There were three types of assassination attempts against Doctor Hooves.
One: Doctor Hooves was provided an adulterated sandwich. Hay laced with arsenic. He started to eat it, was interrupted by his wife, set the sandwich down in front of a sideways rocket engine. Roasted remnants post-qualification-testing wouldn't trouble any pony or beast.
The sandwich ruse was repeated three more times. Probably by the same bad actors. But maybe not.
The second time, he ate the sandwich, accepted a kiss from his wife, who accidentally stumbled and fell tangled in his limbs, forelegs jutting into his stomach inducing a gag reflex. He vomited out the poison.
The third time, he was bumped by his wife, who simultaneously smashed a glass container full of rocket fuel as she flared her wings in surprise. That dropped sandwich was consumed a few hours later by a burly subordinate, who fell violently ill but ultimately survived, likely because the poison had been formulated for a smaller body frame. A nameless government investigator who smelled of chocolate and bon bons reported that the under-dosage must have been done under hopes no toxicology test would be undertaken, or if it was, that the chemicals would be mere traces by the time somepony ran a test.
The fourth attempt saw Doctor Hooves ingest the sandwich. His wife was... away.
Two: A diamond dog performed a beg-salute, legs dangling like a rabbit's as it stood on both hind legs. Doctor Hooves looked over at his smiling wife who was clapping her hooves together at the performance, then trotted over and provided the dog a bit. The dog's eyes flared wide open and it grabbed in its teeth not only the proffered bit but also the carrying hoof. Doctor Hooves cried out and released the bit, which migrated down the dog's throat and caught in it. The dog jerked back, wheezing.
"It's having a seizure!" Mrs. Hooves exclaimed, flaring her wings. "We have to save it!"
Doctor Hooves, wincing from the pain and the slobber nevertheless sprung into action and tried to perform the Heimlich maneouver. "I am not certain if this works on its anatomy--" he said.
"We have to try. I'll get help! Poor thing." Mrs. Hooves rose into the air.
Despite Doctor Hooves's best efforts, the dog choked to death.
Medical professionals and a couple of sunglass-wearing professionals from the government looked into the demise but didn't examine too deeply as there was little reason to suspect espionage. The professionals had arrived because the diamond dog was a foreigner and thus the strange death might be diplomatically sensitive.
At the moment of the diamond dog's death, the space race had still been in its infancy--matters would progress quickly over the next year, and much work had been done behind the scenes but Equestria did not yet know about that.
Griffons from the Griffonian Socialist Creatures Republic (GSCR) had only a week before, on National Liberation Day, announced their intent to surpass the "arrogant ponies" and to demonstrate that even though ponies "controlled vast capital, that the labor of hard-working intelligent griffons will certainly outmatch the intellectually and morally bankrupt ponies now that the bourgeois feudal Princess Celestia had retired after a thousand-year reign of terror." Despite the violence of the statement, few ponies took it seriously. Most ponies' familiarity with griffons came from Griffonstone--a failed state hyper-capitalistic vassal protectorate of Equestria. The GSCR was across an ocean, a vast former colony of Griffonstone that had been lost, from Equestria's perspective, for hundreds of years after the Griffonian Empire's collapse.
A bit was found in the grass beside the dog, and it was quickly established that the dog must have spit out the one it had bit. Thus, nopony investigated the full circumstances of the demise too deeply, and the unidentified dog, whose existence the diamond dog embassy denied having any knowledge of, though they did accept a hefty donation in memorial of the dog, was buried without further incidence.
I ensured Doctor Hooves received proper medical treatment for his chipped hoof. He made a full recovery!
Three: Piranhas with teeth polished with poisoned toothpaste were released into Doctor Hooves' self-constructed hot tub. The piranhas ended up poisoning each other to death and neither Doctor Hooves, nor his wife entered the water after she side-eyed the bobbling heads while Doctor Hooves was preparing to back into the tub.
Princess Twilight Sparkle ordered an investigation after the piranha attempt. And, in the course of that investigation, she discovered residue from a poisoned sandwich. Ponies were interrogated, among them the Doctor's close assistant, Roseluck. Peeved, Doctor Hooves filed a complaint with Mrs. Harshwinny who duly filed it away and never looked at it again.
Most accused ponies were cleared. Two were dismissed: a married couple with close contacts to griffons influenced by the GSCR--they'd been caught bird-clawed passing mini-camera-captured research. Their ingenious mini-cameras were seized and the Doctor and Twilight's special mathemagician consultant Moondancer marveled.
