The Mind-Bogglingly Big Journey Ahead

Jan 12, 2021

By David Dezell Turner, Lucy Intern

“Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.” — Douglas Adams, The Hitchhiker’s Guide to the Galaxy

Lucy is traveling to the Trojan asteroids, which are almost 500 million miles (800 million kilometers) from the Sun, more than five times as far from the Sun as Earth. Even light, which zips around at over 670 million mph (1.1 billion kph), takes almost 45 minutes to travel that distance. Lucy will not be traveling this enormous distance in a straight line, however. The spacecraft will need to fly past the Earth twice for gravity assists, then past (52246) Donaldjohanson in the main asteroid belt, then past five Trojan asteroids in the Greek L4 swarm, and finally past Patroclus and Menoetius in the Trojan L5 swarm. In total, Lucy will travel almost 4 billion miles (6.3 billion km)! Even if you had the Thrust SSC, the fastest car to ever travel on land, traveling that distance at its top speed of 763 mph (1228 kph) would take you over 585 years!

The trajectory of the spacecraft. The spacecraft is in red, the Earth and its orbits are in blue, and Jupiter and its orbit are in green. Each target is named as the spacecraft flys by and will change from white to gray after the encounter.

Fortunately, Lucy will be racing through the solar system at an average speed of 39,000 mph (63,000 kph) and will reach a blazing top speed of 92,000 mph (150,000 kph). A runner traveling at Lucy’s top speed could finish a marathon in just over a second. Even so, the spacecraft will take over 11 years to make its complete Trojan tour.

The Trojans are moving quickly too. The two Trojan swarms remain on either side of Jupiter as it speeds around the Sun at an average of 29,000 mph (47,000 kph). Relative to its Trojan targets, Lucy will fly by at speeds ranging from 13,000 mph (21,000 kph) to 20,000 mph (32,000 kph). Even moving at Lucy’s slowest relative flyby speed, you could travel from New York City to Los Angeles in under 12 minutes.

The mission therefore requires tremendous precision, as Lucy must locate and photograph most of these asteroids from 620 miles (1000 km) away, all while whizzing past them. Leucus, for instance, is 21 miles (34 km) wide; during the flyby, Lucy will travel that distance in under six seconds. Since Lucy will be hundreds of thousands of miles from Earth, the Mission Operations Center cannot instantly communicate with the spacecraft. As the spacecraft is making its closest approach to each Trojan target, a radio signal sent from Earth (which travels at the speed of light) would take on the order of 45 minutes to reach it.

For that reason, every part of the flyby must be painstakingly planned months in advance so that each step can be executed automatically. This planning process, overseen by the Science Operations Center (SOC), is extremely complex. “It’s a lot of pressure because it’s a one-time thing,” says Emma Birath, Lucy’s Uplink Lead. “It’s not an orbiting mission, so you’ve just got to get it right.” Fortunately, Lucy isn’t starting from square one. Many of Lucy’s SOC team members (including Birath) also worked on the New Horizons mission to Pluto and the Kuiper Belt, which taught them several lessons about crafting efficient flyby sequences. On New Horizons, each instrument was triggered when a specific amount of time had passed. Unfortunately, this meant that if the spacecraft followed a slightly different trajectory than expected, the instruments could be triggered too far from or too close to the targets. To prevent this, Lucy will instead use range to trigger its flyby sequences, which means each instrument will be triggered when the spacecraft is a specific distance from each asteroid. Like New Horizons, the Lucy Mission uses data sets called SPICE kernels, which contain predictions of the spacecraft’s precise position and velocity as a function of time. The SOC team must manage several slightly different SPICE kernels, each representing a slightly different trajectory. Deviations from the expected trajectory could happen for several reasons, including if the spacecraft must launch at a different time within the launch window, or if there are variations in the firing of the spacecraft’s engines. Even the smallest changes could adversely affect the execution of the flyby sequences, so each flyby sequence undergoes several tests to ensure mission success. One example is the perturbed file test, in which the team intentionally gives the software a different SPICE kernel from normal to check whether the spacecraft could still have a successful flyby. The team also checks the flyby sequences for potential block collisions, which is when a set of commands is called while another set of commands is running. The spacecraft would just ignore the new set of commands, which could prevent the collection of important data, an issue the team works diligently to prevent. The flyby sequences undergo several drafts, and the final drafts must be completed at least 60 days before the spacecraft makes its closest approach to each asteroid. Updates can still be made after this point (if we need to change the exposure of one of the spacecraft’s cameras, for example), but these updates must be made at least four days before closest approach.

By observing each asteroid using occultations or the Hubble Space Telescope, we can ensure their orbits are known as precisely as possible before Lucy arrives. However, even with thorough observations beforehand, we still cannot be completely certain of each asteroid’s exact position, so during flight, Lucy’s Terminal Tracking Camera (T2CAM) will help the spacecraft locate the asteroids and point its instruments toward them. Even from 620 miles (1000 km) away, Lucy will be able to take high-resolution photographs of the asteroids. L’LORRI, Lucy’s LOng Range Reconnaissance Imager, will be able to see elephant-sized objects from that distance. That’s like standing at one end of a football field and being able to clearly see the head of a pin at other end!

Though space will forever be “vastly, hugely, mind-bogglingly big,” Lucy’s incredible speed and keen eyes will make these astronomical distances seem a little bit smaller.