When some companies go public, instead of just offering common shares, they offer a bundled Unit which my also include rights and/or warrants. In general, the components tend to begin trading individually once the IPO goes occurs.
Rights tend to have a shorter expiration whereas warrants are effectively long term options (1-2 years or more). Rights and warrants offer investors the opportunity to buy more stock. The warrants effectively pre-commit the company to selling more stock in the more distant future at the warrant's strike price.
In the case of your stock, one unit consisted of one common share and 1/3 of a warrant to purchase one share of common stock by 10/25/24 at $11.50. This info is from a quick Google search. I did not read the final SEC prospectus so I do not offer this as complete information - though I don't think that there were additional components in the original unit.
Since SPCE/U consists of one share (SPCE) and 1/3 of a warrant (SPCE/WS) , it is indeed worth more than a share of the common (SPCE)
If all three securities are still trading, the value of the unit should be equal to the total value of the warrant and a common share but since the stock market is an auction, it can vary mildly during the day. It won't vary much since any deviation outside of U = C + W will present an arbitrage opportunity and price will be driven back in line.
