# What happened to the Viking 2 Orbiter?

What was the ultimate fate of the Viking 2 Orbiter? According to NASA:

The orbiter developed a leak in its propulsion system that vented its attitude control gas. It was placed in a 302 x 33176 km orbit and turned off on 25 July 1978 after returning almost 16,000 images in 706 orbits around Mars.

But this doesn't say what happened to the orbiter. Did it crash into Mars's surface? If so, do we know where it crashed? Or is it still up there, orbiting? And what does that orbital notation (302 x 33176 km) mean? NASA's catalog says of the Viking 1 Orbiter:

On 7 August 1980 Viking 1 Orbiter was running low on attitude control gas and its orbit was raised from 357 x 33943 km to 320 x 56000 km to prevent impact with Mars and possible contamination until the year 2019. Operations were terminated on 17 August 1980 after 1485 orbits.

So Viking 1 is still orbiting until 2019 (which is coming up, I wonder if they have any plans on that). But Viking 2's orbit was lower, so could it have already crashed? The landers were sterilized, but the orbiters were not, so they could contaminate the surface, if there are any microbes alive on them (not that other spacecraft haven't crashed on Mars).

• The $m$ x $n$ notation indicates an orbit with a periapsis of $m$ and apoapsis of $n$. – DylanSp Jul 18 '16 at 17:03
• @DylanSp Thanks, that makes sense. It still means that Viking 2 was in a lower orbit, though. – Phiteros Jul 18 '16 at 17:04
• – called2voyage Jul 18 '16 at 17:41
• You shouldn't take the "2019" too seriously. That was their best estimate at the time, but there are a lot of things that they didn't know then about the Mars gravity field and blooming of the atmosphere at very high altitudes. – Mark Adler Jul 18 '16 at 17:46
• In the case of spacecraft, the $m\times n$ notation indicates altitude at periapsis and apoapsis. 302 km from the center of Mars is deep inside Mars' core. – David Hammen Jul 19 '16 at 1:46

## 1 Answer

It seems pretty likely that it is still in orbit. MRO is in an orbit of about 370-400 km, which is quite low, but still doesn't use significant fuel to keep it up. 250 km is considered too high for aerobraking. Almost certainly Viking 2 is still orbiting Mars, probably in a similar orbit to what was last known. The only real question is if sunlight pushed it into an orbit where it started to dip in to Mars's atmosphere, at about 150 km.

There has, however, been some debate recently as to if the Viking Orbiters are still around. Most of the models indicate that both orbiters are still orbiting to this date, however some of them indicate that they might have crashed in to Mars already.

In any case, the likelihood of any kind of contamination is very small. An uncontrolled re-entry would expose the spacecraft to tremendous heat, the UV radiation will kill anything living on it prior to the time, and the high velocity of impact will also sterilize it. The future (Or present) crash sites of the Viking orbiters are likely to be more sterile than Curiosity, Spirit, Opportunity, and the other landers that have arrived on Mars since Viking.

• That's a rather complex report, but if I am understanding it correctly, it is all dependent on the orientation of the craft (which influences how much solar radiation pressure they experience) and the gravitational model of Mars used to calculate the orbits. The authors of this article seem to indicate that, based on no evidence of the orbiters crashing and new gravitational models, both orbiters are still in orbit. Am I reading this correctly? – Phiteros Jul 18 '16 at 17:44
• If you read that paper correctly, the answer is "we don't know". – Mark Adler Jul 18 '16 at 17:45
• @MarkAdler "It seems pretty likely" - I don't think Pearson is saying that we know. – called2voyage Jul 18 '16 at 17:51
• Well, there is the idea of using satellites orbiting Mars to try and track tiny objects, in the hopes of finding them, but I'll believe it when I see it. – PearsonArtPhoto Jul 19 '16 at 2:42
• Well, if you ask me, it's a smashing good idea! – Mark Adler Jul 19 '16 at 4:31