 As you no doubt recall, if you've been following this blog, Rosetta's Philae comet lander made a successful (and eventful) landing on Comet 67P on 12 November 2014. After a non-nominal landing it found itself in an unfortunate location and in an unusual attitude such that the performance of its solar arrays was critically compromised. As a consequence, once its batteries were almost exhausted, little Philae entered an automatic process which placed it into a period of hibernation - and I would have guessed that that would be the last we'd hear from it. The only hope was that as the comet moved closer to the Sun on its orbit, the sun's intensity and the lander's temperature would increase, making it at least possible that the batteries would recharge.  Well, much to the surprise of the ESA scientists and engineers, Philae 'spoke' briefly for 85 seconds, the signal being received at 20.28 UT on Saturday 13 June 2015 at the European Space Operations Centre. Among other things, it indicated that its temperature was a balmy -35 degrees C, and it currently had 24 W available from the solar array. The simple brief message suggests that Philae may yet recover from its long sleep. The comet is not yet at perihelion (the closest approach to the Sun - which occurs on 13 August). So hopes are high that Philae's work is not yet done.  Image of the comet nucleus taken by Rosetta's NavCam on 5 June 2015 while at a range of 208 km. Solar heating is causing jets of gas and dust, which form the comet's characteristic tail as it approaches the Sun. Image courtesy of the European Space Agency. In the mean time, the 'mothership' Rosetta continues to orbit the nucleus, imaging and recording every detail as the nucleus begins to warm up .
 Artist's impression of New Horizon's encounter with Pluto and Charon on 14 July 2015 When I were a lad I grew up with the ‘old Solar System’, which had nine planets, and with mnemonics to help me remember their names and sequence – such as “My Very Easy Method Just Speeds Up Naming Planets”. Up to the present time, all of these bodies have been visited or orbited by spacecraft – bar one – humble Pluto. However in the early noughties, NASA decided to rectify this deficit, and the process culminated with the launch of the ‘New Horizons’ spacecraft on 19 January 2006. However, ironically in the August of the same year the International Astronomical Union decided to demote Pluto from full ‘planet’ status to a ‘dwarf planet’ producing the ‘new Solar System’ comprised of 8 planets (and in the process ruining all those long-standing mnemonics of old!). So New Horizons has been travelling across the Solar System for the last 9 ½ years or so, and on the day of writing (11 June 2015) finds itself 34 days and about 40 million km (25 million miles) from its destination. Travelling at a speed relative to Pluto of about 13.8 km/sec (8.6 miles/sec) it will finally reach its closest approach to its target on 14 July 2015. Note that this is a fly-by mission – New Horizons does not have the rocket power or propellant to allow it to slow down and enter orbit around Pluto. So this means that operations to observe and measure the Pluto system is a rather brief affair, beginning on 4 July and concluding on the 20th. After the encounter the spacecraft will continue on its trajectory and will ultimately leave the Solar System.  Image of Jupiter and Io taken by New Horizons during its Jupiter swing-by in February 2017 As a consequence, the trajectory, and in particular the flight time from Earth to Pluto had to be carefully chosen. One of the main drivers is a trade-off between spacecraft reliability on the one hand, and ensuring that there is sufficient time to observe Pluto during the encounter on the other. Current technology and build techniques allow spacecraft to be manufactured with a typical expectation that they will continue to operate reliably for something like 10 to 15 years. Once they reach about the 10 year mark there is a probability that there may be sub-nominal operation of some components. However, with redundancy built into the design, it is reasonable to expect that the spacecraft’s performance is not critically compromised. So in choosing the New Horizon flight time, this important aspect of spacecraft reliability needed to be considered. For example, the mission designers could have chosen a larger boost out of Earth orbit to decrease the flight time to, say, 5 to 6 years, so increasing the probability of reliable operation at Pluto. The down side of this strategy would be an increased speed relative to Pluto, and consequently a shorter period for close approach operations. On the other hand a flight time of, say, 15 years would slow the spacecraft at encounter and so increase the opportunity to collect data. However the longer mission would then result in poor reliability and potentially spacecraft failure prior to the encounter.  The Pluto system as seen by Earth-based telescope The mission target Pluto was discovered in 1930, and is a small object about 2400 km (1500 miles) in diameter. It is currently about 30 times more distant from the Sun than the Earth, which means that the solar intensity is about 1/1000 th of that at Earth. As a consequence it’s cold (Pluto’s surface temperature is around – 230 degrees C). Images from the Hubble Space Telescope, and other ground-based systems, currently give Pluto a total of 5 moons. Charon is the largest of these, being almost the same size as Pluto itself, making the Pluto-Charon system to closest thing to a double planet in the Solar System. The mission poses many challenges to the spacecraft designers and operations team. The thermal control design needs to accommodate the extreme variation in environmental temperature – from the significant solar heating experienced at Earth orbit to the intense cold at Pluto. The relatively low light intensity at Pluto impacts the imaging operations. One of the main factors is the light-time between the Earth and the spacecraft. A signal from Earth to New Horizons will take about 4 ½ hours, so the close approach operations cannot be controlled from Earth. This means that all imaging and other data gathering tasks have to be carefully planned prior to the encounter, and then all the time-sequenced commands required to carry out these tasks have to be uplinked to the spacecraft, so that whole process can be controlled automatically.  Schematic of the New Horizon's encounter with Pluto on 14 July 2015 Closest approach to Pluto will occur at 11.50 UT* on 14 July, and data will be gathered by the spacecraft’s imaging systems – looking principally at Pluto, Charon, Nix, Hydra and looking for possible ring systems. Other information characterising Pluto’s surface composition, atmosphere and surface temperature will also be stored on board. Onboard storage is critical as there is no real-time communication with the ground.
After the encounter this stored data will need to be downlinked to Earth. The spacecraft has a 2.1 m (83 inch) diameter high gain antenna to help with this task. However, because of the extreme distance of propagation and the low power availability onboard the spacecraft, the data rate will be typically about 2000 bits per second (2 kbps). Comparison of this with your typical broadband data rate, say 20 million bits per second (20 Mbps), shows that your broadband’s performance is about 10,000 better than the downlink from New Horizons. The task is also limited because the large ground-based antennae of the DSN (Deep Space Network) need to be shared with other spacecraft missions. As a consequence, the process of downlinking the data products will probably take until the back end of 2016 – so don’t hold your breath! But it will be worth the wait! * UT (Universal Time) is the time system used by the astronomical community. It is essentially equivalent to GMT (Greenwich Mean Time). For those in the UK, British Summer Time BST = UT + 1, so the closest approach to Pluto will occur at 12.50 BST on 14 July 2015. |
AuthorGraham Swinerd - I hope to use this page to highlight current major events in space and spacececraft. Archives
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