New planetary missions are desperately needed to visit Uranus and Neptune since these ice giant worlds haven't been visited since Voyager's flybys in the late 1980s. Scientists said in a new analysis that such a spacecraft would reveal much about these solar system siblings, but it could also peer much deeper into the universe. Astronomers may be able to detect ripples in gravity caused by some of the most violent events in the universe by carefully observing the variations in radio signals from one or more such spacecraft.
Voyager 2's images of those outer worlds, which came to us in the late 1980s on its "Grand Tour", are the only ones available to us.
There have been probes sent to Mercury, missions to Jupiter, Saturn (including landings on Saturn's moon Titan), and spacecraft that have explored asteroids and comets and driven rovers on Mars.
But Uranus or Neptune remain untouched.
The worlds that we now refer to as "ice giants" exist alone on the outer fringes of our celestial neighbourhood as they are saturated with water and ammonia ices.
They are unique in the solar system, and an entire generation of planetary scientists has studied them with only ground-based telescopes and occasional glimpses from the Hubble Space Telescope.
Neptune, even at its closest, sits over 4.3 billion kilometres from Earth, so some of that delay has been out of our hands. The extreme distance to Neptune and Uranus makes it incredibly hard to launch payloads there.
However, there is an opportunity soon to come.
A window when Jupiter lines up so that it provides a much-needed velocity-boosting gravitational assist to the outer system and significantly reduces travel time. The mission could reach Jupiter in around two years if it were launched in the early 2030s on a sufficiently powerful rocket such as NASA's Space Launch System. A single spacecraft would then become two separate spacecraft, one headed to Uranus (reaching it in 2042) and the other heading to Neptune (achieving orbit two or three years later).
Those orbiters could stay en route for over a decade if luck prevails, just as Cassini did at Saturn.
In an updated paper uploaded to the preprint server arXiv.org recently published in the Monthly Notices of the Royal Astronomical Society Letters, the same space probes could also provide insights into another type of science, gravitational waves. Papers on these issues have recently been uploaded to the preprint server arXiv.org for submission to the Monthly Notices of the Royal Astronomical Society.
As the spacecraft travels through space, scientists and technicians must update the trajectory and check its status, releasing information to the spacecraft in a wireless fashion.
Physics researchers measure passing gravitational waves with lasers reflected along miles-long tracks on Earth. While passing through the Earth, the waves distort objects, compressing and stretching them in alternating series. The gravitational wave detectors are sensitive to these waves by altering the distances between distant mirrors, affecting the ray paths in small amounts.
Radio communications from a faraway space mission back to Earth have the same effect. The distance to the spacecraft would stay steady if a gravitational wave passed through the solar system so that the probe would be ever-so-slightly closer to us once, then farther away a few times, then closer again. There would be a Doppler shift in frequency of the radio communication if the spacecraft were continuously transmitting throughout its voyage.
However, astronomers would have sharper observations of the shift if two such spacecraft were acting simultaneously.
Basically, these sputtering probes may be the largest gravity wave observatories on the planet.
A huge technological challenge will be measuring the frequency of spacecraft radio communications with a high degree of precision. Recent research suggests that we must be able to measure this with an accuracy at least 100 times better than we were able to achieve for the Cassini mission to Saturn.
A visual representation of a gravitational wave
This may seem like a lot, but Cassini was been designed decades ago and we've drastically improved communication technology in that time. As it stands, the Laser Interferometer Space Antenna, like the Laser Interferometer Laboratory in London, is currently being implemented by physicists, and it involves similar technologies. Since we're still about a decade away from an ice giant mission, we could spend even more time developing the necessary technology.
When that level of sensitivity is cracked, the extreme length of that gravitational wave detector's "arm" (literally billions of times larger than our current sensor's) could uncover a large number of extreme phenomena happening in the universe.
Owing to its tremendous size, this "ice giant observatory" would be sensitive to an utterly distinct type of events than what we can observe today.
Several dozen mergers of black holes with intense mass variations are likely to be observed during the lifespan of such a mission, and one merger that is expected to be spotted is a merger involving a supermassive black hole.
With existing gravitational wave detectors, we simply cannot observe such events.