reap the rewards during the next bull market by investing now. And there's one ETF that could help you make a lot of money: the Vanguard S&P 500 ETF (VOO -0.67%).

Why buy an S&P 500 ETF right now?

The S&P 500 ETF strikes a balance between risk and reward. It's one of the safest investments out there, and it's almost guaranteed to see positive long-term returns. At the same time, though, it can help grow your money exponentially.

The Vanguard S&P 500 ETF tracks the S&P 500 itself, which means it includes the same stocks as the index and aims to replicate its performance. By investing in this ETF, you'll own a stake in 500 of the largest, strongest companies in the U.S.

With an S&P 500 ETF, you're almost guaranteed to make money over time -- even if the market is volatile. The index has experienced dozens of bear markets, recessions, and crashes over the decades, yet it's recovered from every single one so far.

In fact, data from Crestmont Research also shows that historically, there's never been a bad time to invest in the S&P 500. The index's rolling 20-year total returns have been positive every single year since 1900. That means if you had invested in an S&P 500 ETF at any point after 1900 and held it for 20 years, you'd have made money -- regardless of how severe the volatility was during that time.

The Vanguard S&P 500 ETF, in particular, is a strong choice because of its low fees. This fund boasts an expense ratio of just 0.03%, which is substantially lower than the average ETF. Over time, that could save you thousands of dollars in fees.

How much can you earn with this investment?

S&P 500 ETFs may be one of the safest investments out there, but they can still help you make a lot of money over time.

Historically, the S&P 500 itself has earned an average rate of return of around 10% per year. That means all the annual highs and lows have averaged out to roughly 10% per year over decades.

If you were to invest, say, $200 per month while earning a 10% average annual return, here's approximately how much you could accumulate, depending on how many years you consistently invest:

Number of Years Total Savings
20 $137,000
25 $236,000
30 $395,000
35 $650,000
40 $1,062,000

Data source: Author's calculations via Investor.gov.

By giving your investments as much time as possible to grow, it's possible to become a stock market millionaire with this ETF -- even if you can only invest a couple of hundred dollars per month.

The key is to do your best to ignore the market's short-term fluctuations and stay invested for the long haul. By keeping a long-term outlook, it won't necessarily matter what the market does over the coming weeks or months.

The market may be rocky right now, but a bull market is on the way. By investing now, you'll be well positioned to take advantage of it. And with the Vanguard S&P 500 ETF, you can turn a few hundred dollars per month into hundreds of thousands of dollars or more.