The novel concerns people with incurable injuries and defects (biocompensators) who volunteer for the first interstellar flight.Turned down by the medical authorities, they are refused after they steal a ship to plead their case directly to the people of Earth. Returning to their asteroid hospital, they rig a working gravity drive and escape from the solar system. They head towards the Centauri system, solving problems such as overcoming a limited supply of needed pharmaceuticals and improving the drive. Earth sends out a ship, the Star Victory with a better version of the new drive in a bid to beat them there, on the theory that any aliens they encounter might make the mistake of thinking of them as normal humans. The Victory does beat them by a few days, arriving a year after they left, but they settle on a Mars-like planet near Alpha Centauri, before the Victory makes contact with butterfly-like aliens on a methane gas giant in the Proxima Centauri system. The aliens are somewhat similar in technology, but lacking a reliable gravity drive. They want a reasonable sampling of humans to study, but the crew of the Victory are all specialists and they are not welcome. The victory leaves the system to the Accidentals, promising to return, someday.

Question: What do the aliens lack?
Answer: A relilable gravity drive.
Alternative answer: A reliable gravity drive