The GSCR griffons had better technology than ponies had realized. Their political system was supposed to have doomed them to be backwards, but they somehow manufactured an advance that Equestria couldn't have imagined. Although Moondancer and Doctor Hooves salivated over the advanced technology, other government agents were assigned to reverse-engineer the cameras for Equestria's benefit. Roseluck and Mrs. Hooves kept Doctor Hooves on target and physically steered him back to his work.
Mrs. Harshwinny, Overseer of day-to-day operations, also stuffed her snout into the investigations. Mrs. Hooves found herself banned from project grounds in a technical sense if not a practical one--everypony knew whose spouse she was and the ones who were closest to him weren't particularly interested in keeping her out. It was impossible to say she was harmless since she caused innumerable accidents. But she definitely wasn't a spy. And she had important experience to impart. The pilot candidates, more or less, listened. But the emphasis on more or less was definitely on the less with respect to ones not named Spitfire or Soarin.
The government's leading theory was all attempts could be traced back to one culprit--the space-seeking griffons.
"I'm not so sure," Mrs. Hooves had commented, eating toast her husband had prepared in the morning, and nibbling a buttery muffin she had made last night. "Multiple creatures may wish the program ill. There could be doomsday cultists like the Rosy Crucifixtions or those Tartarus Gate minotaurs. It's against their religion to go to space."
"Who has time to listen to ravings of madmares and stallions?" Doctor Hooves shrugged.
"I think I have some pamphlets Roseluck's sisters gave me," Mrs. Hooves busied herself in a drawer and came out with a stack. "Here, I think. Sorry there's a lot of Neighponese restaurant menus mixed in. It might take a while to sort." She started turning over advertisements, squinting and cocking her head.
"I'm so close to success," Doctor Hooves said.
"That's great," Mrs. Hooves smiled. "But if your life is at risk, what are you weighing it against?"
"Knowledge! Invention! Wisdom!"
"It has been fun," Mrs. Hooves smiled. "But is that enough when the risk is really high? I don't want to lose you."
"Hope! It will bring hope. If we can leave Equestria, we can find resources elsewhere, friends elsewhere. Live elsewhere. More room! More experiences."
"I can get behind hope..." she looked back at her bubble cutie mark. "It's who I am."
"Yes, certainly," Doctor Hooves nodded.
"At least I think it's who I am. Though, I haven't always felt a lot of hope. What if my bubbles mean something else... like something related to--I dunno. I feel bad for saying this but maybe that hope you expressed is a bit ambitious. We can do a lot more on Equestria here to make friendship with other creatures... to make life better for everycreature! We can do things that will help everycreature every day!"
"Never fear, Dear! I could always continue work on the Time--"
"But what about the horrible event last time..." Mrs. Hooves trailed off cracked a smile. "This project is good," she stroked Doctor Hooves' mane back and forth. "Maybe I spoke too hastily. I'm just bubbly and silly sometimes."
"Don't say that in such a discouraging tone, Dear. You're no sillier than me." They nuzzled snouts, then Mrs. Hooves returned to stroking her husband's mane.
"It's simply that... the time device is a little dangerous. Remember Minuette?"
Doctor Hooves could feel her shiver through her fetlock. "She did make it back..."
"Eventually. Barely. Umm... safe timey wimey plans... seem a bit far off." She forced a dry giggle. "But promise me you'll quit these rockets if it becomes too dangerous. The benefits aren't worth the life of anypony. Could we live with somepony's death on our souls?"
"No," Doctor Hooves frowned.
"Besides, is there really a problem for ponies if griffons reach space first?"
"Then I won't have achieved anything."
"Really?" She looked around at pieces of rockets and equipment.
"We did so much together," he said. "I'd like to finish what we started. A wondrous capstone!"
"But, before you said you cared most about helping ponies. And you're so good with Dinky."
"She's wonderful. I can't believe I never considered having a foal. Was too wrapped up in my studies. Snug as a bug eating up a book, I was! Self absorbed! Shame!"
"Well... we came together. And it's right now."
"Indeed. Indubitably. Inarguably, yes."
"And we have another wonderful accomplishment right here," she navigated one of his hooves to rest against her belly.
"You mean--a--oh--"
"Mmm... we're going to have a foal."
"Oh, Derpy," Doctor Hooves kissed her. She set aside her pamphlets.
Back to the sandwich.... I wasn't there to help... and mission control was panicked because my husband was going to die....