Doctor Anderson woke up to her phone pinging. Careful not to disturb her husband she checked it, and saw that it was a new supernova. She slipped out of bed and went to the kitchen, where she logged into the observation database.

Almost at once she could see something wrong. The report only came from the visible light observatories. None of the radio telescopes - not even the wide-field one - had reported anything.

By morning it was clear that this was not any sort of object that was normally her field of interest. It was, however, not one she was going to stop studying. For starters, it was clearly inside the solar system - well inside the orbit of Jupiter. For another, it was accelerating - at about 1.5G if the repeat observations were correct. It was also getting brighter - a lot brighter, almost as if it were getting bigger.

Her husband found her staring at her laptop in the morning, a half-empty and stone cold cup of tea next to her.

"We have to get the kids up for school."
Without looking up "Can you? I've got something here" she replied.
He looked over her shoulder. "Um, that's not an FRB. Or a supernova," he paused, clearly doing maths in his head "That's a powered trajectory."
"Yep. It's not heading for us, we don't think. It looks like it is planning a slingshot."
"So how come you're heading it up?"
"I spotted the anomalous behaviour first."
"And by anomalous, that it was accelerating."
"And getting brighter. We're re-pointing a couple of the orbital scopes."
"I'll get the kids ready."
"Thanks - you're a wonder."

By the time breakfast was over, she was able to show her family a picture - a flower-shaped object, with a dark core, and fourteen bright petals around it. She printed three of them off, so that the kids and her teacher husband could take them with them.

"Our initial estimate is that it is about two hundred metres across. And it is holding a constant 1.5g acceleration. That is an insane amount of energy."

By tea time, there had been no response to radio transmissions, and there had been no change in the trajectory. The following morning brought a dramatic change, though. Overnight the acceleration had changed to a deceleration.

"It will stop just inside the Mars orbit around about midnight tomorrow. We've got a good look at it now, though."

The petals now appeared bluish, and the dark core now appeared to be a grey colour.

The following night, after getting the family up, all four of them clustered around the laptop watching the feeds from the telescopes as the craft finally slowed to a constant velocity. As they watched, the petals suddenly folded back. A few minutes passed - a sudden burst of violet light - and it was gone.

"Mummy where is it?" the youngest asked.
"I don't know, sweety. I just don't know."
"Did it explode?"
"There's no sign of debris, so no" It had vanished completely..
"Maybe they were recharging their engine."
"Maybe. Maybe."