Yesterday SpaceX succeeded in the first Falcon Heavy launch, but the payload, a Tesla car has no real useful purpose (except for company prestige).
Thus, can it be considered as space junk ? How long will the car stay in orbit and has it been designed for an atmospheric disintegration? Is the car equipped with a propulsion system to change its trajectory in case of imminent collision risk ?
First the payload does have a purpose: it is a boilerplate, and those have a purpose, namely to "test various configurations and basic size, load, and handling characteristics of rocket launch vehicles".
Second, you are asking...
is the car equipped with a propulsion system to change its trajectory in case of imminent collision risk ?
No, it is not. The payload is not in Earth orbit any more. It is in an elliptical heliocentric orbit. The likelihood that is will ever be a collision risk for anyone or anything is infinitesimal.
Generally — as this NASA page states — we only consider objects in Earth orbit to possibly be space junk, or "orbital debris" as the more technical term for such things are.
1). What are orbital debris?
Orbital debris are all man-made objects in orbit about the Earth which no longer serve a useful purpose.
The reason only objects in Earth orbit are considered "orbital debris" is because only those are of relevance to us. We do not expect to ever run into man-made objects that are not in Earth orbit, simply because the probability of a collision is so small and the number of them is so low, that combined it is not worth the effort to try to prevent any such collisions. By comparison we do not even try to protect against meteor/meteorite strikes even though the Earth is hit by such — the size of the Roadster or larger — several times each year. If that does not bother us enough to warrant taking measures to prevent it from happening, why would the Roadster warrant it? It simply does not.
If you personally want to call the Roadster "space junk" you may do that of course, but I will counter that by saying "Yeah but it is harmless space junk".
Yes, it's space junk: after about 6 hours, the second stage will stop working and there will be no way to change the trajectory of stage and payload. So it's a non-functional satellite, i.e. junk. An object whose course cannot be controlled, and a potential future navigation hazard.
It's not in Earth orbit, so it's unlikely to cause a problem here. There is no propulsion system that will work after about 6 hours, and no sensors to detect an imminent collision.
If it reenters at some point (because its orbit intersects that of Earth), reentry will be fast enough that it'll burn up. If it entered Mars' atmosphere, it might survive reentry and crash on the surface.
Footnotes:
${}^1$ That the term "Space Junk" (as used in this answer and which is probably the right answer) has a different generally agreed meaning in spacecraft lingo than just plain "Junk" has been pointed out in this answer as well as in in this comment.
It started as visual art (we watched it on YouTube, it was beautiful! (makeshift GIF, looking for something better)).
And now, it will forever be conceptual art. Even the Wheel of Cheese was art.
Tongue in Cheek from The Verge: Elon Musk made history launching a car into space. Did he make art too? - An investigation, with a very official point system
Will Voyagers, with their plaques and records that Carl Sagan and a host of others word so hard to make happen be simply "space junk" when we loose contact? Will New Horizons, carrying Clyde Tombaugh's ashes and artifacts be junk as well? Are you sure? Perhaps consider the points made in this answer. "One person's junk is another person's treasure..."
The Pioneer Plaque, junk as well? Or is there in fact still a purpose to these spacecraft's existence?
The spacecraft also serves as a symbol. It has a purpose, people will visit it again in the future if it hasn't collided with something, there's absolutely no question about that. There are already plans to revisit the Apollo landing sites.
It is a statement, a symbol, an artifact. It's more than discarded rocket body number n, it's a red sports car in space!
In the industry, the concern with space junk is whether or not certain objects are a navigational hazard. If the Falcon Heavy payload were on a collision course with an active spacecraft, then it would definitely be a navigational hazard, as it has no way to redirect itself. That said, there really isn't a lot to avoid out where it is going.
As can be seen in the other answers, the issue of whether or not the payload can be considered "defunct" is more subjective, and in fact it is an issue that is unlikely to be resolved by some formal definition. Space junk is loosely defined for a reason. We talk about it from a collective perspective. Individual items simply aren't relevant until they are an active threat. That isn't to say that individual items aren't tracked--they are. That is part of the mitigation strategy for the broader issue.
If you define "space junk" as any human artifact anywhere in space serving no useful purpose, then Starman has become space junk, but so have a number of NASA or other space agency probes that have either malfunctioned, or exhausted their propellants and/or power supplies (RTGs), so the Voyagers are destined to become "space junk" in interstellar space.
If you define "space junk" as a synonym for "orbital debris" (human artifacts in Earth orbit serving no useful purpose, as is usually the case), then no, Starman is not because it is not in Earth orbit.
Junk in space only matters if
Orbital debris matters because there is so much of it.
It's still transmitting imagery, so by that standard it's not junk (yet). Once the batteries drain it will be inert. It has no propulsion capability to speak of, not even reaction control thrusters to change attitude.
It's on an orbit where aphelion will be in the asteroid belt; the odds of it posing a hazard to future space flight is low. If, in the coming years, it encounters Earth and re-enters, it will burn up fairly easily.
It's less of a hazard than dead sats and debris in MEO.
