After soaring higher in 2024, shares of Rocket Lab USA (RKLB -1.51%) haven't enjoyed the same upward trajectory so far in 2025. For those who have hopped aboard for a ride with the space stock, this year's poor performance -- a 26% plunge -- is surely disconcerting as the S&P 500 index has remained about flat.

But the stock price is hardly the most important figure to monitor. Let's take a closer look at what should be a greater concern for investors to watch as Rocket Lab continues trying to shoot for the stars.

Are customers lining up to lift off with Rocket Lab?

From private companies like Varda Space Industries to BlackSky to government contractors, Rocket Lab provides launch services to a variety of customers. In fact, Rocket Lab proclaims that its Electron rocket was the third-most frequently launched rocket in 2024 after SpaceX and China. While it's certainly a notable achievement, the company's ability to retain interest from customers is critical for its continued success. With this in mind, current (or potential) investors will want to gauge the company's backlog.

In 2023, Rocket Lab succeeded in steadily growing its backlog through the year. At the end of 2023, the company's backlog stood at $1.046 billion -- the result of steady growth from the second and third quarters of 2024 when the company ended the periods with backlogs of $534.4 million and $582.4 million, respectively.

Success in securing future business from customers slowed down, however, in 2024. After reporting a backlog of $1.05 billion at the end of the third quarter of 2024, Rocket Lab ended 2024 with $1.067 billion in backlog, a 2% year-over-year increase.

What's a growth investor to do now?

While there were some bright spots in Rocket Lab's recent fourth-quarter 2024 financial results -- 2024 revenue rose 78% year over year, for example -- the slow backlog growth is concerning.

Investors may want to sit tight, waiting to see if Rocket Lab succeeds in debuting its Neutron rocket this year -- an achievement that can help the company remain on a growth trajectory.