[mrsmith]

"drag due to interstellar gas"
Impact and accretion of cosmic debris would about fit their measurements IMHO.

"Look here brother-who you jiving with that cosmik debris?
Now is that a real space probe or is that a Sears space probe?"

[joebuck]

"Now is that a real space probe or is that a Sears space probe?"

LOL. I don't know, let me examine it with my handy pair of zircon encrusted tweasers.