SPOILERS FOLLOW for Star Control 2. (Though if you haven't finished it in the ten-plus years since its release, you're taking way longer than necessary.)

The Slylandro are a race of sentient gas bags, who live in the outer layers of a gas giant they call "Source". They live for thousands of years, spending the whole time aloft as there is no solid ground to rest on. Indeed, even drifting too high or low in the atmosphere would be fatal to them, due to the difference in pressure. For this reason they cannot survive the rigors of space travel.

Though they are intelligent, they have no technology of their own. In their history, some individuals have attempted to craft physical tools from the carcasses of dead beasts, but every attempt was doomed to failure. Eventually the object would become too heavy and drag the crafter down to his death.

The Slylandro do have access to a couple of tech items acquired from other races. Most important of these is an automated Von Neumann probe purchased from the Melnorme. "Catalog item 2418-B" is a self-replicating robot vessel, programmed to make peaceful contact with other races, gather data on the wider galaxy, and return to Source to report. Along the way it will gather raw materials and construct more probes, so that exploration will happen more quickly.

Unfortunately, the Slylandro made a programming error: the priority for self-replication was set much too high. The probe does seek out other ships and transmit a message of peace, as intended. But as soon as communication ends, the probe shifts into resource gathering mode and attempts to break down the target into its component molecules. This process is highly inconvenient for the crew of the target starship.

In the time period covered by Star Control 2, the probes start out as a minor nuisance to the player, because there are few of them. As time goes on they become increasingly common. If left unchecked they will be so ubiquitous that completing the game becomes practically impossible. (Notionally, they will continue replicating until they destroy all life in the galaxy.) To prevent such a disaster the player must discover the Slylandro homeworld, inform them of their error, and have them reprogram the probes to self destruct.

The probe's appearance is that of two tumbling red spheres connected by an energy field. It has an odd form of inertialess drive, always travels at top speed, and can reverse direction instantaneously. Its sole weapon is a close-range lightning bolt, which does reasonable damage but burns energy rapidly. To recharge, the probe must approach an asteroid and use a secondary beam to harvest the rock as fuel.

When attacked by a probe, your strategy is to stay out of reach of its weapon. Ships with very heavy weapons, like a fully upgraded Precursor flagship, can destroy it before it closes. Fast ships like the Spathi Eluder should run away and snipe. Slower ships like the Earthling Cruiser must expect to get flanked and take significant damage, but can then fire upon the probe as it withdraws to recharge.