Here are the relevant quotes:

"Other people have done huge amounts for me!" Harry said. "My parents took me in when my parents died because they were good people, and to become a Dark Lord is to betray that!"

Professor Quirrell was silent for a time.

"I confess," said Professor Quirrell quietly, "when I was your age, that thought could not ever have come to me."

"I'm sorry," Harry said.

"Don't be," said Professor Quirrell. "It was long ago, and I resolved my parental issues to my own satisfaction.

[...]

"Have you heard of the Pioneer program?" Harry said. "They were probes that would fly by different planets and take pictures. Two of the probes would end up on trajectories that took them out of the Solar System and into interstellar space. So they put a golden plaque on the probes, with a picture of a man, and a woman, and showing where to find our Sun in the galaxy."

Professor Quirrell was silent for a moment, then smiled. "Tell me, Mr. Potter, can you guess what thought went through my mind when I finished assembling the thirty-seven items on the list of things I would never do as a Dark Lord? Put yourself in my shoes - imagine yourself in my place - and guess."

Harry imagined himself looking over a list of thirty-seven things not to do once he became a Dark Lord.

"You decided that if you had to follow the whole list all the time, there wouldn't be much point in becoming a Dark Lord in the first place," Harry said.

"Precisely," said Professor Quirrell. He was grinning. "So I am going to violate rule two - which was simply 'don't brag' - and tell you about something I have done. I don't see how the knowledge could do any harm. And I strongly suspect you would have figured it out anyway, once we knew each other well enough. Nonetheless... I shall have your oath never to speak of what I am about to tell."

"You have it!" Harry had a feeling this was going to be really good.

"I subscribe to a Muggle bulletin which keeps me informed of progress on space travel. I didn't hear about Pioneer 10 until they reported its launch. But when I discovered that Pioneer 11 would also be leaving the Solar System forever," Professor Quirrell said, his grin the widest that Harry had yet seen from him, "I snuck into NASA, I did, and I cast a lovely little spell on that lovely golden plaque which will make it last a lot longer than it otherwise would."

...

...

...

"Yes," Professor Quirrell said, who now seemed to be standing around fifty feet taller, "I thought that was how you might react."

...

...

...

"Mr. Potter?"

"...I can't think of anything to say."

"'You win' seems appropriate," said Professor Quirrell.

"You win," Harry said immediately.

"See?" said Professor Quirrell. "We can only imagine what giant heap of trouble you would have gotten into if you had been unable to say that."

They both laughed.

A further thought occurred to Harry. "You didn't add any extra information to the plaque, did you?"

"Extra information?" said Professor Quirrell, sounding as if the idea had never occurred to him before and he was quite intrigued.

Which made Harry rather suspicious, considering that it'd taken less than a minute for Harry to think of it.

"Maybe you included a holographic message like in Star Wars? " said Harry. "Or... hm. A portrait seems to store a whole human brain's worth of information... you couldn't have added any extra mass to the probe, but maybe you could've turned an existing part into a portrait of yourself? Or you found a volunteer dying of a terminal illness, snuck them into NASA, and cast a spell to make sure their ghost ended up in the plaque -"

"Mr. Potter," Professor Quirrell said, his voice suddenly sharp, "a spell requiring a human death would certainly be classified by the Ministry as Dark Arts, regardless of circumstances. Students should not be heard talking about such things."

And the amazing thing about the way Professor Quirrell said it was how perfectly it maintained plausible deniability. It had been said in exactly the appropriate tone for someone who wasn't willing to discuss such things and thought students should steer away from them. Harry honestly didn't know whether Professor Quirrell was just waiting to talk about it until after Harry had learned to protect his mind.

"Got it," Harry said. "I won't talk with anyone else about that idea."

"Please be discreet about the whole matter, Mr. Potter," Professor Quirrell said. "I prefer to go through my life without attracting public notice. You will find nothing in the newspapers about Quirinus Quirrell until I decided it was time for me to teach Defense at Hogwarts."

That seemed a little sad, but Harry understood. Then Harry realized the implications. "So just how much awesome stuff have you done that no one else knows about -"

"Oh, some," said Professor Quirrell.