In November of 2010 (and not October as you see above – comets are fickle beasts) the spacecraft EPOXI (better known as Deep Impact) made a flyby of the comet 103P/Hartley (Hartley 2). It found a rather odd world – peanut shaped, for one. For many of the pictures from the flyby, go here.
As you can see from the above widget, the comet is both small and has an extremely short orbit – barely making it beyond Jupiter. Click on the equals sign at the bottom of the output, by the way, to search for further information, for example where it was in November 2010.
More recently, papers and news articles have been written based on the data collected by Deep Impact. They find that the comet contains a larger amount of Carbon Dioxide than other comets (bear in mind that it was only the 5th comet visited by spacecraft) and is out-gassing quite a lot. Older research also shows that the comet will only last around 700 more years at the current rate of mass loss.
You can see where this is going, can’t you?
If it looks young, maybe it is young
What’s his evidence for this? Let’s see…
Basically, as he quotes:
For starters, its nucleus contains an abundance of carbon dioxide (CO2—or, in solid form, dry ice). This is a volatile material—it burns easily—and so scientists would expect much more of it to have burned away in the 4.5 billion years since the comet formed along with the rest of the solar system.
The author of the source article probably meant ‘sublimates’ where they wrote ‘burns’. This is when dry ice turns directly from a solid into a gas. However, heat is needed for this, and Mr Thomas seems to over look the possibility that Hartley 2 might only have been in it’s current, short orbit for a very short period of time, most likely through the influence of Jupiter. As the transcript of a podcast on the comet says:
From the observations over the years, we know that it had a close approach to Jupiter in 1982, and again in 1993 when it passed a mere 0.37 astronomical units from Jupiter. That close approach changed the comet’s orbital period from 6.26 to 6.39 years and its perihelion distance from 0.95 to 1.058 astronomical units, from just inside the Earth’s orbit to just outside. The aphelion, or far point, is just outside of Jupiter’s orbit.
It seems likely that previous effects on the comet like that have been the cause of the comets movement into the inner solar system. This means that the current rate of out-gassing has not been sustained over the lifetime of the comet. This in turn means that attempts to extrapolate back the amount of material that has sublimated off the comet in recent times to millions of years ago are fundamentally flawed from the outset.
It could well be the case that the current activity from the comet is a kind of last-gasp for a small, dying comet. Every comet so-far photographed from spacships have been unique – as I said, there have been only five so far. Try again, Mr Thomas. Try again…
For some more pictures and science from the flyby, click here.