A somewhat challenging space alien identity puzzle

Miscellaneous and Personal Stuff I Must Share
1

It’s years in the future, and the One-World Earth (OWE) government has built 2 spacestations in the Cosmos. These are Spaceport X and Spaceport Y.

The OWE operates 20 spacerunner ships that do nothing but travel between the the Spaceports carrying secret space information back and forth. Each spacerunner craft has its own pilot, and no other pilot is allowed to fly that spacerunner. The spacerunners must travel on a special path known as a space highway. The space highway is a fairly direct narrow avenue between the two Spaceports. Much like cars on our divided roadways of today, spacerunners travel back and forth on the spacepath. Spacerunners travelling from Spaceport Y will pass immediately to the side of those travelling from Spaceport X, and vice-versa. In fact, the spacerunners pass so close it is common practice for the pilots to wave to each other as they go by.

Only 1 spacerunner may leave each Spaceport each day, and that’s at 3PM EST. Each day at 3PM EST a spacerunner leaves Spaceport X and heads directly for Spaceport Y. At the same time, a spacerunner also leaves Spaceport Y and heads for Spaceport X. It takes exactly 9 days to the minute to complete the trip, and spacerunners always travel at a constant speed. Therefore, when a spacerunner leaves Spaceport X, the one that left Spaceport Y 9 days earlier is just arriving. The just departing pilot always waves to the just arriving pilot. It’s good manners.

Now everything was going fine for a while, but suddenly lost cargo and sabotage have become a problem. Intelligence for the OWE discovers that 9 of the 20 OWE pilots have been replaced by evil duplicate twin space aliens! They probably made the switch at the “peace” conference at Spaceport Y a few months ago. It’s impossible to tell from appearance the loyal pilot from the space alien duplicate. That’s why you have been called in. The newest spacerunner, #21, has just been built. Your mission is to pilot this craft between the two spaceports and pick up whatever information you can. Your special spacerunner #21 has been equipped with a Super-Duper Space Alien Pilot Detector. When you aim it at another ship as it passes by, you’ll be able to tell if the pilot is an earthling or a space alien. Good thing you have special “alertness” pills to keep you awake for 9 straight days.

So now it is 3PM EST. As your spacerunner leaves Spaceport X you aim the identifier at the other spacerunner just arriving from Spaceport Y. It looks like Albert at the controls, but can you be sure? You wave and turn on the detector. Yep, it’s Albert true and real, your Space Alien Pilot Detector confirms. You wave and smile. You are glad good old Albert has not been replaced by an evil space alien.

As you travel toward Spaceport Y you’ll pass other spacerunners heading back to Spaceport X. You’ll aim the detector at each. You know that any spacerunners not in flight are placed in solitary confinement, and their pilots disappear to the spacebars only to return when it’s time to fly again. You’ll have no chance to check any pilot not in flight.

The OWE Intelligence wants to know how many space alien imposter pilots can you GUARANTEE you will be able to identify by the time you arrive at Spaceport Y. How many?

2

Four, of course.

That is SO obvious. :rolleyes:

3

Wow, talk about a complicated set up. Glad it doesn’t have any plot holes. Because I know that if I were actually trying to solve this problem, there would be no way I’d consider sticking around the hanger and pointing the gun at the pilots as they enter the craft. As the man in charge of the spaceport, I know I wouldn’t even consider halting their travels until I was sure they were good guys. As an actual good guy, I know I would love to be accused of being a bad guy and I would just sit there and do nothing to clear my name. That would be silly!
So my answer, which I can guarantee is wrong: zero. There are 11 good guys. He’s met one. That leaves 10 and 9 meetings left. It is possible, though not probable, that he’ll meet all good guys the entire way there and still not know which one of the ten left is a good guy.

4

I think Enderw24 is right. The answer is zero. On the way to Spceport Y you meet Albert and 9 others, then you hang around till it’s your turn again without scanning anyone. When you take off again back to Spaceport X, you meet Albert and the same 9 guys on the way back. You can’t guarantee you’ll catch any of the bad guys.

Jim

5

No correct answers so far. Hmm…is it time for a hint? This riddle is a version of another puzzle that’s been around a long time–with just a slight twist. You certainly can land at Spaceport Y with a lot more information. Try again…

6

I knew my answer was wrong. The reason is that it’s too obvious of an answer. The problem is that I don’t see any reason why it isn’t the correct answer. You asked for a guarantee. There are ten meetings and 11 good guys. It is possible to meet every good guy with one left over by the time you reach spaceport Y. There is no guarantee.

7

He can guarantee just one. As he leaves in ship #21, he passes #1 at the spaceport gate. One day out he passes #2, and so forth until he reaches the other gate and waves at #10. If he spots NO aliens, the remaining nine are the bad guys. If he sees only ONE, he has a lingering doubt about the unseen 9 (eight of nine are bad). If he sees TWO, the odds of pinpointing the badguys go from eight of nine to seven of nine. He can GUARANTEE just one.

8

So the ability to count is overrated.

#21 sees 10 other craft, leaving 10 unknowns. If none are seen, there are nine badguys out of the ten remaining.

9

I guess this puzzle was a little more than “somewhat challenging.” Oh well, from my few month’s experience at these boards I’m sure someone will figure out the correct answer. I’ll be ready to tip my hat to the one that does first!

10

Assuming we’re talking about nonrelativistic speeds, I should encounter a ship every half day. Remember, I’ve travelled a half-days distance while the approaching ship travels the same distance, so I’ll see another ship twice each day.

I should be able to scan 18 ships by the time I arrive, leaving only two unscanned. If I find 7 aliens, I’ll know the other two are aliens, so I’ll find them all. If I find eight, I’ll be unsure which of the other two are bad guys.

Therefore, I can guarantee at least 8 discovered by my arrival at Y.

11

OK, Saltire, recall you scanned Albert as you departed. Number of ships passed equals one every 12 hours PLUS the one at ZERO hour. As you scan at the docking port, you’ve scanned the 19th ship. If you see eight bad guys, the last one is also bad. The answer is nine. Good ROI on the Super-Duper Space Alien Pilot Detector.

12

Saltire is almost right - as you leave, there are nine ships in flight in the opposite direction, plus Albert whom you have already scanned, and since you will be travelling 9 days, 9 more ships will be launched before your arrival. You will scan all 19 before you arrive, leaving you with deadly 100% accuracy.

13

I knew I’d be off by one. I always do that in these sorts of things.

14

Congrats and a tip o’ the hat to Uncle Bill