LiteBIRD Newsflash

Just a quick post to pass on the news that the space mission LiteBIRD has been selected as the next major mission by the Japanese Space Agency JAXA and  Institute for Space and Astronautical Science (ISAS).

LiteBIRD (which stands for `Lite (Light) satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection’) is a planned space observatory that aims to detect the footprint of the primordial gravitational waves on the cosmic microwave background (CMB) in a form of a B-mode polarization pattern. This is the signal that BICEP2 claimed to have detected five years ago to much excitement, but was later shown to be a caused by galactic dust.

It’s great news for a lot of CMB people all round the world that this mission has been selected – include some old friends from Cardiff University. Congratulations to all of them!

I’m not sure when the launch date will be, but the mission will last three years and will be at Earth-Sun Lagrange point known as L2.It will be a very difficult task to extract the B-mode signal from foregrounds and instrumental artifacts so although there’s joy that it has been selected, the real work starts now!

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BepiColombo goes to Mercury

You may have missed the news that the joint Japanese-European (JAXA-ESA) Mission BepiColombo was successfully launched on October 19th 2018 and is now on its way to the planet Mercury, where it will arrive in December 2025.

As it happens I’ve just finished delivering a set of lectures on gravitational dynamics, part of which was devoted to orbital mechanics. One of the problems I worked out during these lectures was the Hohmann Transfer Orbit which is the simplest way to get a spacecraft from Earth to, e.g., Mars (which take 8 to 9 months to reach).

Since the radius of Mercury’s orbit is 0.39 AU (compared with Mars’s 1.52 AU) you might think it would take a similar time to reach Mercury, but designing a trajectory that results in a controlled encounter with Mercury in an efficient manner is much harder than for Mars (largely because Mercury is moving much faster). The solution to this problem involves a series of encounters with VEarth, Venus and Mercury each of which results in an adjustment to BepiColombo’s orbit until it finally encounters the target planet at a reasonable speed. This approach takes over seven years, but it saves an enormous amount on fuel – using the gravitational boost from planetary encounters instead of firing rockets.

Here’s a video showing this complex but fascinating example of orbital mechanics in action:

p.s. the dates in the video correspond to the originally planned launch date of 5th October, so are off by a couple of weeks.

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The End of Hitomi..

Time for a gloomy Monday update to my recent post about the Japanese X-ray satellite Hitomi.

First here’s a new plot of the debris (via Jonathan McDowell):

This shows more pieces of debris than the one I showed previously, and also demonstrates that some of the pieces are in rapidly-decaying orbits. A rough estimate suggests that some of these – those in the lower right of the diagram- will burn up in the atmosphere within a week or so. This behaviour is consistent with them being rather light fragments, on which the effect of drag is greater, and consequently possibly rather small.  Their behaviour does not therefore necessarily imply anything too catastrophic about the main spacecraft.

However, there is now strong evidence that the main spacecraft actually did break up fairly completely rather than shedding a few pieces of casing or whatever. Two of the brightest pieces are of roughly equal size and, ominously, the original identification of one of them with the main part of the spacecraft has been shown to be wrong. Furthermore, no signals have been received from the onboard beacon for six days now. It all sounds very terminal to me.

So what happened? Of course I don’t know for sure, but the above picture suggests the possibility of an explosion (possibly violent outgassing of cryogens needed for the instruments near the rear of the main body of the vehicle). The structure to the rear of the vehicle is a deployable optical bench used to increase the focal length of the telescope for hard X-ray work. This could well have broken off during such an explosion, as could all or part of the solar panels used to supply power to the satellite.

The Japanese Space Agency JAXA has not officially given up on Hitomi (formerly known as ASTRO-H) but I think the hopes of most commenters I’m aware of have now faded away.

It’s all very sad.

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The Trouble with Hitomi

One of the stories I’ve been following a bit while taking a break from blogging has been that of the Japanese X-ray satellite Hitomi (formerly known as ASTRO-H), which was launched into a low-Earth orbit on February 17th 2016, experienced a “communication anomaly” on Saturday March 26th. It has now become clear that this was more than a simple communications glitch. Astronomer Jonathan McDowell posted this diagram on Twitter that showed a sudden decrease in the orbital period of the satellite:

Students of orbital dynamics will know that a decrease in orbital period corresponds to a decrease in the semi-major axis of the orbit, so Hitomi actually fell during this episode. It dropped only slightly – look at the % change on the graph – but by enough to be very worrying.

The plot thickened still further when radar detected five pieces of debris near the satellite and visual observations indicated the spacecraft to be tumbling rapidly. That suggested a very grim picture.

Putting the evidence together it seems that some kind of explosive event – possibly connected with out-gassing of cryogenic material from one of the on-board experiments – had damaged the satellite, changed its orbit and set it spinning uncontrollably.

Since then ground stations have picked up some signals from Hitomi, which is good news,  but these broadcasts are just from the on-board beacon. It has not yet proved possible to communicate with the attitude control system which is the only way to get it back into a stable state.

Obviously it’s touch and go as to whether the Japanese Space Agency JAXA will be able to regain control of Hitomi, but at least there’s more hope than on Saturday when many of us thought the vehicle had fallen apart. In fact the pieces of debris reported may be rather small (ten cm or so is detectable) and the main body of the telescoper may be intact. Maybe.

Update: April 1st. Tracking facilities are now reporting 11 pieces of debris, and also suggesting the object whose period is plotted in the above graph may not be the main part of the spacecraft. This does not sound good.

Update: April 2nd. The debris from Hitomi has now spread out due to different orbital speeds. The two largest pieces are both spinning out of control. I would say at this point that hope of a recovery has now disappeared. It’s very sad.

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