While refueling at a gas giant in a remote system, sensors indicate a very faint incoming transmission. At first it seems to be a random series of numbers, but computer analysis indicates it is a data stream, compressed using a method long out of date (f rom the Last Imperium if adventure is set in the New Era universe). Triangulation indicates the transmission is just above the pressure horizon (max depth into the atmosphere) for the ship and spacesuits the party is using. The source also appears to be slowly sinking - if the party decides to go down they will only have a limited time to do so. The exact time is up to the GM, but should be short enough to prevent the players from going for help.
If the players investigate, skill rolls for piloting and sensor ops are appropriate. The object will turn out to be a large (1000 ton displacement) sphere with rather obvious dents and other damage, which reveal that the sphere is very heavily armored. Sensors indicate low power. If the players have a neural activity scanner, they will get no readings. There are no obvious markings.
Classic Traveller or Megatraveller Version
The object is a scientific probe used to explore the interior of gas giants, collecting data and taking samples. It is designed to descend and ascend on a regular basis - apparently the scientific team that was collecting the data is no longer here for some reason (see below). The probe will open for maintnence and sample retrieval if a code is transmitted - the players should be given some way of figuring this out (a small panel on the hull, an entry in a databank on their own ship, decoding the data transmission, etc.)
1) The scientific team left because it thought the probe was destroyed - it was only damaged and took much longer to come to the surface. If the players go on board, they may be able to fix the probe and download the data collected. This will be a race against time, as the probe slowly sinks into the atmosphere. If the players find the scientific organisation that placed the probe, they will be rewarded in some suitable way.
2) As 1 above, but the probe is damaged beyond repair. The players may be able to salvage some of the sensor equipment.
3) As 1 above, but pirates are using the probe as bait - they will attack the players during the salvage operation.
4) As 1 above, but the probe has brought with it some Blobs (tm) from down below. The blobs are unintelligent, fearless, and in pain - they attack the players, of course. Adjust number and strength of Blobs (tm) to challenge players.
5) As 4 above, but these are Smart Blobs (tm). In fact, it's a team of Smart Blob (tm) scientists and explorers (Smart Blob (tm) player characters, of course), and they and their friends down below captured the probe and are now using it to explore the u pper atmosphere. They are excited to meet these strange carbon-based life forms, and would like to set up trade relations. If they play their cards right, the players could wind up rich and famous.
6) As 5 above, but these are Evil Super-Smart Blobs (tm). They want to take the players with them as samples, and will stop at nothing. Good luck surviving high pressure.
New Era Additions
The above scenarios can be complicated by the presence of a Virus in the probe's computer. The exact type is up to the GM, but a "Hobbyist" strain is most appropriate. The hobbyist virus, if contacted, will be happy to talk about the gas giant and how fun it is going up and down in the atmosphere, and will even cooperate with the players if they help fix its broken systems.
Note: Blobs (tm) appear courtesy of GDW's Azhanti High Lightning game; Smart Blobs (tm) and Evil Super-Smart Blobs (tm) appear courtesy of several twisted campaigns run by yours truely.