"Breaching in five, four, three, two, one...we have arrived in the first gaseous layer, ladies and gentlemen." Navigator Epulum commented. A peach mist covered every window. The group members congratulated each other. The youngest, Vinalia fistbumped the air as the rest laughed good-naturedly at her enthusiasm.
[Taking samples.] A robotic voice notified them. For now, they would be staying here, taking some samples and then they would try to breach the second layer. Should it prove caustic, they would go back.

Their victory was short-lived, however. A strange being appeared in front of them, unexpectedly clear despite the barely transparent gases. They looked a lot like a human, but it's skin was grey and it's body seemed only halfway finished, the rest ending in some kind of gas. Their long, white hair flowed as if they were in water and to every member of the crew, their red eyes seemed focused on them. They opened their mouth, speaking.
"Open communicative frequencies." The captain commanded.
"Communicative frequenties opened."

"-hear me?" A voice rang out. The voice was understandable, close to normal, but not completely. It distorted sometimes, as if it was unstable. It made their hair stand on end. They had not yet activated their side.
"Should I..?" The captain nodded. Ludi pressed a button and flipped several switches.
"Affirmative, you are heard." They seemed to smile at this.

"Good. I have a message for you: turn back, if you value your lives."

"We do not mean to do harm."
"Turn back. You are intruders." The group wasn't going to give up so easily, after traveling so far.
"We are merely doing tests here, to see what composes the gases of Jupiter...this gas giant." He elaborated.
"Do you not understand? Leave, at once. We do not want you here."

The crew discussed.
"To be honest, I don't want to tangle with whatever that is."
"I know, but we have a mission."
"What can he do to us anyway?"
"Please don't say that, I don't want to know."
"Guys, he's still looking at us."
"Fine. We'll turn back and return on the other side. If we're lucky, it won't even notice."

A button and...wait.
"I left communications on." Ludi whispered. Captain Ides face-palmed.
"I will have to force you out, then." The sound turned into static. The being turned completely gaseous, then moved with incredible speed to slam into the spaceship. They kept on doing this, trying to push them back. They were no match when they started up the engine, however.

Suddenly, the banging stopped.
"Do we have any visuals on it?"
"No, captain." She cursed.
[Ventilation system breached]
Dark gas seeped from the vents. They held their breath as Vinalia tried to eject the poisoned air and replace it. Breathing meant death.

Soon, there was only one. The ship was theirs. They set a course for earth. Following them back would have been easier, but this should do.

"Finally, something new to destroy."

A/N: 500 words, exactly. The code names of the crew are festivals or days dedicated to the God Jupiter.